II STRUCTURE AND FUNCTION OF THE BRAINFrederick A. King
III BRAIN STIMULATIONWilliam Hodos and Joseph V. Brady
IV ELECTROENCEPHALOGRAPHYDonald Lindsley
The study of the evolution of the brain and behavior is a paradox. We cannot study behavior in fossils or the internal structure of the brain of organisms long dead. In our lifetimes, we experience but a very brief interlude in the long evolutionary process. What leads us to believe that it is possible to investigate the evolution of the brain and its relation to behavior?
An answer lies in the encouragement investigators obtain from examining the brains of those contemporary organisms which have close links with their fossil ancestors. Patterns in the brain are found which are common to these and different species suggesting parallels with common ancestry. Comparative behavior provides similar encouraging parallels. In practice, it is reasonable to suggest that comparative neurology and comparative behavior have information to offer which is relevant for evolutionary theory and, conversely, that evolutionary perspective provides a framework for making sense out of comparative material.
Historical highlights and approaches
C. L. Herrick was moved by such considerations when he founded the Journal of Comparative Neurology in 1891. His original vision was that the study of comparative neuroanatomy could be integrated with the study of comparative behavior.
Students of C. L. Herrick, such as C. J. Herrick and G. E. Coghill, continued in the tradition of the elder Herrick after his death. C. J. Herrick (1948) concentrated on the connections found in amphibian nervous systems, relating his information to the findings of workers who used other forms and interpreting the possible significance of the described connections. Coghill (1929) spent much of his life in the study of the development of the nervous system of the salamander, in conjunction with a study of the development of behavior. Both Coghill and C. J. Herrick worked in an era of considerable interest in comparative neuroanatomy. Much of the work of the era was summarized by Ariëns Kappers, Huber, and Crosby (1936).
Concurrently, others studied comparative behavior effectively without a simultaneous study of the structure of the nervous system, but they did not overlook the possibility of relating behavior to the nervous system. Among these were Yerkes (1916), Hunter (1913), Loeb (1918), Noble (1931), and Parker (1919). Characteristically, their studies were concerned with discriminative capacities, learning abilities, tropisms, stereotypical behavior, and sensitivities in various species. [See the biographies ofHunterandYerkes.]
A third group, frequently employing methods of intervention in the nervous system as a means of studying the relation of the nervous system to behavior or various units of behavior, was led by such outstanding men as Lashley, Sherrington, and Pavlov. Lashley (1929) was noted for his studies of the role of the mass of cerebral tissue in learning and intelligence. Sherrington (1906) was concerned primarily with the problems of the organization of the nervous system in the regulation of reflex actions. Pavlov (1927) had a similar broad concern but an entirely different approach, stressing the general concept that the study of the elicitation or suppression of reflexes by systematically paired, concurrent stimulation was the key to understanding the role of the cerebral cortex. [See the biographies ofLashley; Sherrington;Pavlov.]
Studies of comparative neurology and comparative behavior continue today both as separate ventures and as combined enterprises. Although the work of many of the men whose names are mentioned here and of numerous contemporary workers has never been specifically addressed to the problems of the evolution of the brain and behavior, much of the data is relevant.
The information accumulated and available is overwhelming but riddled with hidden error, gaps, and misconceptions. Since the work on behavior and the nervous system is being done on various species, we are obliged to make explicit our views of the relationships of these organisms, including man, if we hope to extrapolate results from one species to another. Specifically, we must clarify the evolutionary perspective with which comparative work is to be viewed.
Attributes of the central nervous system
The common origins of the various vertebrate species are reflected in the organization of the brain and spinal cord. Man, as would be expected, possesses the neuroanatomical characteristics of the vertebrates in general, the mammals in particular, and the primates especially. Those portions of man’s nervous system which are common to the vertebrates have been described as primitive, and those which man shares with mammals—pre-eminently with the primates—are referred to as recent portions of the nervous system. For example, man possesses “paleocortex” (primitive outer cell layers of the forebrain) and “neocortex” (more recently evolved outer cell layers of the forebrain). Among the primates, progressively greater proportions of neocortex are found in successively evolved representatives (Clark 1960).
Sensorimotor and central systems of connections
The portions of the nervous system most directly significant for behavior are the connections formed by the processes of the billions of nerve cells. Microscopic examination of suitably prepared biological material reveals the connections to be systematically organized. Systems of connections exist within the central nervous system which allow identification of the sensory routes, the motor pathways, and the organization of central connections. It is a common feature of the vertebrates that the sensory pathways have a continuity with motor pathways at various levels. We speak, therefore, of sensorimotor continuities which form the basis of simple and complex reflexes. Together with the action of central systems, the outcome of activity of higher continuities may become considerably more elaborate than the reflexes.
First-level sensorimotor continuity
The simplest reflex is formed by the continuity of sensory cells and motor cells, the sensory cells sending their processes directly to the motor cells. Such reflexes are represented in the spinal cord and in the brain stem and are common to all vertebrates. The reflex closing of the jaws as they tend to open of their own weight is an example of the simplest reflex, the two-neuron or monosynaptic reflex. Stretch receptors in the jaw muscles conduct the excitation directly into the brain via sensory neurons. The muscle stretched is the same muscle that is thereby stimulated to contract.
The simplest reflexes are segmental reflexes, the afferent and efferent neurons belonging to the same segment or segment fraction. Vertebrate segmentation is comparable in terms of embryological origins, and the concept of segmentation is used to include the structures related to each of the fifth, seventh, ninth, and tenth cranial nerves. The jaw muscles, for example, which possess an afferent and efferent supply from the same segment (fifth nerve), tend to remain reflexly in a steady state as they respond to the steady pull of gravity.
The significance of the reflexes of a segment fraction is more far-reaching than indicated thus far. The first-level reflexes are the reflexes of sustained activity (tonic reflexes), and they are generally opposed by the second-level reflexes. The reflexes of the first and second levels form a reciprocal dichotomy. (We are dealing here with only the first half of the dichotomy.) Tonic reflexes are supported by cutaneous stimulation as well as by stretch stimulation. The skin is systematically supplied by spinal nerves and cranial nerves in a segmental pattern. In all vertebrates, particular portions of the body surface belong to distinct segments or segment fractions. As in the stretch reflex, it is reasonable to suggest that moderate stimulation of a given area of the body surface results in reflex action which is prolonged and keeps the stimulated body surface in continuing contact with the stimulus. The extensor muscles of the limbs, in response to stimulation of the toe pads, reflexively support the standing posture of the quadruped and thus enhance the continuing stimulus. The radial nerve which innervates the dorsal surface of the hand also supplies the extensor muscles in the upper limb. The extensor posture of the ape’s forelimb, as he rests his weight on his knuckles, is consistently categorized in these terms. The median nerve supplies the palmar surface and the flexor muscles of the hand. The grasp reflex of the human infant is understandable in a similar way. The value of the grasp reflex to the primate clutching his mother’s hair as she carries him through the trees can be readily appreciated.
Second-level sensorimotor continuity
The reflexes of the second level form the other half of the reciprocal dichotomy. Each of the first-level reflexes can be inhibited and replaced by an antagonistic action when the stimulus strength changes from moderate to intense. A thorn in the knuckle of the ape prevents extension and produces flexion. A beesting to the palmar surface will break the grasp reflex and produce opening (extensor action) of the hand. These are examples of the basic withdrawal reflexes which depend upon inhibition of the simpler reflexes in order to appear. They are polysynaptic reflexes which also depend upon sensorimotor continuities in the spinal cord and brain stem. It is reasonable to regard these second-level reflexes as discharging motor neurons in segment fractions adjacent to the stimulus and inhibiting motor neurons in the same segment fraction as the originating stimulus.
The second-level continuities are common to all vertebrates. In vertebrates without appendages the form of the first-level and second-level reflex organization need not differ fundamentally from that found in the other vertebrates. It is characteristic of all vertebrates that the cutaneous portion of the fifth nerve, which provides the nerve supply to the face, extends its fibers into the cervical spinal cord, where contact is established with motor neurons controlling the neck musculature. The arrangement allows for reflex turning of the head toward or away from the side of cutaneous stimulation. When the stimulus to one side of the face is strong, the neck musculature reflexly turns the head away from the stimulus. When the stimulus is moderate, the neck musculature turns the head toward the side of the stimulus. It is instructive to note with regard to the above statements that the larval form of the primitive eel, Petromyzon marinus, shows reactions comparable to those of the human infant. The infant turns its head toward the nipple as it contacts the cheek but turns its head away if the cheek is pinched. The larval form of Petromyzon marinus burrows into the mud of the river bed. The burrowing is accomplished by undulatory movements of the entire body as the organism penetrates the mud head first. It is logical to expect that, as it burrows, it presses the side of its head first to one side and then to the other side of the hole it makes as it burrows. It would be consistent with effective burrowing for the larva to continue to press to one side until the cutaneous stimulus became strong on that side and then to reverse its pressing, the pressing continuing on the other side until the stimulus there became strong, and so on to produce successive undulations starting near the head and proceeding tailward down the body until the animal has gone far into the mud by the forward propulsion of these undulations.
Copulatory reflexes, which are, of course, essential for species propagation, may be viewed as special instances of alternating first-level (tonic, thrusting) and second-level (withdrawal) reflexes.
Third-level sensorimotor continuity
A continuity of sensory to motor fibers is established through the medial reticular formation of the brain stem in all vertebrates. Sensory fibers of the dorsal root ganglia connect with interneurons of the spinal cord. Many of these interneurons send their processes directly to the large cells of the medial reticular formation. It should be said parenthetically that the brain-stem reticular formation consists of a meshwork of cell bodies and fibers in the core of the brain stem, and it is in the medial part thereof that the large cell bodies are situated. The large cell bodies send processes back to interneurons of the spinal cord, where contact with motor neurons can be made. The medial reticular formation is also influenced by cranial nerves, which directly contact the dorsolateral portion of the reticular formation. The dorsolateral portion, in turn, connects with the medial reticular formation.
The significance of the third level of continuity lies in providing a route through which existing reflex activity may be enhanced and competing, antagonistic reflex activity may be weakened. It is a route through which stimulation arising in one segmental fraction may influence sensorimotor action in more widely distributed segments or many segments at once. In concert with intersegmental spinal connections, the third level stabilizes the stereotyped whole-body postures of terrestrial forms and the stereotyped whole-body movements of aquatic forms. In stabilizing the upright posture of the vertebrate, the reticular role is heavily dependent on the action of the vestibular nerves. Vestibular reflexes, stimulated by the pull of gravity, define the symmetrical, upright posture of the head and body. Magnification of the action is achieved through level 3. Except for primates in which even higher levels of sensorimotor continuity are required for effective standing, level 3 is adequate for maintaining an exaggerated, crude, upright position.
When the organism is ill or weary, however, the upright posture collapses. The mechanism of the action is unknown, but it is evident that the vestibular reflexes and the actions of level 3 are considerably diminished under such circumstances. It is possible that a source of inhibition on the medial reticular formation rather than simply an exhaustion of reticular activity enforces the resourcerestoring condition of rest. A logical origin of the inhibition may be the visceral afferent input reaching the dorsolateral reticular formation directly through the vagus or glossopharyngeal nerves, which carry afferent responses to a visceral “crisis” (chemical imbalance).
Fourth-level sensorimotor continuity
Interneurons of the spinal cord, serving spinal afferents, and neurons of the brain stem, serving cranial nerve afferents, distribute axons to the cerebellar cortex. The cerebellar cortex, in turn, connects with deep cerebellar nuclei, which send axons to the medial reticular formation. These connections form the fourth level of sensorimotor continuity. The arrangement is one which allows sensory feedback to have a regulative impact on the actions of the medial reticular formation. Broadly speaking, sensory elements responding to muscle tension and elements responding to moderate cutaneous stimulation are sources of negative feedback. Elements responding to intense stimulation such as pain are sources of positive feedback, which serves to sensitize all reflexes except the first-level reflex to the local stimulus.
The vestibular nerve connects directly with the fourth level. Typically, the result of vestibular activity is dependent on asymmetry of stimulation of the vestibular receptors on the two sides. Thus, if the right side of the head is lower than the left, the extensors of the right side are stimulated, while the flexors of the left side are stimulated. The action is magnified by the vestibular contribution to the fourth level, the fourth level acting on the third level. The balance between the two sides of the body which is achieved is characteristic of the role of the cerebellum in equalizing the muscle tone (sensitivity of postural reflexes) on the two sides of the whole body during stationary postures.
Although the cerebellar level is represented in all vertebrates, it becomes progressively more elaborate as the vertebrates themselves exhibit greater differentiation of segmental structures.
Fifth-level sensorimotor continuity
The midbrain tectum and associated central gray also receive directly from rostrally conducting interneurons of the spinal cord and brain stem. The tectum sends fibers to the reticular formation and upper spinal cord. The fifth level of sensorimotor continuity is thus formed. Optic, auditory, vestibular, and somatic sensory systems converge on the midbrain tectum and/or central gray. These sensory systems provide guidance for primitive, whole-body reactions. The reactions, like the isolated reflexes of the first and second levels, fall into two mutually exclusive categories. They consist of whole-body reactions toward or away from a stimulus. Towardturning and away-turning form one dichotomy, while forward lunging and backward progression form another. The importance of this level for food-seizing, whole-body approach (including sexual approach), and whole-body withdrawal is evident. Coincident with approach and withdrawal, the midbrain (central gray particularly) is implicated in the linkages which define “distress” or “well-being” in terms of concurrent intrinsic reactions such as “distress calls,” “cooing” vocalization, autonomic changes, and endocrinological reactions.
Fifth-level connections are common to all vertebrates, but they are especially well developed in birds. The fifth level is probably most important to mammals when they are as yet immature. When the seventh level (recently evolved forebrain level) reaches maturity in mammals, it provides a more elaborate and flexible guidance system for wholebody activities. In the long history of the vertebrates, the functioning fifth level probably made possible the evolution of the seventh level, which, in a sense, supersedes the fifth level. In the ontogeny of the mammal, the immediately functioning fifth level allows for the more gradual development of the seventh.
Sixth-level sensorimotor continuity
The same order of sensory interneurons which entered into connection with levels 3 and 5 connects with the primitive (mid-line and intralaminar) thalamus. The primitive thalamus, besides having numerous central interconnections, connects with the striatum and hypothalamus. Both the striatum and hypothalamus connect with the midbrain level. It is suggested that the striatum paces somatic activity and the hypothalamus paces the concurrent visceral activity, both the striatum and hypothalamus being dependent on the connections through the primitive thalamus. The primitive thalamus is, thus, critical in adjusting the rate of reaction of the organism. Perhaps because of its systematic central connections, the presumptive outcome of the activity conducted through the primitive thalamus would be to increase or decrease the rate of sustained withdrawal or approach at the same time concurrent visceral activity is intensified or retarded. To be of survival value, the outcome must be in accord with the demands placed on the organism, as these demands are represented by the convergence of sensory input to the thalamus.
Level 6, together with its associated primitive forebrain connections, is represented in all vertebrates. The contribution of the olfactory nerve is represented at the sixth level as it reaches the striatum, hyypothalamus and paleocortex (or its counterpart in primitive forms) almost directly. On theoretical grounds, olfactory stimulation is thus in a position to determine the rate of visceral and somatic activity. The contribution to the cortex is likely to play a vital role in the memory of specific odors. Various odors are, or become, important for determining whether the organism hastens its approach or retreat. Other substances indicate whether the organism is in territory where it usually comes to rest.
Seventh-level sensorimotor continuity
Sensorimotor level 7 and its associated central connections evolved out of those in level 6 for the most part, but links with all other levels are apparent.
Just as there are second-order neurons connecting directly with the primitive thalamus, so there are second-order sensory neurons connecting directly with the neothalamus, the neothalamus being defined as those portions of the thalamus which have major projections directly to the neocortex. The neocortex is understood as that part of the cerebral cortex which is found in mammals only. Parts of the neocortex, in turn, send fibers to the striatum, hypothalamus, midbrain tectum, cerebellar cortex, reticular formation, the interneurons of the spinal cord, and, in primates, the motor cells themselves. The last connection indicates the direct way in which the neocortex can control motor activity. Level 7 is a sensorimotor level richly interwoven with reciprocal central connections of the neothalamus and neocortex. Within broad limits, every level below level 7 can be inhibited or facilitated by the activity of level 7.
Level 7, together with its associated, regionally specialized, central connections, contributes a vast enrichment of permutations and combinations to the sensory and central control of sensorimotor integration. The regionally specialized connections, influenced by lower levels, limit the likely character of organized activity. Reflexes and tendencies of lower levels are incorporated in body-wide activities.
However, the regional specialization in the forebrain is, at present, only crudely identified. Agreement on the significance of various neocortical regions, for example, is not universal. Regions receiving and processing somatic, visceral, optic, auditory, gustatory, and olfactory input are delimited. Zones especially important for speech have been outlined. Organization of motor activity has been linked with the frontal areas. The connections of the forebrain important for such functions as learning, memory, emotion, attention, and selfprogramming (establishing a hierarchy of preferred directions and order of complex behavior) are not at all clear. [See the biographies ofBrocaandFlourens.]
Unraveling the behavioral significance of the organization of the forebrain is an enormous challenge. It required many millions of years to evolve the existing advanced vertebrate brain. We should not expect to find the task of understanding the present end products an easy one. It will require many years of integrated, cooperative investigation, thought, and communication to reconstruct the evolution of the brain and to understand its full significance for behavior.
[Other relevant material may be found in Evolution,articles on Human Evolutionand Evolution AND Behavior.]
AriËns Kappers, Cornelius U.; Huber, G. Carl; and Crosby, Elizabeth C. 1936 The Comparative Anatomy of the Nervous System of Vertebrates, Including Man. 2 vols. New York: Macmillan.
Clark, Wilfred E. LEGROS 1960 The Antecedents of Man: An Introduction to the Evolution of the Primates. Chicago: Quadrangle Books.
Coghill, George E. 1929 Anatomy and the Problem of Behaviour. Cambridge Univ. Press.
Herrick, Charles J. 1948 The Brain of the Tiger Salamander, Ambystoma tigrinum. Univ. of Chicago Press.
Hunter, Walter S. 1913 Delayed Reaction in Animals and Children. New York: Holt.→ Also published as Volume 2, no. 1, of Behavior Monographs.
Lashley, Karl S. 1929 Brain Mechanisms and Intelligence: A Quantitative Study of Injuries to the Brain. Univ. of Chicago Press.
Loeb, Jacques1918 Forced Movements, Tropisms, and Animal Conduct. Philadelphia: Lippincott.
Noble, Gladwyn K. 1931 The Biology of the Amphibia. New York: McGraw-Hill.
Parker, George H. 1919 The Elementary Nervous System. Philadelphia: Lippincott.
Pavlov, Ivan P. (1927) 1960 Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. New York: Dover. → First published as Lektsii o rabote bol’shikh polusharii golovnogo mozga.
Riss, Walter; and Scalia, F. 1966 Functional Pathways of the Central Nervous System. Unpublished manuscript. -→ Contains an extensive bibliography and also presents the conceptual framework for the discussion found in this article.
Sherrington, Charles S. (1906) 1948 The Integrative Action of the Nervous System. 2d ed. New Haven: Yale Univ. Press.
Yerkes, Robert M. 1916 The Mental Life of Monkeys and Apes: A Study of Ideational Behavior. Behavior Monographs 3, no. 1.
Young, John(1950) 1962 The Life of Vertebrates. 2d ed. Oxford: Clarendon.
The primary function of the central nervous system, which includes both spinal cord and brain, is the process of neural integration for the expression of adaptive responses. Sensory information, arising from physical stimuli in the external and internal environment, reaches the central nervous system via the peripheral and cranial nerves in the form of bioelectrical nerve impulses. These sensory impulses are integrated with central neurophysiological activities of the brain to produce appropriate motor response patterns.
The nerve cell
The primary morphological element for information transmission in the nervous system is the neuron, a special type of cell with projections extending from the cell body. In physiology, those processes which conduct electrochemical impulses toward the cell body are called dendrites and are essentially protoplasmic extensions of the cell body. They usually extend a short distance and tend to branch profusely. In this fashion a great increase in the surface area of the nerve cell is provided, so that many other neurons may be linked with it. There are, however, large numbers of neurons in the nervous system which have no dendritic processes. The single projection which carries information away from the cell body is the nerve-action potential and the return to the resting potential require from 0.5 millisecond to 2.0 milliseconds. When an action potential is generated in a segment of a fiber, the resulting entry of ions into the cell from outside produces depolarization of the adjacent membrane as well. This marginal depolarization acts as the stimulus to a further breakdown in permeability along the membrane, and the process continues as a chain reaction along the length of the fiber. In this manner the nerve impulse is propagated over the length of the fiber in a fashion not unlike that in which ignition travels along the fuse of a firecracker. The range of velocity of the nerve impulse is very wide, varying from approximately 1 to 300 meters per second, depending upon the diameter of the fiber and other factors. The size of the nerve-action potential is not a function of the strength of the stimulus applied to the nerve. It is true that the referred to as the axon or nerve fiber. The axon is frequently of considerable length, and although it may possess collateral branches along its extent, the greatest branching is at its termination. Axons either terminate in muscle or form junctions with other neurons. Nerve fibers are generally covered by a fatty sheath called myelin, and in the case of axons lying outside the central nervous system, the myelin is in turn surrounded by a thin membrane known as the neurilemma. The neurilemma participates in regeneration of peripheral axons following injury or transection of the fiber, and since there is an absence of neurilemma in the central nervous system, regeneration does not normally occur there to any significant degree. While there are many structural types of nerve cells and great diversity in the arrangement of their projections, neurons may be functionally divided into three classes. These are sensory (afferent) neurons, motor (efferent) neurons, and associational or internuncial neurons. (Figure 1 provides a diagram of a motor neuron.) Exceptions to this schematization as discussed below are special efferent fibers which are found in sensory systems but do not serve a motor function.
The nerve impulse
The ability of neurons to conduct nerve impulses along their length is largely a function of the cell membrane. When the neuron is in the resting state the cell membrane is said to be polarized—that is, a steady electrochemical potential difference is maintained over the membrane by virtue of its selective ability to prevent sodium ions from passing from extracellular to intracellular space and to permit potassium ions to move inward through the membrane. As a result of this differential, the concentration of ions inside the cell becomes electrically negative with respect to the outside, and the magnitude of this difference in potential is usually between -50 and -90 millivolts.
When an appropriate stimulus is applied to a nerve fiber, the membrane at the point of stimulation suddenly becomes permeable to sodium ions, and these move across the membrane and inside the fiber. This process of membrane depolarization is represented by a rapid reversal of the resting potential from, for example, —70 millivolts to +40 millivolts. This change of about 110 millivolts is referred to as the nerve-impulse, or nerve-action, potential. Immediately following depolarization, the permeability of the membrane to potassium increases, and potassium ions shift from inside to outside the cell causing a reversal of the nerveaction potential and a restoration of the normal resting level (repolarization). The occurrence of the nerve-action potential and the return to the resting potential require from 0.5 millisecond to 2.0 milliseconds. When an action potential is generated in a segment of a fiber, the resulting entry of ions into the cell from outside produces depolarization of the adjacent membrane as well. This marginal depolarization acts as the stimulus to a further breakdown in permeability along the membrane, and the process continues as a chain reaction along the length of the fiber. In this manner the nerve impulse is propagated over the length of the fiber in a fashion not unlike that in which ignition travels along the fuse of a firecracker. The range of velocity of the nerve impulse is very wide, varying from approximately 1 to 300 meters per second, depending upon the diameter of the fiber and other factors. The size of the nerve-action potential is not a function of the strength of the stimulus applied to the nerve. It is true that the
magnitude of the nerve-action potential may vary from fiber to fiber, but within a neuron and under constant conditions it is usually invariant, occurring in full amplitude or not at all, although there are certain exceptions to this rule. Also of importance is the fact that the nerve impulse does not decrease in size as it sweeps along the fiber. This all-or-none principle applies only to the nerveaction, or spike, potential of the axon. There is another kind of potential change which immediately precedes the generation of the nerve impulse whenever a stimulus—physiological or artificial—is applied to a neuron. This is the local excitatory potential, which is graded in amplitude and, thus, does not function in an all-or-none fashion. The size of the excitatory potential is proportional to the magnitude of the stimulus applied and represents a partial depolarization of the cell membrane. It is propagated only in a weak and decremental fashion, if at all. If the stimulus is not very strong, the local potential will decay rapidly and no further neural events will occur. If, however, the stimulus is sufficiently strong, the local potential will increase in amplitude and reach the threshold for triggering the all-or-none nerve-action potential.
The spike potential is followed immediately by a brief period during which the axon is absolutely refractory and will not respond to any stimulus, regardless of magnitude. However, the fiber very soon enters a period of relative refractoriness before returning to the resting level. During the relative refractory period the fiber will respond to stimulation, but only if it is of greater strength than is required by the resting nerve. Although the refractory period lasts only a matter of milliseconds, it is nonetheless clear that the rate of discharge possible in a given fiber is limited by the presence of periods of absolute refractoriness. The exact duration of the refractory period is a function of nerve diameter and structure, and the recovery rate in the most rapidly conducting fibers permits them to discharge nerve impulses at a frequency of approximately one thousand per second.
The synapse is a functional connection between two neurons in which the axon endings of one cell make contact with the surface of a dendrite or cell body of another neuron. Since there is considerable branching toward the end of the fiber, an axon may establish junctions with many cells. Reciprocally, the body of a neuron may have in contact with it a very large number of axon endings arriving from many other cells. The axon terminals, which are often referred to as end buttons or knobs, make a slight indentation in the postsynaptic cell body. There is, in fact, a narrow separation between the axon terminals of the presynaptic cell and the dendrites and cell bodies of the postsynaptic neuron. This separation is only a few hundred angstroms in width and is referred to as the synaptic cleft.
When the action potential reaches the axon branches and endings resting on another neuron, a chemical substance is released by the presynaptic terminal. Electron microscopy has revealed that the presynaptic terminals contain tiny vesicles which are related to the formation and release of these transmitter substances. The released chemical agent then acts upon the membrane of the postsynaptic cell to alter its permeability to ions. The presynaptic terminals may be either excitatory or inhibitory. In the case of excitatory synapses the transmitter substance presumably produces a depolarization of the postsynaptic membrane, which is represented by a graded potential. If the input to the postsynaptic cell becomes sufficiently strong, a nerve-action potential will be generated when threshold is reached, and it will be propagated down the axon. This will occur when there is spatial and temporal summation of a number of slowly decaying graded potentials which have been produced sequentially and simultaneously by a volley of incoming signals over many presynaptic terminals. In the case of inhibition at the synapse, the chemical transmitter produces hyperpolarization and stabilization of the postsynaptic cell membrane. As a consequence there is decreased responsiveness to excitatory volleys and depression of activity in the cell. It is presently believed that the transmitter substance released at excitatory terminals is different from that released at inhibitory terminals and that the excitatory substance produces depolarization by a nonspecific increase in permeability to all ions, while the inhibitory substance produces hyperpolarization through a specific increased permeability to potassium ions. The excitatory and inhibitory substances themselves have not as yet been identified. Two substances involved in synaptic transmission (acetylcholine and norepinephrine) have been known for some time, but their precise relationship to excitatory and inhibitory actions is not well understood.
Development of the central nervous system
On the back, or dorsal, surface of the embryo there lies a sheet of cells called the ectoderm. In early development this layer becomes thickened, and because cells lying toward the edge of this plate have a more rapid growth rate than cells in the middle, a neural groove is formed. As this groove becomes deeper, it eventually closes off on top, forming a neural tube which extends from head to tail of the embryo. The tube then detaches itself from the remainder of the ectodermal plate. It is from this neural tube and a group of detached ectodermal cells lying along its side—called the neural crest—that the whole adult nervous system is formed.
In the developing embryo the brain first becomes apparent as an enlargement at the anterior end of the tube; the remainder of the tube becomes the spinal cord. With further growth the embryonic brain differentiates into three vesicular subdivisions: the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon).
Six weeks after fertilization there is further subdivision of these primary vesicles. The prosencephalon divides into an anterior portion, the telencephalon, or endbrain—which will eventually form the cerebral hemispheres (cerebral cortex, rhinencephalon, and basal ganglia)—and a posterior part, the diencephalon, or “twixtbrain” (thalamus and hypothalamus). The mesencephalon remains relatively small and undifferentiated, but from it will be derived its constituent components: the tegmentum, colliculi, and cerebral peduncles. The rhombencephalon divides into anterior and posterior portions, the metencephalon, or afterbrain (cerebellum and pons), and myelencephalon, or marrowbrain (medulla oblongata), respectively. The long, tail-like remainder of the neural tube will become the adult spinal cord. A schematic representation of these developmental stages of the brain is given in Figure 2.
The cavity which extends throughout the length of the embryonic neural tube forms, in the fully developed fetus, the ventricles of the brain and central canal of the spinal cord, which contain cerebrospinal fluid.
In mammals, one of the most remarkable events during development is the expansion and elaboration of the cerebral cortex. The degree of development of the cerebral mantle is greatly out of proportion to that of the rest of the brain. Its relationship to brain stem is not unlike that of the cap of a mushroom which has grown over, around, and partially down the sides of the stem. In addition, in higher mammals the surface of the cortex is further increased by numerous foldings or convolutions. A groove created by these foldings is referred to as a fissure or sulcus, and the ridge formed between two fissures is called a gyrus.
Fully two-thirds of the cortex in humans lies buried in fissures. The importance of the cerebral mantle in human behavior is to some degree expressed by the fact that it weighs as much as all other central-nervous-system structures combined.
Organization of structural elements
The term “gray matter” is used to refer to neurons and, specifically, to areas where a large number of cell bodies are collected together, since their natural color is a brownish gray. In contrast, most nerve fibers appear distinctly white, as they assume the color of the myelin sheath which covers them. When bundles of nerve fibers course together in the central nervous system, they are referred to as white matter, and these pathways of fibers connecting one area of the nervous system with another are called tracts within the central nervous system and nerves in the peripheral nervous system. A tract which connects a structure on one side of the brain with the homologous area in the opposite hemisphere is called a commissure.
A distinct aggregate of cell bodies is referred to as a nucleus when it lies within the central nervous system and as a ganglion when it is in the periphery. In many areas of the brain, neurons are distributed in a less concentrated fashion and occasionally they are arranged in layers, or lamina—as in the cerebral cortex, for example. The term “center” has frequently been applied to those areas or nuclei in which it has been demonstrated that a substantial number of neurons play a common role in a particular physiological or behavioral function. This term is roughly correct if it is taken to mean that the area is an important link or locus of elaboration in the execution of the response under consideration, but it has often been misused to imply that a center functions in an autonomous manner or in isolation from other areas of the brain. Regions near to and distant from an important nucleus or center may influence and modulate the particular behavior by virtue of connections with the center. No central-nervous-system structure operates entirely independently of other areas or is solely responsible for producing a response. For example, by priming the center with excitatory or inhibitory impulses remote structures participate in the determination of whether or not a response is to occur at any given point in time. Under special circumstances, such as injury to or destruction of the center, other areas may become capable of producing the behavior through alternate pathways. In this framework, the center is important because it is an area for the collection and integration of neural information concerned with a behavioral mode and because it may be a critical link in the normal creation of a response.
In the peripheral nervous system a distinction is made between somatic and autonomic motor nerves, both of which have their origin of action in the central nervous system. The somatic motor nervous system consists of those efferent motor nerves which have their cell bodies in the spinal cord or brain but whose axons extend into the striated muscles attached to the skeleton. The autonomic nervous system is concerned with responses of smooth and syncytial muscles. These include heart, blood vessels, glands, gastrointestinal tract, genitourinary structures, irises, and other internal structures. Cell bodies of the neurons of the peripheral autonomic nervous system lie in ganglia outside of the brain and spinal cord but are under the influence and control of autonomic fibers originating within the central nervous system and exerting their influence through synapses in the ganglia. The distinction between somatic and autonomic nervous systems is sometimes a useful one, but fundamentally it is artificial. The reason for this is that within the central nervous system the areas and pathways responsible for somatic and autonomic responses are not clearly differentiated. While it is true that a nucleus, area, or pathway within the brain may function predominantly in either autonomic or skeletal muscular responses, it is also the case that neural activity in such a structure is likely to affect both types of peripheral response mechanisms.
The cell bodies of sensory neurons entering the spinal cord lie in ganglia outside the central nervous system, and their fibers may be either somatic or visceral, depending upon the site of termination in the periphery. Somatic afferent fibers convey tactile, pressure, pain, or temperature information from skin, muscle, tendon, and joint receptors to the cord and brain. Visceral afferents transmit sensory impulses from the internal organs, glands, and blood vessels. Sensory activity in the visceral afferents is in close functional relationship to the autonomic nervous system, as is somatic sensory activity to the somatic motor system.
A simple reflex arc consists of at least two or three neurons and provides a mechanism whereby a relatively fixed and rapid behavior pattern may occur in response to an appropriate sensory stimulus.
A two-neuron reflex arc comprises a receptor and an afferent neuron bearing information about the stimulus to the central nervous system and a motor neuron with which the sensory cell synapses and through which the response is produced. This simple reflex pathway is referred to as a monosy nap tic reflex arc, since only one synaptic junction is involved. An example is the well-known knee jerk. This reflex may be produced by a tap on the tendon just below the kneecap. This elicits a slight stretching of the muscle fibers and results in stimulation of muscle-spindle receptors. Sensory impulses are conducted into the spinal cord, where motor neurons are excited, their discharge producing quick, phasic contractions of the muscles and a consequent “jerk” of the leg. Monosynaptic reflexes also include arcs in which there are more sustained or tonic contraction of muscle, and these play an important role in the control and maintenance of posture.
In a three-neuron or multineuronal arc there may be one or many neurons interposed between the sensory and motor cells. In general, the greater the number of these interposed links (interneurons) between the sensory input and final motor outflow, the more complex and less stereotyped the reflex.
The sensory impulses which produce a reflex may also be propagated over fibers that reach the cerebral cortex and, thus, result in awareness of the stimulus. However, this sensation is incidental to, and not critical for, the elicitation of the reflex. Furthermore, although a reflex pathway may function without the participation of other spinal or brain areas, this potential for independence is unusual in the normal activity of the organism. Reflexes which involve only one segment of the spinal cord are referred to as segmental reflexes; but as already pointed out, most reflexes include the participation of more than one spinal segment (intersegmental reflexes) and frequently the brain as well (suprasegmental reflexes). Suprasegmental reflexes may be extremely complex, in the sense that organization and coordination of the relatively simple spinal and intersegmental reflexes are carried out by the higher centers. For example, the postural supporting and shifting reactions, which are required for walking and which involve alternating contraction and relaxation of the limbs, are exquisitely integrated by delicately balanced mechanisms in the brain stem. A reflex may be somatic or autonomic, and it may involve either spinal or cranial nerves or a combination of both.
Examples of reflexes, in addition to the postural adjustments mentioned briefly above, are withdrawal of a limb from a painful stimulus before the pain is appreciated as a sensation; the constrictor response of the iris in response to bright light; coughing; sneezing; gagging; vomiting; adjustments in heart rate, blood pressure, and gastrointestinal activity. It is evident from this representative, but incomplete, listing that reflexes are appropriate moment-to-moment adaptations of the organism upon which its very survival depends.
The importance of reflexes for behavior in general may be elaborated and summarized by the following statements: reflexes are innate in the sense that they are the result of genetically transmitted neural mechanisms; they are hierarchically organized so that very simple reflexes can be combined, by the activity of brain centers, into more complex response patterns; while the most simple reflexes within an organism are fixed and stereotyped, complex reflexes tend to be variable and can be modified by experience; finally, phylogenetically, nervous systems have evolved from diffuse and crude reflex mechanisms to more specialized linkages capable of clearly differentiated response patterns. In higher species the borderline between elaborate adaptive reflexes and complex flexible behavior which can be learned is not clear. In man, evolution culminates in the capacity for a high degree of symbolization and the appearance of language and speech. This capacity includes the ability to abstract general principles from specific items of information and memory and the development of an extraordinarily complex social organization accompanied by the ability to transmit it.
From the foregoing, it is evident why the reflexes are commonly considered the basic units of the nervous system and the functional elements from which all other behaviors are derived. We have observed an apparent structural, physiological, and behavioral continuity extending from the simple segmental reflex to complex human behavior. However, whether this continuity between reflex and symbolic capacity is actual or illusory remains today very much an open question, for although there obviously exist common underlying structures and processes, it is not possible to describe or account for a large part of motivated behavior and learning—much less ideation, reasoning, or abstraction—in terms of basic reflex mechanisms.
The cranial nerves
In addition to paired spinal nerves which are arranged along the vertebral column, 12 pairs of cranial nerves enter the brain stem and cerebrum directly.
From each spinal segment project four types of nerve fibers, which may be differentiated functionally: the general somatic and visceral afferents and efferents. Some of the cranial nerves also contain these same four functional fiber types, but others may carry only one or two kinds. In addition, the term “special sensory afferents” is applied to fibers transmitting taste, olfactory, visual, auditory, and vestibular sensations. The first two of these are considered to be visceral in origin and the remainder somatic. Further, the motor fibers which supply the striated muscle of the head and neck fall in the unique category of special visceral efferent fibers. Table 1 describes the origin (or termination) and primary functions of each nerve. (The cranial nerves are traditionally designated by Roman numerals as well as by name.)
It should be noted that three of the cranial nerves are purely sensory in their activity; the Ist, IInd and VIIIth. The Vlllth cranial nerve actually mediates two sensory modalities, hearing and equilibrium. By rigorous definition only the XIth cranial nerve has solely a motor function. Nerves V, VII, IX, and X contain mixed afferent and efferent fibers, and both the sensory and motor components of these nerves play critical, or substantially important, roles in the effective functioning of the organism. The sensory components of nerves III, IV, VI, and XII are of considerably less practical significance.
It is important to realize that the whole of the central nervous system is a continuous entity in which the component structures rarely, if ever, operate in total isolation. Although we speak of the spinal cord and higher structures as mediating or being responsible for particular functions, they almost invariably do so with some degree of cooperation from many other areas. As a general principle the simplest reflexes and behavior patterns are present at the lower levels of the central nervous system, and more complex activities appear in an integrated form only when higher structures are functioning in a normal manner.
A basic function of the spinal cord is the conduction of nerve impulses to and from
|Table 1—The cranial nerves|
|NUMBER||NAME||ORIGIN (OR TERMINATION)||FUNCTIONS|
|I||Olfactory||Telencephalon (ventral rhinencephalon)||Smell|
|III||Oculomotor||Midbrain||Ocular muscle proprioception||Eye movement Eyelid elevation Contraction of iris Accommodation of lens|
|IV||Trochlear||Midbrain||Ocular muscle proprioception||Eye movement|
|V||Trigeminal||Midbrain and pons||Touch, pain, and temperature sensation from face, jaw, head, ear, cornea, sinuses, teeth, gums, tongue, mucous membranes of mouth, and meninges Proprioception of masticatory muscles||Muscle movement of jaw, middle ear, and palate|
|VI||Abducens||Pons||Ocular muscle proprioception||Eye movement|
|VII||Facial||Pons||Taste (anterior two-thirds of tongue)||Muscle movement of face, scalp,|
|VIII||Vestibulocochlear (a) Vestibular division (b) Cochlear division||Medulla||Equilibrium Hearing|
|IX||Glossopharyngeal||Medulla||Taste (posterior one-third of tongue)||Swallowing Salivary secretion|
|X||Vaaus||Medulla||Cutaneous sensation of external ear Sensation from pharynx, larynx, trachea, and esophagus Visceral afferents from thoracic and abdominal cavity linings and organs||Control of activity of heart, gastrointestinal tract, blood vessels, and other visceral organs of thorax and abdomen Control of pharyngeal, laryngeal, and esophageal muscles|
|XI||Spinal accessory||Medulla and spinal cord||Muscles of pharynx, larynx, and palate Muscles for flexing neck, rotating head, and raising shoulders|
|XII||Hypoglossal||Medulla||Position sense of tongue||Tongue movements|
higher levels of the central nervous system. Except for nerve activity carried over the cranial nerves, all remaining sensory information and motor outflow travels through the spinal cord and the nerves which extend outward from it to the periphery. Spinal functions have often been determined by studying animals in which the cord has been experimentally severed or in humans in whom spinalcord transection has been the result of accidental injury. In man there are 31 pairs of nerves arranged along the length of the spinal cord, which is encased in the bony vertebral column. Each pair of nerves lends to the cord the appearance of external segmentation. These 31 segments have been designated as follows, proceeding from the top (rostral) portion of the cord to the bottom (caudal) end: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. If the spinal cord is completely transected above the fourth cervical segment, death occurs immediately, since the nerve fibers originating in the respiratory centers of the lower brain stem are severed from their connection with the muscles of respiration. Humans may survive interruption of the spinal cord at lower levels. If the transection is complete, voluntary movements are permanently lost below the level of the transection. The ability of the isolated cord to maintain postural reflexes and motor activity depends upon the species of organism and the type of reflex under consideration. For example, in lower forms such as amphibians and reptiles, reflex withdrawal of limbs in response to noxious stimuli and reflex swimming movements are retained to a remarkable degree. In some mammals, such as cats and dogs, occasional and usually brief periods of reflex standing may occur if adequate assistance and support are provided. Other basic postural and motor reflexes, such as running movements, may also be present, but these, as well as the standing reflexes, are highly variable.
In man, although the motor neurons of the cord are still connected with skeletal muscle, all reflexes are lost during the period referred to as spinal shock, which often lasts for several weeks after transection. Gradually, some reflexes reappear—first flexion of the limbs, then the reflexes of extension. These soon become exaggerated, since, following transection, reflexes originating in the cord below the cut function without the normal inhibitory influences from higher levels of the central nervous system. Eventually, either flexion or extension spasms will predominate clinically. All sensation, which is normally conducted from the periphery to levels of the cord below the transection, is permanently absent as the result of severance of sensory fibers in the cord ascending to the brain. Autonomic functions—for example, those of the urinary bladder and rectum—are seriously disturbed immediately following interruption of the cord but often show some recovery, indicating the ability of the cord to integrate, at least partially, certain aspects of these reflexes. In summary, it is clear that although certain basic postural, motor, and autonomic reflexes remain or recover after section of the cord, these have only limited autonomy and require the higher centers for their full and adaptive coordination.
The brain stem
In textbooks and research literature the term “brain stem” is used differently by various authors. The most encompassing definition includes all of the central nervous system above the spinal cord except the cerebral and cerebellar cortices. In this context the brain stem is composed of the medulla, pons, midbrain, thalamus, hypothalamus, and the basal ganglia of the telencephalon. At the opposite extreme many authorities would include only the medulla, pons, and midbrain as constituents of the brain stem. Still others would add the diencephalon (thalamus and hypothalamus) to this list but omit the basal ganglia from the definition. Frequently, a distinction has been made between the lower brain stem (medulla and pons) and the upper brain stem (midbrain and diencephalon).
Externally, the most caudal portion of the brain stem, the lower medulla, appears as a continuation and expansion of the spinal cord. Internally, however, at the microscopic level, there are many important differences between brain stem and cord. These include the appearance of nuclei serving the cranial nerves and the development of large cell aggregations and relay nuclei with highly complex interconnecting fiber systems. Part of these form the reticular formation, the phylogenetically ancient core of the brain stem, which begins in the medulla and projects its most rostral fibers into the diencephalon. Functionally, as mentioned above, the brain stem represents an organization of physiological and behavioral response patterns that not only provides for the suprasegmental coordination of spinal reflexes, but adds distinctive integrating features of its own. Each of the main subdivisions of the brain stem will be discussed in more detail. Figure 3 depicts the location of the brain-stem components.
Medulla oblongata and pons. The medulla contains, in addition to relay nuclei and fiber tracts conveying information between higher stations of the brain and the spinal cord, aggregates of neurons which are of great importance for the basic
maintenance of the organism. Certain of these are concerned with visceral or autonomic functions, and in many of these activities the medulla and pons collaborate closely. These areas are fundamental to the normal regulation of cardiac activity, constriction and dilation of blood vessels, respiration, and gastrointestinal functions of motility and defecation. Protective reflexes, such as coughing, sneezing, gagging, and vomiting, and ingestive reflexes, such as sucking, swallowing, and salivation, are mediated through these same lower-brain-stem levels. This is not to say that higher-brain structures do not also modulate and provide an even more elaborate functional assembly of these reflexes, for they do; but the medulla and pons together can carry out most of these functions in a manner adequate for survival of the organism in the absence of the rest of the brain.
As already noted, suprasegmental regulation is also essential for appropriate postural adjustments and motor coordination. The medullary nuclei and tracts play an intermediary role in the control of spinal neurons concerned with muscle tone. The role of the reticular formation, which has its lower levels in the medulla and pons, will be discussed in a later section. The pons lies immediately above the medulla, and cell groups within it exert control over the medullary respiratory centers. The pons also aids in the elaboration of the expressive aspects of ingestive and emotional behaviors, since nuclei of the cranial nerves responsible for jaw and facial movements and the secretion of tears and saliva are present at this level. Additional inhibitory and facilitatory control over postural mechanisms of the skeletal musculature is also imposed upon medulla and cord by the pons. Finally, structures and fiber tracts within the pons act as intermediaries in integrating neural activities of the cerebellum and cerebral hemispheres, and transverse fibers extending across the pons provide communication between the two hemispheres of the cerebellum.
Midbrain. The midbrain, like the pons and medulla, contains nuclei of cranial nerves and many fibers in passage, both of which provide communication between higher and lower levels of the central nervous system. In addition, the roof, or dorsal, region of the midbrain is referred to as the tectum and contains two pairs of protuberances, the anterior, or superior, colliculi and the posterior, or inferior, colliculi. These bilateral pairs of elevations are collectively known as the quadrigeminate bodies and are essentially aggregations of gray matter. The remainder of the midbrain is composed of the centrally located tegmentum—which includes the midbrain portion of reticular formation—and large fiber bundles, which for the most part are laterally placed. The quadrigeminate bodies are primarily sensory reflex centers. The superior colliculi receive fibers directly from the retinas of the eyes, and although in higher mammals other pathways and structures mediate complex processes such as pattern perception, the colliculi nonetheless participate with adjacent regions in certain reflexes such as blinking in response to novel visual stimuli, pupillary constriction to light striking the eyes, and certain reflex conjugate eye movements.
The inferior colliculi receive fibers from the auditory pathways which enter the medulla and ascend through the pons to the midbrain. These collicular midbrain stations are concerned with startle reflexes and eye, head, and body-orienting reactions to unexpected noises. An additional level of skeletal muscle control is also contributed by other nuclei within the midbrain.
The cerebellum, consisting of two large hemispheres and a midline structure called the vermis, is perched over the brain stem and attached to it by heavy fiber tracts, the cerebellar peduncles (see Figure 3). The cerebellum also has connections with the spinal cord and cerebral cortex. The outer surface of the cerebellum is covered by a cortex, as is the cerebrum, and its interior consists of fiber tracts and several subcortical nuclei. One of the more clearly understood functions of the cerebellum is its role in producing smooth, welltimed, and precise motor movements. It also plays a prominent role in maintaining appropriate posture and balance by utilizing sensory information from the vestibular mechanisms of the inner ear and proprioceptive, or position sense, receptors in the musculature of the body. Projections from tactile receptors in the skin are another major source of input to the cerebellum. Experimental studies of electrical stimulation of the cerebellum indicate it is also active in the regulation of respiration and certain autonomic responses.
Sensory information from every modality, with the exception of olfaction, is relayed through thalamic nuclei before being transmitted upward to the telencephalon. Figure 3 shows the location of the thalamus. From these thalamic sensory nuclei, impulses are projected to appropriate and specific receiving areas in the cerebral cortex. In addition, neural information exchanged between the cerebral cortex, on the one hand, and the brainstem reticular formation and cerebellum, on the other, reaches thalamic stations en route. It is important to emphasize that the thalamic relay nuclei, like other synaptic stations in the central nervous system, do not simply send on raw information in an unchanged form from one part of the brain to another; but, in a manner not well understood, modify the neural discharge patterns and their temporal relationships in such a way as to provide simultaneously for the apparently contradictory processes of both increased selectivity, on the one hand, and integration of neural information from several sensory modalities, on the other. The crucial role of the thalamus in functions of the reticular system will be discussed in a later section.
The hypothalamus is perhaps best thought of as a major center for the integration of a variety of motivated behaviors, such as feeding, drinking, sexual activities, and emotional responses. It also is of major importance in the regulation of autonomic responses which are not only critical for the moment-to-moment physiological equilibrium of the organism but are also components of the motivated behaviors themselves. Figure 3 shows the location of the hypothalamus.
The functional assembly of these visceral responses and motivated behaviors depends upon the integrity of the hypothalamus. For example, in a series of classical experiments which were designed to determine the central-nervous-system level essential for coordinated emotional behavior, all structures above the hypothalamus were severed or removed surgically in experimental cats. Following recovery from the operation, an intact and wellcoordinated pattern of defensive-aggressive display was still present. When, however, a brain-stem transection was made behind the hypothalamus, the integrated rage response—which in the cat consists of baring of the teeth, hissing, spitting, extrusion of the claws, striking out with the claws, arching of the back, elevation of body hairs, and sometimes urination and defecation—was no longer present in an integrated pattern. Disjointed and isolated elements of emotional behavior in response to tactile stimuli were still occasionally expressed through receptive and motor mechanisms in the brain stem below the hypothalamus, but the coordination of these responses, which makes the behavior protective and adaptive, was permanently lost. It is worth noting that the defensive-aggressive response pattern consists of an assortment of skeletal motor and autonomic reflexes which are exquisitely combined and organized by the hypothalamus into a precisely timed sequence of actions. It should be pointed out, however, that although the hypothalamus is critical for the integration of this and other behaviors, it too responds to higher influences from the basal ganglia and cortex of the cerebral hemispheres.
Autonomic activities. The regulation of autonomic activities is maintained by the interplay of two major regions of the hypothalamus. Experimental studies have demonstrated that electrical stimulation of the posterior hypothalamus results in the expression of a variety of visceral activities that, in general, mobilize the organism for action and the expenditure of energy. The hypothalamic, lower-brain-stem, and peripheral structures which mediate this autonomic pattern of mobilization are referred to as the sympathetic system, and the activities of these regions are conveyed to peripheral organs through the thoracic and lumbar segments of the spinal cord. The second subsystem of the autonomic nervous system is the parasympathetic, which has its outflow to the periphery through the cranial nerves and sacral region of the spinal cord. Parasympathetic responses are integrated primarily in the anterior part of the hypothalamus, in general are opposed to sympathetic actions, and are associated with conservation of energy, relaxation, and sleep. Sympathetic and parasympathetic fibers often innervate the same peripheral structure, and while one system may elicit excitation of the function of that organ, the other system may produce inhibition or a decrease in activity. In the sympathetic nervous system, the chemical mediator at the end organ is norepinephrine, an adrenalinlike substance. In the parasympathetic system, the substance released at the junction of nerve endings and smooth muscles is acetylcholine. An exception exists in the case of the sweat glands, which are all innervated by sympathetic fibers but utilize acetylcholine as the chemical transmitter in temperature-reduction sweating and norepinephrine in emotional sweating. The sympathetic system possesses preganglionic fibers which synapse in ganglia arranged in chains running parallel and close to the cord, just outside the vertebral column. From these ganglionic synapses, postganglionic fibers extend to the end organs. An exception to this is the innervation of the adrenal medulla, which secretes adrenalin into the general circulation and thus affects other sympathetic organs. The adrenal medulla receives preganglionic fibers directly from the central nervous system. The parasympathetic preganglionic fibers, on the other hand, are very long, since the ganglia of this system do not lie beside the cord but are in close proximity to the end organs; thus, parasympathetic postganglionic fibers are relatively short. [SeeInfancy, article onTheEffectsOFEarlyExperience; Stress.]
Usually an autonomic response pattern is a highly complex and integrated sequence of both sympathetic and parasympathetic reflexes. For example, in the male, successful completion of the sexual act requires first a parasympathetically evoked relaxation of the arteries of the erectile tissue of the penis, which produces a slowing of venous outflow and, consequently, erection. The flow of semen into the posterior portion of the urethra and final ejaculation is a sympathetic response. During this process another sympathetic reflex in the sphincter of the bladder prevents spermatozoa from entering that structure and simultaneously blocks urination. Finally, after completion of ejaculation an increase in sympathetically produced tone of the arteries of the cavernous tissue elicits detumescence and a decrease in bladder-sphincter tone, permitting normal urination.
Metabolic and endocrine response. In addition to sympathetic and parasympathetic regulating areas in the hypothalamus, there are reciprocal control areas for other behaviors. For example, the ventromedial nucleus of the hypothalamus is an inhibitory center for eating behavior, while a more lateral region is concerned with excitation of this behavior. Evidence of the existence of critical areas such as these often comes from experimental studies in which small regions of the brain are either destroyed or electrically stimulated in experimental animals. For example, in the case of the ventromedial nucleus, stimulation produces immediate cessation of eating for the duration of the stimulus, and destruction of this nucleus results in overeating and, eventually, obesity. The overeating following this lesion presumably occurs because the excitatory center in the lateral hypothalamus is released from the inhibitory control of the ventromedial nucleus. Conversely, stimulation of the lateral hypothalamus elicits the onset of eating, and a lesion in this area leads to starvation.
As part of its controlling role in the expression or inhibition of eating, drinking, sexual behavior, and additional metabolic functions, the hypothalamus exerts an important influence over the major endocrine gland, the pituitary, which lies at the base of the brain and is connected with the hypothalamus by a stalk containing neural fibers which extend from the hypothalamus down into the posterior lobe of the gland (see Figure 3). The anterior lobe of the pituitary is influenced by the hypothalamus through a vascular system, so that chemical substances released by secretory cells in the hypothalamus may act upon the anterior pituitary. The pituitary, in turn, plays a central role in the control of the several other endocrine glands with which it has reciprocal relations. When hormones of these glands enter the general circulation, they act on not only the pituitary itself but the hypothalamus as well, to modify its activity. Thus, an equilibrium of bodily functions concerned with metabolism and consummatory behavior is maintained by a circular system of neural and hormonal checks and balances, with the hypothalamus and pituitary performing the central integrating steps in these processes.
Basal ganglia. Just as the cerebellum possesses nuclei which lie within and below its cortex, so the cerebral hemispheres (telencephalon) contain aggregates of gray matter which lie beneath their cortices. These subcortical structures are collectively known as the basal ganglia and are composed of the caudate nucleus, putamen, globus pallidus, claustrum, septal nuclei, and the amygdala, which consists of a complex of five closely related nuclei lying within the temporal lobe. Frequently, the term “lenticular nucleus” is used in referring to the putamen and globus pallidus collectively. The term “corpus striatum” is used to include the lenticular nucleus along with the caudate.
The traditional view of the basal ganglia and, in particular, the structures constituting the corpus striatum is that they are primarily concerned with coordination of somatic motor movements, especially control of postural reflexes and adjustments. These structures are complexly connected with certain motor areas of the cerebral cortex, the thalamus, the reticular formation, and motor structures lower in the brain stem. The corpus striatum, with these other areas, forms the so-called extrapyramidal motor system. This series of structures is distinguished from the pyramidal system, whose fibers also originate in motor areas of the cortex but course directly to the motor neurons of the cranial nerves and spinal cord. The pyramidal system is responsible for individual and distinct phasic movements of the body and limbs, while the extrapyramidal system functions to produce coordination of these. Thus, normal somatic motor patterns require the integrated activity of three apparently separate systems, which actually share overlapping structures and functions: the pyramidal system, the extrapyramidal system, and the cerebellar system discussed earlier. Recent studies indicate that parts of the corpus striatum may also play a role in the mediation of attention, learning, and emotional responses, although the extent and specificity of these activities is not yet clear. [SeeAttentionandEmotion.]
The amygdala appears rather distinct from the rest of the basal ganglia, in terms of its location in the temporal lobe, its connections, and its functions. This complex of nuclei receives fibers from the olfactory pathways and has strong connections with the hypothalamus. Functionally, it is prominent in the control of autonomic activity, emotional responses, and probably feeding behavior. To a certain degree, it appears to replicate some hypothalamic functions. The septal nuclei are also connected with the hypothalamus and, like the amygdala, exert an influence over autonomic activity and affective behavior. Experimental evidence suggests that the septal and amygdaloid nuclei carry out some of their activities by exercising modulatory control over hypothalamic integrative mechanisms. Investigations of the claustrum have as yet revealed little of its functions.
The cerebral cortex
The cerebral cortex is a convoluted, or folded, mantle of gray matter arranged as layers of cells over the surface of the cerebral hemispheres. It virtually covers and surrounds the upper levels of the brain stem. The cortex has an average thickness of about 2.5 millimeters, but this varies considerably from one region to another. Fibers leading to and from subcortical areas and connecting cortical areas with each other make up the white matter within the hemispheres. The corpus callosum, a massive bundle of fibers, crosses the midline of the hemispheres and provides a direct connection between the right and left cerebral cortices.
The cortex represents the most recent and highest development of the nervous system. In man it is proportionally larger than in any other species, and to the cortex are attributed those activities which are most distinctly human: complex learning and reasoning; symbolization, abstraction, and generalization; and the development of language and speech. While a substantial portion of the cortex is given over to sensory receiving areas and regions concerned with the initiation and control of motor responses, the higher integrated activities just mentioned depend upon other cortical regions —the so-called cortical association areas—as well;these regions are critical in the mediation of complex perceptions and cognitions.
At the gross anatomical level, the cortex is divided into lobes. In addition, areas which are presumably different in their cellular architecture have been traditionally assigned numbers (Brodmann’s areas). In some instances Brodmann’s areas correspond to functional zones, but in other cases they do not. The relationship of the lobes and functional areas is shown in figures 4 and 5.
The frontal lobe, which extends from the anterior tip of the hemispheres to the central fissure, contains two major subdivisions: the posterior portion, which is primarily motor in function, and the anterior part, which is association al. Electrical stimulation of the strip of cortex(Brodmann’s area 4) just anterior to the central fissure produces discrete and isolated motor responses of separate body parts. This is the primary motor cortex, and it is organized somatotopically, or in terms of body regions. For example, beginning at the uppermost part of the motor strip and progressing downward along its surface, electrical stimulation will successively produce movements of the toes, foot, leg, trunk, thorax, shoulder, fingers, neck, and face. These responses are mediated by the direct connection of motor nerve cells in area 4 to spinal motor neurons or cranial-nerve nuclei. Stimulation of the right motor cortex will produce these responses on the left side of the body, and vice versa, since the pyramidal tract in which these fibers travel decussates or crosses over to the opposite side of the brain before reaching the motor cells of the spinal cord. Destruction of parts of the primary motor strip will produce paralysis of that part of the body which is represented in the injured portion.
The premotor area (6) has fewer connections with the pyramidal tract than does area 4 but sends a large number of fibers to the extrapyramidal motor system via the basal ganglia and reticular formation. Often, stimulation of this area does not produce an observable effect, although complex motor responses and patterns have been reported even in the absence of area 4. Some investigators assign a predominantly inhibitory function to area 6. Area 6 also extends mesially over and down
along the midline. Its extent there is referred to as the supplementary motor cortex, since slow, coordinated responses can be elicited by stimulation. These consist of the gradual assumption and maintenance of a particular posture, in contrast to the rapid, phasic, and highly localized responses obtained from area 4. Area 8 is often called the frontal eye field, since horizontal and vertical conjugate eye movements result from stimulation of this region.
It is of interest that a variety of autonomic responses may be obtained from all of these motor areas in addition to other parts of the cortex such as the tip of the temporal lobe and the undersurface of the frontal lobe (orbitofrontal cortex).These responses are, in the main, examples of critical modulation and control of amygdaloid and hypothalamic integrative mechanisms for visceral activity. However, certain vascular responses originating in the motor cortex may bypass these subcortical centers and directly control some activities of the blood vessels.
The anterior portion of the parietal lobe immediately behind the central fissure (Figure 5) is called the postcentral gyrus and contains the primary somatosensory receiving region (areas 3, 1, and 2). These areas receive sensory information directly from the posterioventral nucleus of the thalamus and, thus, are important in the initial perception, discrimination, and localization of stimuli exciting nerve endings and receptors on the surface of the body and from within its somatic musculature. However, a primitive consciousness of certain somatic sensations probably exists in man at the thalamic level, even in the absence of cortex. These thalamic sensations are at best poorly differentiated and not well localized. For example, a patient with a lesion in the primary somatosensory receiving area of the cortex may indicate that he appreciates the fact that stimulation has occurred but may have great difficulty in distinguishing degrees of intensity of stimulation or in localizing the stimulated point on the body.[SeePain; SkinSenses AND Kinesthesis.]
Fibers carrying the somatic sensations of touch, pressure, pain, proprioception, and warmth and cold are not well separated in either the thalamus or cortex, but the cortical receiving area is topographically organized in a manner similar to that of the motor cortex: lower extremity representation at the uppermost portion of the sensory area, with upper extremities and head represented at the basal portion of the region. Our knowledge of topographical precision within these sensory areas is primarily the result of electrophysiological mapping studies in which punctate physical stimuli are applied to the surface of the body and the resultant electrical potential changes, called evoked sensory potentials, are recorded with small electrodes on the surface of the cortex. A distinct and localized evoked potential occurs at the specific cortical area which represents the part of the body being stimulated.
Although the reader may have gained the impression from the foregoing that the central fissure forms an absolute boundary between motor and sensory cortex, this is not actually the case. Motor responses can be elicited from the sensory areas, and sensations have a degree of representation anterior to the central fissure. However, the responses become fewer and weaker as we progress from motor into sensory cortex or vice versa. It also is appropriate to point out at this juncture that taste, in addition to the somatosensory modalities mentioned above, is also represented in the parietal lobe at the base of the postcentral gyrus. [SeeTaste AND Smell.]
There also exists a secondary somatosensory area which yields a mirror image of the topography of the primary region. It is also located near the base of the primary region, but its functions are still obscure. The remainder of the parietal lobe is primarily associational in function.
Visual impulses, originating in the retina, reach the occipital lobe of the cortex after relay through a specific thalamic nucleus called the lateral geniculate body. Figure 5 shows this visual receiving area, or striate cortex (area 17), to be located on the lateral surface of the occipital pole. However, much of area 17 extends into the midline occipital cortex and lies along the mesially located calcarine fissure. The visual cortex, too, is topographically organized. Stimulation of the eye with fine beams of light while recording evoked potentials in area 17 demonstrates that there is a point-to-point representation in the cortex for retinal areas. In addition, fibers from the medial half of each retina cross over before reaching the lateral geniculate body and go to the opposite side of the brain, while those for the lateral half of the retina do not. Through this arrangement, visual stimuli appearing in the right visual field (thus producing stimulation of the medial retina of the right eye and the lateral retina of the left eye) are represented on the striate cortex of the left hemisphere. Conversely, objects in the left visual field are represented in the right hemisphere. [SeeVision.]
Complete destruction of area 17 in man leads immediately to permanent total blindness. This is not quite the case for lower mammals, since even after total removal of the visual receiving area these species are still capable of primitive brightness discrimination, although pattern, or form, vision is lost. The occipital association cortex has traditionally been thought to be concerned with visual integration. However, this statement is something of an oversimplification, since the function of these regions is considerably more than that of the organization of visual perception.
The primary receiving area for audition (area 41) is located on the upper surface of the temporal lobe (Figure 5). Its input arises from the medial geniculate body of the thalamus, but since the auditory paths cross at several points before reaching the medial geniculate body, bilateral representation for hearing is a more complex matter than that for vision. In view of multiple crossings of auditory fibers in the brain stem en route to the lateral geniculate body, it is remarkable that tonotopic organization is present in area 41. That tonotopic organization is retained from receptor to auditory cortex is indicated by the fact that when small areas of the cochlea, which are responsive to a relatively narrow range of tone frequencies, are stimulated, evoked potentials appear in an orderly spatial arrangement over the auditory cortex. The temporal association areas are extremely complex in their function and will be discussed along with the other association areas.[SeeHearing.]
The receptors for the sense of balance or equilibrium are located in the labyrinthine portion of the inner ear, and the cortical representation for equilibrium lies in or near that for audition, area 41 of the temporal lobe.
Considered from the phylogenetic point of view, many sensory and motor functions have undergone a process known as corticalization. That is, functions which in lower forms appear to be mediated subcortically depend in higher species on the integrity of the cortex. The instance cited, showing visual losses to be greater in man than in lower animals following destruction of the striate cortex, is an example of corticalization. In the motor sphere this evolutionary process is equally evident. For example, in the rat the motor cortex is not well organized either cytoarchitectonically or physiologically. If it is completely removed, it is difficult for even the sophisticated observer to discern any motor impairments. In the cat or dog, where there is greater organization and finer differentiation of motor areas, motor weakness and a substantial loss of movement follow injury to this region. The recovery, however, is usually prompt although not necessarily complete. In primates—especially man—the reliance upon cortex for normal activity is even more crucial, and motor-cortex injury produces greater and more enduring paralysis than in any of the species described above.
The association regions of the cerebral cortex are a comparatively recent evolutionary development. For example, the relatively simple and unconvoluted cortex of rodents is primarily sensory and motor in function. In these species areas of association cortex are few in number and very small in size. In carnivores which are more advanced in the evolutionary hierarchy there is a substantial increase in the amount of cortex given over to associational functions. When the phylogenetic level of primates is reached there is an abrupt and dramatic increase in the extent of association cortex. This is especially evident in man, in whom the greater part of the cerebral cortex is associational.
It has been pointed out that upon the association cortex depend those higher functions which best distinguish man from the other primates and primates from lower mammalian species. While the level at which man is able to segregate and assemble elaborate perceptions and memories for the solution of complex problems is indeed impressive, it is not unique. Other primates and mammals considerably down the scale from man also have the capacity to reason, but in an attenuated form and to a lesser degree. Man’s superior ability to manipulate symbols, to isolate conceptually a dimension of an object from that object(abstraction), and to derive a principle through the categorization of these abstractions (generalization) is quantitative rather than absolute. It is reasonable to surmise that the magnitude of these gifts is related to the extent and degree of elaboration of association cortex.
How then does the association cortex perform these remarkable integrative tasks? Although the precise neurophysiological and chemical mechanisms are far from clearly understood, it is recognized that for any behavior above the level of a simple innate reflex, associational systems, cortical and subcortical, must first bring together stored information (memory) and incoming sensory data from many modalities. Then, through the as yet poorly understood neuronal processes of data selection, interaction, and matching and comparison, a choice is made which, represented by a particular pattern of neural activity, “commands” the motor system to effect a specific response. There are several factors which, when taken together, are assumed to determine the “decision” of associational mechanisms. The most basic of these are the strength and pattern of the sensory input and the efferent connections of the participating association areas. Of course, ongoing central activities which represent the effects of past experience and the motivational state of the organism will also function as determinants of output by interacting with the effects of sensory stimulation. During this process the responsivity of individual neurons in associational systems will be controlled also by the existing level of arousal and attention within the brain.
It has been pointed out that the sensory and motor cortices actually overlap. There is also no absolute separation of associational functions from either sensory or motor regions. Indeed, there is evidence that some integrative activities commonly attributed to the association cortex are also exhibited by sensory and motor areas. The complexity and inseparability of sensory, motor, and associational functions is emphasized not only by these facts but by the existence of descending fibers from many cortical areas and other regions of the brain which impose their activity upon lower sensory stations, even those as remote as the receptor. Through this arrangement, ongoing central activity controls and modulates incoming sensory impulses long before they reach the receiving areas of the cerebral cortex. Furthermore, the ongoing activities in these higher levels of the brain which modulate sensory activity are themselves modified by the sensory input. Thus, it is evident that complex feedback circuitry exists throughout the central nervous system for the control and balance of sensory, motor, and integrative activities.
Frontal lobe. The prefrontal region, or association cortex of the frontal lobe, lies rostral to those areas from which motor effects can be elicited. Neurons of the prefrontal region make no fiber contribution to the pyramidal motor pathways, nor do they receive impulses directly from thalamic sensory-relay nuclei. They do, however, have connections with other subcortical structures, including the thalamus, hypothalamus, corpus striatum, midbrain, lower brain stem, and cerebellum, as well as with many cortical areas.
In monkeys, experimental injury to portions of the prefrontal cortex may result in a number of behavioral alterations, including hyper activity and stereotyped pacing, increased distractability and inattention, response perseveration, and possibly disruption of short-term memory. A combination of these disturbances may account for the difficulty that monkeys with lesions of the prefrontal cortex have in solving problems which require sequential responding or in holding and retrieving information over a delay period between stimulus presentation and response, during which delay the stimulus object is not visible to the animal. As is generally the case with injury to the association cortex in monkeys and lower species, symmetrical bilateral lesions produce much greater defects than do unilateral ones. In fact, in some instances unilateral lesions may result in no observable loss of function. Another consequence of prefrontal damage is in the sphere of emotional behavior. Normal monkeys and chimpanzees working on very difficult discrimination problems often display emotional disturbances, including violent temper tantrums, as a manifestation of the frustration resulting from the many errors made. After prefrontal lobotomy, a surgical procedure which severs the connections between the prefrontal lobe and subcortical areas, the animal no longer becomes upset with its errors and works calmly and quickly on the problem, even though the lobotomy may have actually increased the frequency of errors.
These observations on emotional behavior led to the development of the prefrontal lobotomy in man for the relief of neurotic and psychotic symptoms. This operation has been almost entirely replaced in recent years by the verbal and experiential therapies, electroshock treatment, and the psychotropic drugs. However, variants of the lobotomy procedure are still occasionally employed in the alleviation of intractable pain in terminal diseases. The behavioral effects of lobotomy in man are highly variable, and reports have been frequently contradictory. There is no clear evidence that prefrontal lobotomy significantly alters the intelligence of man, and it is possible that the defects which have been shown by some studies may actually reflect motivational and attitudinal, rather than intellectual, changes.
The prefrontal lobotomy has been considered most effective in disorders characterized by emotional tensions, such as schizophrenic anxiety, agitated depression, and obsessive tension. Since after prefrontal surgery the patient becomes relatively unconcerned about his previous problems, anxieties, and pain, this has been the rationale for this operation in the above-mentioned disorders as well as in cases of untreatable pain. This should not be taken to mean that the patient is no longer capable of displaying emotion. Indeed, he may be emotionally labile and occasionally hyperreactive, but the aspect of suffering and caring about his psychological and physical difficulties (which may still exist) is virtually eliminated. [SeeMentalDisorders, Treatment OF, article onSomaticTreatment.]
This solution to the problem of anxiety has its unfortunate aspects as well, for the normal sense of responsibility for one’s self, for one’s work and home, and toward others, may practically disappear. This disintegration of social conscience may be accompanied by a high degree of distractability, inability to plan ahead, and inappropriate or socially unacceptable emotional reactions. It is possible that some of the signs produced by prefrontal-lobe lesions represent a release of diencephalic activity from cortical control.
Parietal, temporal, and occipital lobes. In monkeys, proprioceptive and tactile roughness and form discriminations which had been learned preoperatively were impaired after bilateral destruction of the posterior parietal cortex, but retraining was possible except in the case of very complex tactile-form discriminations. These findings of somatosensory-discrimination losses attributable to parietal-association-cortex damage are in accord with the spatial proximity of this region to the primary somatic receiving area.
In the temporal lobe of cats, bilateral lesions have been made in association cortex adjacent to the primary and secondary auditory receiving areas. While such lesions do not disrupt a simple intensity discrimination, there is a disturbance of both frequency and tonal pattern discrimination. However, the frequency discrimination can be relearned, but the pattern discrimination cannot. The severity of disruption in this instance is clearly related to task complexity, which falls neatly into the order of intensity discrimination, frequency discrimination, and pattern discrimination. This is a good example of the increasingly important role association cortex plays as behavior becomes more elaborate.
It is useful to treat the parietal, temporal, and occipital association cortices together, since in many instances the functional boundaries are not clear and lesions may produce similar or closely related functional losses. Relatively large lesions of the parieto-temporo-occipital association cortex in monkeys result in a loss in previously acquired visual-form-discrimination ability, with some impairment of relearning as well. It is to be emphasized that in order to produce consistent deficits, the lesions usually have to be quite extensive. More restricted lesions do not have this effect. In connection with this point, it is interesting that bilateral injury to the inferotemporal, or ventral, cortex, which is not adjacent to the visual receiving areas but does have fiber connections with the occipital cortex, is crucial for the retention of visually guided discrimination behavior. Somasthetic and auditory discriminations do not appear to be affected by lesions in the inferotemporal region.
It is important to note at this point that many of the foregoing deficits appear specifically related to the memory of learned tasks, since relearning can often take place after the cortical removals mentioned above. In a few instances the ability to learn is disturbed as well. It has been emphasized that bilateral lesions are usually required to produce a significant deficit in memory or learning in monkeys. In the realm of human behavior, however, certain functions appear to be highly lateralized, and unilateral damage to the cortex may produce profound defects.
The fact that the left and right sides of the brain appear grossly identical does not mean that there is necessarily equivalence of function for the two hemispheres. For example, damage to the right parieto-occipital cortex of man often produces impairment of performance of nonverbal perceptual motor tasks, while similar injuries to the left hemisphere may be without consequence with respect to this class of behavior. For many complex human activities there is a dominant hemisphere, but for a particular individual the dominant hemisphere is not necessarily the same for all such activities. Because the pyramidal motor pathways cross as they descend, individuals who are unequivocally right-handed are cerebrally dominant in the left hemisphere for this function. In fact, most people, whether righthanded or left-handed, show left cerebral dominance for speech, but not all do; for example, individuals who are dominant for speech in the right hemisphere are more likely to be left-handed. Thus, for a particular individual the hemisphere which is dominant for speech may or may not be the one that is dominant for handedness.
If the dominant hemisphere is injured, there is a tendency for the homologous cortical area of the opposite hemisphere to assume its function. Thus, if the area of the left hemisphere which controls speech is severely injured, this function will be lost, but it may be recovered if comparable regions of the right hemisphere can take over. Whether recovery of an activity occurs in a particular case is largely a function of the age at which the damage occurs. Plasticity is especially pronounced in early life and declines with maturity and age. Recent evidence indicates that there are persons in whom speech is represented bilaterally in the cortex. In these cases, it is not yet clear whether both hemispheres have come into use as a result of injury to the one which was originally dominant.
Equipotentiality, mass action, localization
Experiments with monkeys have shown that even lesions which destroy both prefrontal lobes will not produce the prefrontal-lobotomy syndrome described earlier if the operation is done very early in life. This illustrates that the central-nervoussystem plasticity involved in the restitution of certain abilities does not necessarily require the presence of an anatomically intact and comparable region in the opposite cortex. Other, as yet undetermined, structures of the hemispheres are sometimes able to assume the role of regions which are morphologically quite different. Many findings such as these have led to the concept of functional equipotentiality of cortical areas, which is almost certainly an oversimplification. But if the limits of plasticity are taken into account, the “equipotentiality” notion has merit, especially in lower mammals, in which structural and functional differentiation is not so prominent and recovery is more frequent and rapid.
For many years, in neurology and psychology, a lively debate has raged over the degree to which behavioral functions are localized in specific areas of the hemispheres. At one extreme, some scientists have taken the view that functions and subfunctions are highly localizable. Proponents of strict localization have created detailed maps of the cortex relating precise cortical regions to a myriad of specific complex psychological activities. At the other end of the spectrum are those who believe that except for the primary sensory and motor areas, the cortex functions as a whole in practically all psychological activities and that localization is a rare finding. Evidence for this “mass action” viewpoint is often taken from rat studies in which large amounts of several cortical areas had to be removed before a deficit in learning or memory was noted, as well as from the fact that in humans certain psychological-test performances are disturbed regardless of the location of the lesion. The concepts of mass action and equipotentiality are obviously closely related. As has often been the case in similar scientific controversies, the evidence of both sides is frequently valid and not so mutually exclusive as the proponents of the extremes would sometimes have us believe.
Recent studies on human cortical injuries and electrical stimulation of the brains of patients indicate that some psychological functions are rather well localized, although the precise region on the cortex may vary somewhat from individual to individual. For other classes of behavior, specific regions have not been found, and it is reasonable to assume that certain complex intellectual activities require roughly equivalent participation of neuronal circuits from many areas of the cortex.
The cortical areas responsible for speech and language are of considerable importance, since these are the activities above all others that come very close to being truly unique to man. They provide one of man’s immeasurable advantages over other species: the verbal and written communication of acquired knowledge, beliefs, and attitudes or, more simply, the transmission of culture.
The speech and language areas are located in the frontal, temporal, and parietal lobes, usually lateralized to the left hemisphere. Lesions in, or electrical stimulation of, the frontal area, first described by Broca, which lies near, but not in, the primary motor area may result in motor aphasia, and inability to use spoken words properly. In aphasia, which may be categorized as being motor (expressive) or sensory (receptive), there is no injury to the primary sensory receiving apparatus, no paralysis of the muscles used in speech, and no damage to the area of the primary motor cortex controlling these muscles. Aphasia, then, is not a disorder of articulation. It is an ideational or associational disorder involving cortical structures and systems concerned with the use of language in thinking. [SeeLanguage, article onSpeechPathology; the biography ofBroca.]
In cases of injury to Broca’s area and the region immediately above it, there is also frequently present a loss of ability to write (agraphia) associated with the speech deficit. It appears, then, that just as Broca’s area is critical in the organization of sounds and words for the spoken expression of concepts, so is this adjacent superior region important in the arrangement of words for writing.
Lesions in the temporal-lobe portion of the speech area, adjacent to the auditory receiving cortex, produce an inability to understand spoken language (word deafness), although sounds can be normally heard. This loss constitutes a sensory aphasia, as does the inability to understand written language (alexia) as a result of injury to the posterior parietal speech area close to the visual cortex. While this classification is undoubtedly an oversimplification and while it is rare that the aphasic difficulties and lesions of an actual patient correspond perfectly with this schematic presentation, it is a useful frame of reference for further consideration of the problem. [SeeReadingDisabilities.]
Rhinencephalon, limbic system, and reticular formation
”;Rhinencephalon” literally means “nose-brain”it is basically composed of those structures concerned with olfaction. These are the olfactory bulbs and pathways, the amygdaloid nuclei, the threelayered cortex (paleocortex) on the ventral and mesial surface of the temporal lobe, and the hippocampus (archicortex), which is folded under the neocortex, which comprises the remainder of the covering of the hemispheres and has been discussed in the previous section. The paleocortex and archicortex are relatively primitive structures in terms of point of appearance in evolution and degree of morphological differentiation. [See Taste AND Smell.]
Although the involvement of rhinencephalic structures in olfaction has been assumed and studied for many years, it was not until the past few decades that their role in emotional and visceral activity was also recognized. Out of such investigations of structures concerned with emotion has grown the concept of a limbic system. A rather loose term, “limbic system” is often defined somewhat differently by various investigators. It arose from the even older term “limbic lobe,” which Broca used to designate the cingulate cortex in addition to most of the rhinencephalon. Broca assigned no specific function to this series of interconnected structures, which form a ring, or “limbus,” around the brain stem, but noted its presence as a common denominator of the mammalian and vertebrate brain. The modern concept of the limbic system is more inclusive than either that of rhinencephalon or limbic lobe. The limbic system is now generally taken to include many structures, from brain stem to cortex, that are connected anatomically and are involved in the expression of affect and other motivated behaviors of which emotion is a part. Some of these structures are the orbitofrontal cortex, cingulate cortex, hippocampus, ventral paleocortex, septal nuclei, amygdala, several hypothalamic and thalamic nuclei, and certain midbrain areas. The limbic system also has strong connections with the reticular formation.
It should not be assumed that structures of the limbic system participate only in states of emotional experience or expression. Many components of the limbic system are also active in the control of autonomic activities, which, according to the particular instance, may or may not be involved in affect—for example, digestion, cardiac and vascular regulation, and respiration. In addition, one of the central structures of the limbic system, the hippocampus, plays a crucial role in the process of memory storage. Before the importance of the hippocampus in recent memory was appreciated, a few patients underwent bilateral removal of this structure in an attempt to relieve severe seizures or psychotic behavior. Following these operations, the patients usually had a good memory of events that had occurred up to a few months or weeks before the surgery but virtually no memory of events occurring after that time. Some of the patients have been followed carefully for several years and continue to show a profound inability to remember events which have occurred since the operation.
It is clear from these unhappy clinical observations that the hippocampus is vital in recording perceptual impressions and laying down the memory trace. It is also evident that this structure is not crucial for the retention, retrieval, or expression of stored material that has been well established, since memory of events long past was adequate in these patients. It is presently hypothesized that the human hippocampus acts as the first-stage recorder of experience but that transfer of accumulated information to other systems is soon effected for long-term storage.
The reticular formation is an intricate and amazingly complex network of neurons forming a core of the brain stem and extending from the medulla up to the diencephalon. In the thalamus, fibers from the brain-stem reticular formation synapse in cell groups of nonspecific nuclei. These nonspecific nuclei—so called to distinguish them from the specific sensory nuclei of the thalamus—relay information to virtually all areas of the cerebral cortex. The thalamic nuclei and fibers which project reticular activity to the cortex are collectively known as the diffuse thalamocortical projection system. Brain-stem reticular activity is also projected to the cortex via the hypothalamus and certain limbic-system structures.
The reticular formation has both descending and ascending fibers mediating activities which may be placed in four functional categories:(1) control of postural-reflex tonus by inhibition or facilitation of motor neurons in the spinal cord;(2) participation in the control of certain autonomic reflexes and respiration; (3) mediation of efferent activity in the central control of sensory input; and (4) regulation of the general state of excitation in the cerebral cortex. This last function is primarily executed by the ascending components of the reticular formation. Arousal, wakefulness, and sleep are to a large extent controlled by the reticular formation and may be measured by the electroencephalogram (EEG). The EEG is a recording of electrical activity in the cerebral cortex by the application of electrodes to the scalp, skull, or cortex itself. The potentials thus recorded are amplified about a million times for visual inspection on a cathode-ray oscilloscope or inkwriter, and each lead or channel represents the activity of a large number of cortical neurons. When a subject is in a highly aroused or excited state the EEG shows desynchronized low-amplitude (voltage), high-frequency activity (beta waves). A subject in a state of relaxed wakefulness whose eyes are closed displays a more synchronous rhythm that is of higher amplitude and lower frequency (alpha waves). As the individual falls into sleep, the frequency decreases further and the amplitude becomes even greater. This rhythm is most evident in deep sleep, in which the EEG is in a state of hypersynchrony (delta waves). A stage of sleep not recognized until the 1950s, called paradoxical sleep, is reflected in low-voltage, high-frequency activity that is virtually indistinguishable from the excited waking state. There is considerable controversy as to whether paradoxical sleep is deeper or lighter than traditional deep sleep of the delta variety, but there is little question that much dreaming occurs during this stage. [SeeDreamsandSleep.]
The brain-stem reticular formation receives collateral fibers from the laterally placed ascending specific sensory pathways. Increased activity in these specific sensory tracts is crucial in producing a change in the pattern of firing of the brain-stem reticular-formation neurons, which in turn produces desynchronization (activation) of the cortical EEG rhythm. The appearance of this desynchronization of EEG activity is coincident with behavioral arousal and increased awareness. Thus, the sensory collaterals leading into the reticular formation form the anatomical pathway by which a sensory stimulus arouses a sleeping individual to wakefulness. Arousal from sleep can also be produced by direct electrical stimulation of the reticular formation, as well as by sensory stimulation.
There is no question that the arousal function is normally mediated by the reticular formation rather than by the specific sensory pathways. If, in experimental animals, the reticular formation is destroyed at a level rostral to the point at which the sensory collaterals project into it, the subject enters an enduring state of coma, for although specific sensory information will reach the cortex over the lateral pathways and sensory systems will continue to fire into the reticular formation, ascending reticular discharges are blocked by the transection from reaching the cerebral cortex. In this situation, evoked potentials, representing specific sensory information provided to the cortex via the lateral afferent pathways, continue to be elicited; but generalized EEG desynchronization of the cortex is not sustained. These facts are of considerable importance, since without generalized cortical activation produced by reticular-formation activity, there can be little or no cortical integration of the sensory information which arrives over the specific pathways. Under these conditions information is probably neither retained by the central nervous system nor integrated with other incoming sensory data or stored perceptions and memories.
If, on the other hand, the specific sensory pathways are cut at a level above the point at which their collaterals enter the reticular formation, cortical arousal and consciousness is still possible, but the animal is awkward in its movements and limited in its abilities, since the cortex is deprived of specific sensory information.
Frederick A. King
[Directly related are the entriesHearing; Learning, article onNeurophysiological Aspects; Psychology,article onPhysiological Psychology; Senses; Skin Senses And Kinesthesis; Taste And Smell; Vision.Other relevant material may be found inGestalt Theory; Mental Disorders, articles OnOrganic Aspectsand Biological Aspects; Mental Disorders, Treatment OF, article OnSomatic Treatment; and in the biographies ofBroca; Flourens; Lashley.]
Brazier, Mary A. B. (1951) 1960 The Electrical Activity of the Nervous System. 2d ed. New York: Macmillan. → A readable and concise account of neurophysiology.
Brooks, S. M. (1961) 1966 Integrated Science. 2d ed.St. Louis, Mo.: Mosby.
Deutsch, J. Anthony; and Deutsch, Diana 1966 Physiological Psychology. Homewood, III.: Dorsey.
Field, John; Magoun, H. W.; and Hall, Victor E. (editors) 1959-1960 Handbook of Physiology. 3 vols. Section 1: Neurophysiology. Washington: American Physiological Society. → A series of articles on virtually all aspects of neurophysiology and its behavioral correlates.
Gardner, Ernest(1947) 1963 Fundamentals of Neurology. 4th ed. Philadelphia: Saunders. -→ An excellent basic text on the structure and function of the nervous system.
Hilgard, Ernest R. (1953) 1962 Introduction to Psychology. 3d ed. New York: Harcourt.
Luriia, A. R. 1966 Higher Cortical Functions in Man. New York: Basic Books. -→ A review of studies on brain-damaged patients, including original Russian contributions.
Mccleary, Robert A.; and MOORE, ROBERT Y. 1965 Subcortical Mechanisms of Behavior: The Psychological Functions of Primitive Parts of the Brain. New York: Basic Books. → A summary of the subcortical organization of arousal, motivation, and emotion.
Magoun, Horace W. (1958) 1963 The Waking Brain.2d ed. Springfield, 111.: Thomas. → An excellent treatment of the anatomical and physiological bases of waking, sleeping, and attention.
Morgan, Clifford T. (1943) 1965 Physiological Psychology. 3d ed. New York: McGraw-Hill. → A standard and highly readable text on the biological basis of behavior.
Oswald, Ian1962 Sleeping and Waking: Physiology and Psychology. Amsterdam and New York: Elsevier.→A general volume on sleep from both the physiological and the behavioral viewpoints.
Peele, Talmage L. (1954) 1961 The Neuroanatomic Basis for Clinical Neurology. 2d ed. New York: McGraw-Hill. → An advanced text on neuroanatomy with clinical interpretations.
Penfield, Wilder; and Roberts, Lamar1959 Speech and Brain-mechanisms. Princeton Univ. Press. → An exhaustive account of the cerebral mechanisms of speech and language functions, based upon clinical observations of the brain-damaged and upon electrical stimulation of the human cortex.
Pontificia Accademia Delle Science Rome1966 Brain and Conscious Experience. Edited by J. C. Eccles. New York: Springer. -→ A fascinating symposium of the research and thoughts of 22 distinguished scientists on the neurology of consciousness.
Ruch, Theodore C. et al. (1961) 1966 Neurophysiology. 2d ed. Philadelphia: Saunders. →A thorough and advanced text on the electrophysiology and function of the nervous system.
One of the most powerful tools for the study of the interaction between brain function and behavior has been stimulation of the brain. Experimenters have employed a variety of techniques in order to induce an excitation or inhibition of the activity of the nervous system. Some of these include the introduction of chemicals and drugs into the brain. Others have employed heat and cold. However, by far the most widely used technique has been the application of electric currents to brain tissue. The use of these various methods of brain stimulation has resulted in an impressive body of data on the structure and function of the brain and its role in the control of behavior. Using these methods, experimenters have been able to stimulate the brain in one region and record the brain’s response in another region, thus determining the functional connections between brain areas. Others have stimulated various brain sites and observed the overt responses of the organism under study. These responses span the range of behaviors from simple reflexes to complex patterns of responses. Still other investigators have studied the effects of stimulation of specific brain areas on ongoing behavior in order to determine whether that particular brain area is involved in the maintenance of the behavior in question. Space does not permit us to summarize all of the various methods of stimulation that have been utilized. Therefore, discussion will be limited to the effects of electrical stimulation, since this has been the method most generally employed.
The study of the effects on behavior of electrical stimulation of the brain is as old as the study of electricity itself. Count Alessandro Volta (1745-1827), while observing the results of the application of electric currents to his various sense organs, passed the current through his own brain. Fortunately, he survived the experiment. Since that time, but particularly in the present century, many investigators have utilized electric currents to map pathways through the nervous system and to study the effects of stimulation of certain brain areas on behavior. Advances in electronic engineering have made available to the researcher more and more precise instrumentation, and it is now possible to stimulate tiny areas deep within the brain with only minimal damage to neural tissue.
In 1870, G. Fritsch and E. Hitzig reported that an electrical stimulus applied to certain regions of the cerebral cortex could elicit movements of the face, arms, and legs of experimental animals. In 1899, Charles S. Sherrington utilized electrical stimulation to demonstrate the reciprocal innervation of flexor and extensor muscles. Although experiments such as these shed a good deal of light on the functioning of reflex pathways, little was learned about voluntary behavior since exposure of such large areas of the nervous system required extensive restraint and anesthesia. The work of W. R. Hess, however, represented a major technical advance with the introduction of the first practical technique for permanent implantation of electrodes into the brain. The development of these chronic electrodes made possible the study of stimulation effects involving deep structures in awake, unrestrained, unanesthetized animals.
Hess’s electrode technique is still the basic method employed, although many laboratories have carried out modifications to suit their own particular needs. Generally, thin, rigid, stainless steel or platinum wires, insulated except at the tip, are lowered into the brain through a small hole in the skull. The animal is deeply anesthetized. Its head is held in a metal frame called a stereotaxic instrument. This frame, a modification of a device originally conceived by Horsely and Clark (1908), not only provides firm support for the head during surgery, but also allows the experimenter to locate deep cerebral structures with a fair degree of accuracy. This is accomplished by means of a threedimensional system of coordinates that locates any given brain structure by its position relative to some zero point. This is analogous to locating a region deep within the earth by means of its latitude, longitude, and distance from the surface. Stereotaxic atlases, or brain maps, of a number of common laboratory animals and of man are availble. Many of these may be found in a book edited by Sheer (1961) along with illustrations of stereotaxic instruments and detailed descriptions of electrode techniques.
Because of the variability in skull dimensions among animals, the stereotaxic technique is not completely accurate. It is necessary, therefore, at the termination of the experiment to sacrifice the animal and verify the exact location of the electrode tip by microscopic examination of the brain.
An important factor in the study of brain stimulation is the nature of the electrical stimulus. Direct current generally destroys nerve tissue. Indeed, prolonged application of direct current has often been used to produce lesions for experimental or clinical reasons. However, brief pulses of direct current, alternations of positive and negative pulses, and sine waves have all been used with good results in brain stimulation studies, provided that the current is not excessive.
Sleep, wakefulness, and emotion
Using the technique of chronic implanted electrodes, Hess systematically explored the diencephalon of the cat. He has summarized his findings in two recent works (1949; 1954). In brief, Hess observed that stimulation of the massa intermedia of the thalamus resulted in a progressive decrease of activity followed by sleep. The animal could be aroused from this sleep by some external stimulus, but once the external stimulus was removed, the animal returned to sleep. Stimulation of the posterior hypothalamus, however, resulted in immediate wakefulness and a state of excitation. With stronger stimulation in this area, the cat would hiss, bare its teeth and claws, arch its back, and show all the signs of rage and fear. Upon termination of the stimulation, the rage reaction ceased. Moreover, if the cat was provoked during this period of stimulation, a highly organized attack reaction would be directed toward the provocative object. Egger and Flynn (1962) have described a study, however, in which stimulation of the lateral nucleus of the amygdala suppressed the attack reaction elicited by hypothalamic stimulation. [seeEmotion and Sleep.]
A variety of motivational effects have been evoked by electrical stimulation of the brain, particularly in the region of the hypothalamus. Perhaps the most striking of these are the effects on food and water intake.
Food intake. Delgado and Anand (1953) have demonstrated that cats stimulated in the lateral hypothalamic area will perform gnawing and chewing movements and will increase their food intake. It is interesting to note that the increased eating is not necessarily directed toward edible objects. Miller (1960) reports that upon presentation of stimulation to the hypothalamic feeding area, rats will gnaw on blocks of wood, sticks, etc., when food objects are not available. This behavior has been described as “stimulus-bound” eating. Miller also described a study in which rats with electrodes in the lateral hypothalamus had been trained to press a lever for food. The animals were later foodsatiated and stimulated in the hypothalamus. Immediately following the onset of stimulation the animals began to press the food lever. When the stimulation was terminated, the rats immediately stopped lever-pressing.
Suppression of food intake by stimulation has also been reported. An investigation by Wyrwicka and Dobrzecka (1960) demonstrated that stimulation of the ventromedial nucleus of the hypothalamus of hungry goats resulted in immediate cessation of eating.
Water intake. Less work has been done in the area of central nervous system control of other motivational mechanisms by means of stimulation. Chemical stimulation of the hypothalamus has been demonstrated to control water intake effectively (Grossman 1960). There is also evidence that electrical stimulation can influence the thirst mechanism (Andersson & McCann 1955).
Sexual behavior. Vaughan and Fisher (1962) have demonstrated that stimulation of the lateral anterior hypothalamus results in a marked increase in sexual capacity in male rats. MacLean and Ploog (1962) have reported that stimulation of a number of forebrain and diencephalic areas in monkeys results in erection of the penis and ejaculation. [SeeSexualBehavior, article onAnimalSexualBehavior.]
The application of the techniques of intracranial stimulation to the study of the neuropsychology of learning has been a fruitful one. The typical approach has been to compare the performance of experimental organisms on a learning task with and without stimulation of some neural areas. This method has advantages over the traditional lesion technique, which requires elaborate control groups to assess the possible extraneous effects of surgical shock, blood loss, intracranial pressure, etc. In a stimulation study, each animal frequently serves as its own control, since the effects of stimulation are almost always reversible. However, certain learning tasks may necessitate the use of a control group with nonstimulated electrodes. Experiments employing electrical stimulation of the brain to investigate learning are numerous. A few examples of recent studies may serve to illustrate some of the major trends in current research.
Delayed alternation and visual discrimination. Rosvold and Delgado (1956) trained monkeys to perform a delayed-alternation task in which the animals were required to seek a peanut reward under the right food cup on one trial, under the left food cup on the next trial, under the right on the next, etc. A five-second delay was interposed between trials. Failure to alternate responses on successive trials was regarded as an error. The monkeys were also trained to perform a simple visual discrimination. Following stabilization of performance on these tasks, the monkeys were implanted with multiple-lead electrodes. Upon recovery from surgery, the animals were again tested on the delayed-alternation and visual discrimination problems. During a part of the testing session, electrical stimulation was delivered through one of the electrode leads. The experimenters found that performance declined markedly on the delayedalternation test when the lead stimulated was in the caudate nucleus. However, the same stimulation failed to interfere with the visual discrimination task. A later study by Buchwald and others (1961) revealed that stimulation of the caudate nucleus does interfere with acquisition of a visual discrimination task, but does not alter performance once the task has already been learned.
Operant conditioning. In another type of experiment, Knott and others (1960) trained cats to press a lever for meat reward in a modified Skinner box, an apparatus provided with devices, e.g., levers or buttons, whose appropriate manipulation leads to some desired outcome. Electrodes were implanted in a number of deep cerebral structures. Continuous low-intensity stimulation was delivered during the lever-pressing task. The experimenters reported that stimulation of the hippocampus, caudate nucleus, and thalamus failed to alter the leverpressing rate. Stimulation in the septal area, however, resulted in a cessation of response during stimulation. The animals resumed pressing the lever following termination of the stimulation, but only after a prolonged delay. Hypothalamic stimulation generally had the same effect as septal stimulation, except that the post-stimulation delays were shorter. The authors concluded that these data support the notion that certain neural pathways critical to the mechanisms of learning and retention become “occluded” by the stimulation. The data could also be interpreted, however, in terms of interfering emotional responses evoked by stimulation of the hypothalamus and the septal area.
Classical conditioning. A third type of investigation utilizes the classical Pavlovian conditioning paradigm except that both the conditioned stimulus (CS) and the unconditioned stimulus (US) are presented to the subject by means of implanted electrodes. Doty and Giurgea (1961) implanted electrodes in the motor area of the cerebral cortex of dogs, cats, and monkeys. Stimulation of these electrodes served as the US. The unconditioned response was the particular limb movement that resulted from the stimulation of a direct motorpathway. Stimulation in some other cortical area served as the CS. Six to ten pairings of the CS and the US were made daily. The experimenters were able to obtain clear-cut evidence of conditioned reflexes with cortical stimulation as the US.
Aside from their implications for learning theory, the experiments of Doty and Giurgea are important because they suggest the possibility of studying the electrophysiology of the conditioning mechanisms in a simple form with known and well-defined inputs.
Speech and memory
A series of remarkable experiments have been carried out by Penfield and Roberts (1959). These investigations have involved the electrical stimulation of the cerebral cortex of awake humans. The studies were carried out during the course of neurosurgical procedures for the treatment of epilepsy. During the operation, the comfort of the patient was maintained at all times by the careful administration of local anesthesia. In addition to yielding highly detailed maps of the sensory and motor regions of the human cerebral cortex, this research has also indicated the location of areas that are involved in the mechanisms of speech and memory. Penfield has described regions on the temporal lobe in which electrical stimulation will result in the recall of vivid experiences of sights and sounds. The patients report that they feel as if they were reliving those events. Moreover, a specific memory can be repeated by interrupting the stimulation and then quickly reapplying it. Penfield suggests that memory is organized on the temporal lobe in somewhat the same manner in which electrical impulses representing visual and auditory patterns are stored on magnetic tape. Thus, whenever a particular region of the brain is made to yield up its stored memories, the memories are recalled with the same vividness and clarity as when they were originally stored. He further suggests the existence of mechanisms in the brain that inhibit the retrieval of those stored memories, the electrical stimulation somehow bypassing or otherwise nullifying the influence of these inhibitory mechanisms.
Penfield and his coworkers have also mapped areas of the cortex that upon stimulation produce vocalization, hesitation and distortion of speech, repetition of vocal sounds, interference with speech, and complete arrest of speech. In addition, stimulation of certain areas results in an inability to name specific objects although the remainder of the speech mechanism seems to be unimpaired. These areas are mainly included in the parieto-temporaloccipital cortex, although the effects have also been obtained from several other well-defined cortical regions.
In an attempt to study the effects of subcortical stimulation on learning in rats, Olds and Milner (1954) placed a rat with a forebrain electrode in an open field maze. They observed that if the rat received stimulation in a particular place in the maze, it would spend more and more time in that place. The stimulation seemed to have a rewarding effect on the rat’s behavior. Next, they placed the rat in a T maze. The animal learned to go to that arm of the maze in which it was rewarded with brain stimulation. Microscopic examination of the rat’s brain revealed that the electrode tip was in the vicinity of the anterior commissure. Rats used subsequently were trained to deliver the stimulation to themselves by pressing a lever in a Skinner box. These animals would deliver several hundred stimulations per hour to themselves. The stimulation served as the sole reward for pressing the lever; food and water were never present in the box.
Since the original report by Olds and Milner, many of the variables affecting the self-stimulation phenomenon have been explored. Some of the principal variables have been the animal species; the location of the electrodes in the brain; the intensity, frequency, and duration of stimulation; the motivational and emotional state of the organism; and the schedule of reward. In addition, the effects of drugs on self-stimulation have also been studied.
Species. Self-stimulation of the brain has been clearly demonstrated in the following animals: rat., dog, cat, pigeon, monkey, dolphin, guinea pig, gold-fish, and man. At this point, experimenters would be more interested in learning of a vertebrate in which the phenomenon could not be demonstrated than in further additions to the list of positive instances.
The demonstration of self-stimulation in man is of particular interest since this is the only species that can provide verbal reports about the subjective experience of brain stimulation. Sem-Jacobsen and Torkildsen (1960) have reported cases in which electrodes were implanted in the brains of human patients for several months in the course of treatment for Parkinson’s disease. During exploratory stimulation, the experimenters encountered several regions of the forebrain in which the patients seemed to enjoy the stimulation. They would smile or grin and express a desire for repeated stimulation. If given an opportunity to press a button to deliver the stimulation to themselves, the patients would press the button often. Their verbal reports ranged from descriptions of tickling sensations to expressions of satisfaction and euphoria.
Anatomical variables. In general, self-stimulation has been demonstrated in those structures of the brain that form part of the limbic system or have strong anatomical connections with it (Olds 1956). Application of stimulation in purely sensory or motor areas does not appear to have any rewarding effect. Moreover, there are areas in which the effect of the stimulation appears to be punishing, and animals will learn to escape and avoid stimulation in these areas (Brown & Cohen 1959). Many of these negative regions correspond to the areas that, upon stimulation, produce rage and attack reactions.
Within the positive reward system, rates of selfstimulation vary widely from area to area and from species to species. Thus, a rat may self-stimulate 800 to 1,000 times per hour when the locus of stimulation is the septal area, 2,000 times per hour when it is the hypothalamus, and 4,000 times per hour when it is the tegmentum. Furthermore, the reported rates of responding may vary considerably among laboratories because of differences in apparatus. For example, it is possible to obtain higher response rates with a telegraph key lever than with a microswitch lever.
If an animal presses a lever 1,000 times per hour for septal stimulation and 2,000 per hour for hypothalamic stimulation, is it safe to conclude that the hypothalamic stimulation is more rewarding to the animal? An experiment by Hodos and Valenstein (1962) suggests that it may not necessarily be. Rats were given a choice, in a two-lever box, between septal stimulation and hypothalamic stimulation. These were presented at various intensities. When presented with high-intensity septal stimulation on one lever and low-intensity hypothalamic stimulation on the other lever, the animals showed a clear preference for the lever producing septal stimulation once they discovered it, even though their response rates were much higher on the lever producing hypothalamic stimulation. Considering these findings, it would seem hazardous to draw general conclusions about the relative reward values of stimulation in different neural areas based solely upon rate of responding. Directpreference tests at several intensities of stimulation provide data that are less subject to interpretive ambiguities resulting from the interfering influence of motor side-effects produced by the stimulation.
Stimulation variables. Reynolds (1958) demonstrated that as intensity of stimulation increased, the rate of responding increased, passed through a maximum, and then declined. Keesey (1962) confirmed this finding and, in addition, reported similar effects with variation in stimulation frequency and duration.
This decline in self-stimulation rate with high intensities of stimulation should not necessarily be interpreted as a decline in the rewarding properties of the stimulation. Hodos and Valenstein (1962) reported that when given the choice between two intensities of stimulation in the same area, rats consistently chose the higher of the two intensities, even though they self-stimulated at a lower rate for the higher intensity.
Motivational factors. Brady and others (1957) observed that animals would self-stimulate faster when deprived of food or water than they would when they were satiated. This was later confirmed by Olds (1958), who further reported that injections of male sex hormones in castrated male rats also increased the rate of electrical self-stimulation. However, Hodos and Valenstein (1960) failed to find any effects on the septal self-stimulation rate in spayed female rats injected with female sex hormones. Recently, Hoebel and Teitelbaum (1962) have demonstrated an interesting correlation between the hypothalamic areas controlling feeding behavior and those yielding self-stimulation. They suggest that the feeding system may control selfstimulation in a manner similar to its control of food intake.
Emotion. Brady and Conrad (1960b) demonstrated an interesting effect of self-stimulation on emotional behavior. Rats were trained to press a lever for either intracranial stimulation or water reward. Periodically, an auditory stimulus was presented for a period of five minutes. At the termination of this stimulus, a painful electric foot-shock was administered to the animals. When the animals were pressing the lever for water, presentation of the auditory stimulus produced a clear suppression in the rate of responding. This has been described as conditioned “fear” or “anxiety” in previous studies. No conditioned “fear” response to presentation of the auditory stimulus could be elicited, however, when the animals were leverpressing for brain stimulation in the medial forebrain area. Moreover, the possibility that the animals were unable to hear the auditory signal during the brain stimulation has been eliminated by Beer and Valenstein (1960), who showed that rats could, with little difficulty, make auditory discriminations when the auditory signals were presented simultaneously with brain stimulation.
Reward schedules. Sidman and others (1955) presented data illustrating the point that behavior rewarded by brain stimulation on either a fixedratio or a variable-interval schedule generally has the same characteristics as food-rewarded behavior on the same schedule. However, Brodie and others (1960) reported that fixed ratios of more than 20 responses to each stimulation are difficult to maintain in monkeys unless very slow and gradual training is permitted. Fixed ratios of several hundred responses to each reward are not at all uncommon when animals are rewarded with food. Moreover, these researchers report far less resistance to extinction of performance when brain stimulation is the reward than has often been observed when the schedule is equivalent but the reward is food.
In a study of timing behavior, Brady and Conrad (1960a) required monkeys to space their responses at least 20 seconds apart in order to receive a reward. In the case of food reward or anterior thalamic stimulation reward, the animals had no difficulty in delaying their responses for the required period of time and thereby received a large percentage of the possible rewards. The most frequent interresponse time was 20 seconds. However, in the case of stimulation of the globus pallidus, the animals were unable to delay their responses sufficiently and thereby received few rewards. The most frequent interresponse time was 10 seconds. There is some suggestion that the stimulation may have interfered with the mechanism of time perception in these animals.
Pharmacological effects. Olds and others (1956) as well as other workers have demonstrated that some drugs can affect self-stimulation performance. Chlorpromazine and reserpine, both tranquilizing drugs, each depressed the self-stimulation rate in some areas, but not in others. However, phenobarbital was not observed to produce any specific effect on performance.
Some general problems
When an experimenter produces a change in behavior by stimulation of a brain area, there is a great temptation to speculate on the possible role of that area in the mechanisms underlying the behavior. However, such speculations should be made cautiously, because the electrical stimulus may not have the same effect on all neural areas. For example, we have seen that stimulation of the caudate nucleus results in a deficit in delayed-alternation performance and that stimulation of the amygdala suppresses rage reactions. These behavioral deficits are the same as the effects of lesions in those neural areas. Therefore, it seems likely that the electrical stimulation had a suppressing effect upon normal function. On the other hand, we have seen that electrical stimulation of the lateral hypothalamus yields eating responses while ventromedial hypothalamic stimulation results in suppression of eating. These effects are the opposite of those observed when lesions are made. Presumably, in this case, the stimulation was augmenting the activity of the feeding areas. Thus, the effects of stimulation studies alone are not sufficient for determining the role that a cerebral structure may play in behavior.
A second and related problem is that of attempting to generalize from the highly unnatural type of stimulation that experimenters introduce into the brain to the normal types of physiological events present in the nervous system. The two may not necessarily have the same effect on neural tissue, and cautious interpretation of data is required. A detailed discussion of the problems of the interpretation of the behavioral effects of electrical stimulation of the brain may be found in a recent paper by Valenstein (1964).
A third problem is that of the possible interaction between brain stimulation and other environmental variables that may be affecting the behavior under study. We have seen that caudate nucleus stimulation will markedly interfere with visual discrimination performance if the animal is still in the process of acquiring the discrimination. However, the effects will be scarcely detectable once the animal has mastered the problem. Similar difficulties may arise in the study of emotional behavior, memory, and perception. Therefore, a thorough knowledge of the environmental variables that influence behavior is essential before attempting to study the effects of stimulation on behavior.
William Hodos And Joseph V. Brady
[Other relevant material may be found in Language,article on Speechpathology; Learning, article on Neurophysiological Aspects; Mental Disorders, article on Organic Aspects; Psychology, article on Physiological Psychology; Sleep.]
Andersson, Bengt; and Mccann, S. M. 1955 A Further Study of Polydipsia Evoked by Hypothalamic Stimulation in the Goat. Acta physiologica scandinavica (Stockholm) 33:333-346.
Beer, Bernard; and Valenstein, ELLIOT S. 1960 Discrimination of Tones During Reinforcing Brain Stimulation. Science 132:297-298.
Brady, Joseph V. et al. 1957 The Effect of Food and Water Deprivation Upon Intracranial Self-stimulation.Journal of Comparative and Physiological Psychology 50:134-137.
Brady, Joseph V.; and Conrad, Donald G. 1960a Some Effects of Brain Stimulation on Timing Behavior.Journal of the Experimental Analysis of Behavior 3:93-106.
Brady, Joseph V.; and Conrad, Donald G. 1960b Some Effects of Limbic System Self-stimulation Upon Conditioned Emotional Behavior. Journal of Comparative and Physiological Psychology 53:128-137.
Brodie, David A. et al. 1960 Rewarding Properties of Intracranial Stimulation. Science 131:929-930.
Brown, George W.; and Cohen, Bertram D. 1959 Avoidance and Approach Learning Motivated by Stimulation of Identical Hypothalamic Loci. American Journal of Physiology 197:153-157.
Buchwald, N. A. et al. 1961 Effects of Caudate Stimulation on Visual Discrimination. Experimental Neurology 4:23-36.
Delgado, Jose M. R.; and Anand, B. K. 1953 Increase in Food Intake Induced by Electrical Stimulation of the Lateral Hypothalamus. American Journal of Physiology 172:162-168.
Doty, R. W.; and Giurgea, C. 1961 Conditioned Reflexes Established by Coupling Electrical Excitation of Two Cortical Areas. Pages 133-151 in Council for International Organizations of Medical Sciences, Brain Mechanisms and Learning: A Symposium. Edited by Alfred Fessard et al. Oxford: Blackwell Scientific Pubs.; Springfield, 111.: Thomas.
Egger, M. David; and Flynn, John P. 1962 Amygdaloid Suppression of Hypothalamic ally Elicited Attack Behavior. Science 136:43-44.
Grossman, S. P. 1960 Eating or Drinking Elicited by Direct Adrenergic or Cholinergic Stimulation of the Hypothalamus. Science 132:301-302.
Hess, Walter R. (1949) 1954 Das Zwischenhirn: Syndrome, Lokalisationen, Functionen. 2d ed. Basel: Schwabe.
Hess, Walter R. 1954 Diencephalon: Autonomic and Extrapyramidal Functions. New York: Grune. → A comprehensive summary of Hess’s work on this subject, previously published in German, especially Die funktionelle Organisation des vegetativen Nervensystems 1948 and Das Zwischenhirn 1949.
Hodos, William; and Valenstein, Elliot S. 1960 Motivational Variables Affecting the Rate of Behavior Maintained by Intracranial Stimulation. Journal of Comparative and Physiological Psychology 53:502-508.
Hodos, William; and Valenstein, Elliot S. 1962 An Evaluation of Rate as a Measure of Rewarding Brain Stimulation. Journal of Comparative and Physiological Psychology 55:80-84.
Hoebel, Bartley G.; and Teitelbaum, Philip 1962 Hypothalamic Control of Feeding and Self-stimulation.Science 135:375-377.
Horsely, Victor; and Clark, R. H. 1908 The Structure and Functions of the Cerebellum Examined by a New Method. Brain: A Journal of Neurology 31:45-124.
Keesey, Richard E. 1962 The Relationship Between Pulse Frequency, Intensity and Duration and the Rate of Responding for Intracranial Stimulation. Journal of Comparative and Physiological Psychology 55:671-678.
Knott, John R.; Ingram, W. R.; and Correll, R. E. 1960 Some Effects of Subcortical Stimulation on the Bar Press Response. Archives of Neurology 2:476-484.
Maclean, Paul D.; and Ploog, Detlev W. 1962 Cerebral Representation of Penile Erection. Journal of Neurophysiology 25:29-55.
Miller, Neal E. 1960 Motivation Effects of Brain Stimulation and Drugs. Federation Proceedings 19, no. 4: 846-854. → Published by the Federation of American Societies for Experimental Biology.
Olds, James 1956 A Preliminary Mapping of Electrical Reinforcing Effects in the Rat Brain. Journal of Comparative and Physiological Psychology 49:281-285.
Olds, James 1958 Effects of Hunger and Male Sex Hormone on Self-stimulation of the Brain. Journal of Comparative and Physiological Psychology 51:320-324.
Olds, James; Killam, K. F.; and Bach-y-Rita, P. 1956 Self-stimulation of the Brain Used as a Screening Method for Tranquilizing Drugs. Science 124:265-266.
Olds, James; and Milner, Peter 1954 Positive Reinforcement Produced by Electrical Stimulation of Septal Area and Other Regions of Rat Brain. Journal of Comparative and Physiological Psychology 47:419-427.
Penfield, Wilder; and Roberts, Lamar 1959 Speech and Brain-mechanisms. Princeton Univ. Press.
Reynolds, Robert W. 1958 The Relationship Between Stimulation Voltage and Rate of Hypothalamic Selfstimulation in the Rat. Journal of Comparative and Physiological Psychology 51:193-198.
Rosvold, H. Enger; and Delgado, Jose M. R. 1956 The Effect on Delayed-alternation Test Performance of Stimulating or Destroying Electrically Structures Within the Frontal Lobes of Monkey’s Brain. Journal of Comparative and Physiological Psychology 49:365-372.
Sem-Jacobsen, Carl W.; and Torkildsen, Arne 1960 Depth Recording and Electrical Stimulation in the Human Brain. Pages 275-290 in Estelle R. Ramey and Desmond S. O’Doherty (editors), Electrical Studies on the Unanesthetized Brain. A symposium with 49 participants. New York: Hoeber.
Sheer, Daniel E. (editor) 1961 Electrical Stimulation of the Brain: An Interdisciplinary Survey of Neurobehavioral Integrative Systems. Austin: Univ. of Texas Press.
Sidman, Murray et al. 1955 Reward Schedules and Behavior Maintained by Intracranial Self-stimulation. Science 122:830-831.
Valenstein, Elliot S. 1964 Problems of Measurement and Interpretation With Reinforcing Brain Stimulation. Psychological Review 71:415-437.
Vaughan, Eva; and Fisher, Alan E. 1962 Male Sexual Behavior Induced by Intracranial Electrical Stimulation. Science 137:758-760.
Wyrwicka, W.; and Dobrzecka, C. 1960 Relationship Between Feeding and Satiation Centers of the Hypothalamus. Science 132:805-806.
Electroencephalography derives its name from the fact that it provides a graph or recording of the electrical activity of the brain, or encephalon. In human subjects—except when the brain surface is exposed during a surgical operation—the electroencephalogram (EEG) is always recorded from electrodes distributed over the surface of the scalp. A variety of electrodes have been used in the past, but at the present time two principal types are in use. One type consists of silver dish-shaped or cupshaped electrodes, 5 millimeters in diameter, under which a conductive paste or electrode gel serves as an electrolyte between the surface of the scalp and the electrode. The other type consists of a finegauge stainless-steel needle, which is painlessly inserted through the scalp. The electrodes are connected by fine insulated wires to a plug-in box which identifies numbered locations on the surface of the head, usually according to a system agreed upon by the International Society of Electroencephalography (Jasper 1958) and known as the 10-20 system. The locations of the electrodes on the head are adjusted according to the over-all head dimensions by measuring from the nasion (bridge of the nose) to the inion (external protuberance at the back of the skull) and by defining electrode positions at intervals of 10 or 20 per cent of the total distance; measurements from the auditory meatuses over the top of the head are similarly fractionated. Finally, measurements of the circumference of the head enter into the placement of the electrodes. Such procedures insure that electroencephalographers will use a uniform system of deriving the potentials on which to base their reports.
The number of electrodes placed on the head depends upon whether only a survey of representative areas is to be made or whether a detailed comparison of many areas is to be made in order to localize some relatively focal disorder, as in the cases of temporal lobe epilepsy, of a local region of trauma, or of a brain tumor.
From the plug-in box, where electrodes representing the various areas converge, a larger, electrically shielded cable enclosing all of the individually insulated wires carries these to the electroencephalograph. The EEG recording unit consists of voltage amplifiers and power amplifiers and some type of write-out and/or display unit. Since the potential differences between any two electrodes on the surface of the scalp are of the order of millionths of a volt and usually range from 2 or 3 microvolts to 100 microvolts—except in the case of some pathological discharges which may reach 500 microvolts—these minute potentials must be amplified at least a millionfold in order to be recordable or visible on the display unit. The voltage amplifiers are responsible for the amplification; the power amplifiers, as their name implies, provide an increase in the power of the signals so that a relatively insensitive recording unit may be driven by the variations in the alternating current. The amplifiers themselves are usually of the resistancecapacity coupled type with a time constant suitable for amplifying and transmitting the potentials to the recording unit without appreciable distortion. The recording unit is typically an inkwriting oscillograph, although it may be a cathode-ray oscillograph or an optical oscillograph. The latter types require the recording of the deflections of a light beam on photographic film or paper. The inkwriting oscillograph consists of a pen motor which converts the electrical alternations from the brain into mechanical movements of the pen, tracing in the form of an arc the variations of potential as they occur. These oscillations of the tip of a stylus or pen upon paper moving at a uniform rate provide a tracing or recording upon the paper, which is a form of graph, with time represented on the abscissa and voltage on the ordinate. Thus, the frequency, magnitude or voltage, and the pattern of the waves so traced on the paper may be studied and measured relative to standards or norms of expectancy established for normal subjects. Abnormal or pathological deviations may be noted and contrasted with normal patterns.
Now let us return to the history of electrical activity of the brain. Caton (1875), an English physician, was the first to publish an account of the recording of electrical variations from the surface of the exposed brain of the rabbit. His findings were confirmed in a study of the dog by Russian and Polish investigators (see Brazier 1959), during the last quarter of the nineteenth century, but little further use of the phenomenon in interpreting the brain’s activities and functions was made until Hans Berger (1929), a German neuropsychiatrist, published the first report on the recording of the human EEG. Since Berger’s recordings showed predominantly wavelike oscillations, the classical neurophysiologists of this period, being acquainted only with spikelike deflections due to transient electrical potentials observed in peripheral nerves, looked upon his findings with doubt and felt that the waves might result from artifacts rather than brain activity. However, Adrian and Matthews (1934), distinguished electrophysiologists at Cambridge University, England, verified Berger’s findings and put their stamp of approval upon the work. Thus, for the first time it was recognized that at least two types of electrical activity exist in the nervous system: (1) when a nerve or neuron is excited and transmits an impulse, it* is accompanied by a sharp, spikelike negative potential and some wavelike after-potentials; (2) aggregates of neurons with their cell bodies and dendritic processes, such as are found in the cerebral cortex of the brain, give rise to continuous, autonomous electrical changes of wavelike nature.
The spike discharge of the neuron is a discrete event and tends to occur only when the neuron is excited by impulses reaching it from a receptor or another neuron. In contrast the wavelike activity of large numbers of neurons closely arrayed structurally appears not to be dependent upon sensory input or excitation.
Patterns of brain activity
The EEG is composed principally of this wavelike activity, which in the human is continuously present in one form or another as long as the brain is alive. Typically, with the eyes closed and with the subject or patient relaxed and not thinking about anything in particular, Berger found, in the adult, that rhythmic waves, which he called alpha waves, were emitted at a rate of 10 per second. He noted that the frequency of these waves ranged from about 8 to 12 per second in different individuals, including older children and adults, but that very young children had slower alpha waves. We now know that alpha waves first appear in regular sequence over the sensory areas of the brain at about 3 months of age, at a frequency of 3 to 4 waves per second. By 1 year of age they have increased to 5 or 6 waves per second, and by 10 or 12 years they have reached the adult frequency, averaging about 10 per second. Such waves are remarkably constant in frequency, varying less than 1 cycle per second in a given individual, from one time to another—from moment to moment, from day to day, or from month to month. Anything which increases or decreases metabolic rate will shift the frequency, up or down respectively, 1 or 2 cycles per second. For example, in hypothyroidism, where metabolic rate is lowered, the alpha frequency tends to be 1 or more cycles slower than the norm for that individual or the average for a normal group; hyperthyroidism, with increased metabolic rate, tends to increase the alpha frequency. The administration of thyroxine, a thyroid extract used in the treatment of hypothyroidism, increases the alpha-wave frequency from its low level. Artificially increasing the body temperature by diathermy or other means usually increases alpha frequency accordingly; also, natural fevers tend to increase alpha frequency. Some people believe that the alpha rhythm constitutes a basic time clock in the nervous system, regulating or modulating other rhythmic processes of the brain and body and possibly playing a role in our estimation of time [seeTime, article onPsychologicalaspects].
It is normal to have an alpha rhythm when awake, relaxed, and not being stimulated. Also, it is normal for the alpha rhythm to be blocked by any type of stimulation which attracts the attention of the subject and alerts him or arouses him from his temporary quiescence or daydreaming. The alpha rhythm quickly recovers from this blocking and continues with its oscillations until another stimulus occurs. If the same, identical stimulus is repeated a number of times at more or less regular intervals, the subject tends to ignore it, and the brain’s response of blocking the alpha rhythm disappears. This state of adaptation or lack of responsiveness is sometimes referred to as habituation. As soon as the quality of the stimulus changes in either intensity, kind, or another characteristic, the blocking of the alpha rhythm will return, as it does in the case of any novel stimulus which attracts attention [seeAttention].
The alpha rhythm, or other characteristics of the EEG, are not significantly correlated with intelligence or personality traits. For references to numerous early studies of this relationship, as well as more detailed descriptions of the EEG, see extensive reviews by Jasper (1937) and Lindsley (1944).
Sleep. The electroencephalogram has proven to be a useful correlate of psychological conditions. In the case of the transition from a waking state to sleep, in human subjects as well as lower animals, there are a series of fairly well-defined stages. There is an alpha rhythm of about 10 waves per second in normal, adult humans who are relaxed, awake, and free of special sensory stimulation. As drowsiness occurs the alpha rhythm slows slightly, decreases slightly in voltage, and shows brief periods of subsidence. As the drowsiness increases to the point where the subject is not aware of events in his environment, slower waves (3 to 8 per second) begin to appear, interspersed with declining alpha waves, and as the first true state of sleep occurs, there appears in the tracing periodic bursts of waves, called sleep spindles, at about 14 per second. In the next stage of sleep the spindles disappear and more and slower waves of higher voltage appear. Beyond this point, the waves become still slower and more prominent, ranging from 1 to 3 per second; these are called delta waves. At this time the state of sleep is deep, and it is more difficult to arouse the subject from his slumbers by various kinds of sensory stimuli. More recently, however, a still deeper stage of sleep has been described in which muscles of the limbs, body, and neck become more relaxed and the EEG shifts from large and very-low-frequency slow waves to fast, low-amplitude waves similar to those seen when a person is awake and alerted or excited. This state of sleep is sometimes called paradoxical sleep, because while the subject appears to be sleeping deeply and is difficult to arouse, his EEG appears to be that of a waking person. Such periods occur within an hour after going to sleep and last only a few minutes but recur again and again during the course of the night’s sleep, usually several times for durations of 5 to 15 minutes. It was discovered by Kleitman ( 1963) and his associates, Aserinsky and Dement, that rapid eye movements occur during these comparatively brief periods in the course of the night’s sleep; more recently these periods have been referred to as REM-sleep, that is, rapid-eye-movement sleep. If a subject is awakened during one of these REM states, he usually reports that he has been dreaming, but he is less apt to have been dreaming if awakened during the slowwave stage of deep sleep. Consequently, REM-sleep is associated with dreaming, and there is some evidence (Dement 1965) that a person needs a certain amount of this type of sleep to remain mentally healthy. [SeeDreamsand Sleep.]
Contrary to widespread belief, fostered by advertising and commercial agencies selling “learn during sleep” apparatus or programs, no well-controlled and validated studies using the EEG to monitor the presence and depth of sleep show that learning does indeed occur during sleep. Simon and Emmons (1955) have reviewed this subject.
Waking and anesthetic states. As we have seen in the instance of transitions from wakefulness to sleep, variations in the state of consciousness or awareness of one’s environment are correlated with marked changes in the EEG. When a person is alerted, excited, or aroused, EEG activity is low, fast, and not particularly rhythmic; when a person is awake, relaxed, and not very attentive—as in reverie or daydreaming—there are good rhythmic alpha waves; at the onset of true sleep, the alpha waves are displaced by slower waves and spindle bursts at the rate of 14 per second; in deep, quiet sleep without complete relaxation, large, billowy, slow waves are the rule; finally, in the most relaxed state, but with REM and dreaming, there are lowamplitude fast waves again.
During the anesthetic state, in passing from one plane of anesthesia to another, similar sequences occur but vary with the type of anesthetic used. In general, it is believed that in both sleep and the anesthetic state a core structure of the lower brain stem, known as the reticular formation, is depressed or inhibited. The reticular formation can be activated by all types of sensory stimulation; in fact, it is generally referred to as a nonspecific sensory-arousal mechanism, which appears to keep the brain awake and alert, and perhaps even attentive, when properly sensitized to incoming sensory stimuli. It may also be affected by neurohumoral substances and endocrine secretions and by other constituents of the blood stream and cerebrospinal fluid. These physiological and biochemical factors are important in the functioning of all brain and nervous-system cells, and the activity of these neurons is reflected in their electrical activity, of which the EEG provides one measure. [SeePain.]
Although in the past the EEG has not proved very useful in differentiating psychiatric patients from normal subjects or in distinguishing different personality characteristics, it has been a very valuable supplementary tool for the neurologist and neurosurgeon. It quickly proved its value in connection with a variety of neurological disorders.
Perhaps most outstanding in this respect were the epilepsies. The minor attack, or petit mal, has a characteristic signature known as the spike-andslow-wave pattern, which recurs about three times per second. In the major attack, grand mal, where there are convulsions and loss of consciousness, the pattern of the EEG begins with increasedamplitude fast activity associated with stiffening and loss of consciousness, known as the tonic state, and shifts to grouped activity as the convulsive movements occur; finally, after the series of convulsions ceases and the EEG quiets to an almost isoelectric state, a period of coma and marked relaxation exists. Generally, this state of the EEG and behavior lasts only several seconds. Then the EEG develops a deep-sleeplike pattern with large, billowy waves, and the patient appears to sleep deeply for a number of minutes or even a few hours before awakening, usually with return of the EEG to a fairly normal status.
A concussion produced by a blow on the head with temporary or prolonged loss of consciousness is accompanied by large, slow delta waves. A focal injury to the brain, caused by a rupture of a very small blood vessel (petechial hemorrhage) or a penetrating wound, gives rise to aperiodic spikelike discharges in that local region and, thus, helps to localize the source of the damage or injury. Encephalitis and other infections which damage tissue have toxic effects or create pressures which usually produce some kind of aberration in the EEG. Thus, the neurologist and the neurosurgeon find the EEG useful as a supplement or adjunct to their other techniques of detecting and localizing brain injuries or in diagnosing the source or cause of a neurologic disorder. The merit of the EEG is that it is a painless and harmless procedure and has no aftereffects, as do some neurologic test procedures. Almost every major hospital in the country and many lesser ones now have some facility for EEG recordings and for using these not only as a diagnostic tool but also as a tool measuring the effects of anesthetics in the operating room or measuring the effects of particular drugs or treatment procedures.
The most recent development involving the EEG is the recording of average evoked potentials over sensory areas of the brain by means of computer summation of responses that are time-locked to their stimuli; the stimuli are repeated as many as a hundred times in order to build up a reliable average. These responses from sensory areas, and even from association areas, are very small in comparison to the ongoing EEG activity; their voltage generally is of the order of five to ten microvolts, whereas the alpha waves and other EEG activity may be several times larger. Using these computer-averaged responses, the senses of vision, audition, and bodily sensation have been studied (see Sensory Evoked Response in Man 1964). Thus, human sensory experience may be studied by judgmental or psychophysical methods and, at the same time, by electrical recording from the particular area of the brain concerned. Other sensory-related functions have been studied with interesting results. Haider and his colleagues (1964) demonstrated the usefulness of the computer-averaging methods in relation to an experiment on attention and vigilance maintained over time. They found that as attention diminished generally, so that a subject no longer detected certain dim flashes imbedded in a series of brighter flashes with the same percentage of accuracy, the magnitude of the evoked response diminished. As vigilance waned, so did the voltage of the evoked potentials; however, if a subject had a five-minute period in which he detected all interspersed signals, the magnitude of his evoked potentials for that period was as great as at the beginning of the task. In another experiment, Spong and his colleagues (1965) demonstrated that by directing the attention of a subject to a particular class of sensation—vision, for example—and by telling him to ignore audition, even though flashes and clicks alternated as stimuli throughout, the magnitude of the average evoked potential was always greater over the visual area when attention was directed toward visual stimuli and auditory stimuli were ignored; the opposite was true for responses over the auditory area. Thus, there is significant evidence that attention and vigilance have a central response which may now be recorded and studied in relation to psychological set or instruction. In addition, Walter has found by a similar method, using direct-current recording, that giving a subject an instruction and an “expectancy” leads to a build-up of potential until that expectancy is met, either correctly or incorrectly, at which time a discharge occurs (Walter et al. 1964). He calls this a “contingent negative variation” It is a brain process which undoubtedly plays a role in decision making and other higher cognitive functions, and will prove to be a valuable probe for getting at problem solving, thinking, and other functions of a higher level. [SeeAttention; Hearing; Senses; SkinSensesAndKinesthesis; Vision.]
The electroencephalogram has played a very useful role in the hands of the neurophysiologist and neuropharmacologist in the study of the electrical activity of the brain. It has proved to be a very considerable stimulus to brain study in animals and man, because it provided a parameter of measurement not previously possible. The psychologist and neurophysiologist today frequently combine their talents in the study of brain and behavior correlations, and through this work a better understanding of the mechanisms and organization of the brain and nervous system is coming about. Thus, the electroencephalogram is an experimental tool useful in research on the brain, as well as a clinical tool useful in the clinic and hospital.
Donald B. Lindsley
[Other relevant material may be found inAttention; Dreams; Electroconvulsive Shock; Hearing; Learning, article onNeurophysiological Aspects; Mental Disorders, Treatment Of, article onSomaticTreatment; Psychology, article OUPhysiological Psychology; Senses; Sleep; Vision; and in the biographies ofFlourensandLashley.]
Adrian, E. D.; and Matthews, B. H. C. 1934 The Berger Rhythm: Potential Changes From the Occipital Lobes in Man. Brain 57:355-385.
Berger, H. 1929 Über das Elektrenkephalogramm des Menschen. Archiv für Psychiatrie und Nervenkrankheiten 87:527-570.
Brazier, Mary A. B. 1959 The Historical Development of Neurophysiology. Volume 1, pages 1-58 in J. Field et al. (editors), Handbook of Physiology. Washington: American Physiological Society.
Caton, Richard 1875 The Electric Currents of the Brain. British Medical Journal 2:278 only.
Dement, W. 1965 Recent Studies on the Biological Role of Rapid Eye Movement Sleep. American Journal of Psychiatry 122:404-408.
Gibbs, Frederic A.; and Gibbs, Erna L. (1941) 1950 Atlas of Electroencephalography. Volume 1: Methodology and Controls. 2d ed. Cambridge, Mass.: Addison-Wesley.
Haider, Manfred; Spong, Paul; and Lindsley, Donald B. 1964 Attention, Vigilance and Corticalevoked Potentials in Humans. Science 145:180-182.
Hill, Denis; and Parr, Geoffrey (editors) 1950 Electroencephalography: A Symposium on Its Various Aspects. London: MacDonald.
Jasper, Herbert H. 1937 Electrical Signs of Cortical Activity. Psychological Bulletin 34:411-481.
Jasper, Herbert H. 1958 Report of the Committee on Methods of Clinical Examination in Electroencephalography: 1957. Electroencephalography and Clinical cNeurophysiology 10:370-375.
Kiloh, Lester G.; and Osselton, J. W. 1961 Clinical Electroencephalography. London: Butterworth.
Kleitman, Nathaniel (1939) 1963 Sleep and Wakefulness. Rev. & enl. ed. Univ. of Chicago Press.
Lindsley, Donald B. 1944 Electroencephalography. Volume 2, pages 1033-1103 in Joseph McV. Hunt (editor), Personality and the Behavior Disorders: A Handbook Based on Experimental and Clinical Research. New York: Ronald.
Sensory Evoked Response in Man. 1964 New York Academy of Sciences, Annals 112 whole volume. → Robert Katzman was the editor of this issue.
Simon, Charles W.; and Emmons, William H. 1955 Learning During Sleep? Psychological Bulletin 52: 328-342.
Spong, Paul; Haider, Manfred; and Lindsley, Donald B. 1965 Selective Attentiveness and Cortical Evoked Responses to Visual and Auditory Stimuli.Science 148:395-397.
Walter, W. Grey et al. 1964 Contingent Negative Variation: An Electric Sign of Sensorimotor Association and Expectancy in the Human Brain. Nature 203: 380-384.
The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or as complex as the centralized system of mammals. In all nervous systems the functional unit is the nerve cell or neuron, a cell specialized to transmit and receive a stimulus.
To survive, animals have to respond to changes in their internal and external environment. General responses are found in animals that have a simple nervous system and can only process information in a limited way. An example of this type of nervous system is found in the common freshwater Hydra, a cylinder-shaped invertebrate. It has a nerve net of neurons between the outer and inner layers of a sac-like body. The nerve net transmits impulses in all directions with no means of processing the information to make a specific response. In flatworms, such as planaria, there is a simple centralized nervous system. Here, neurons are organized into structures called ganglia that act to receive stimuli from the sensory structures and transmit them by way of a ladderlike arrangement of nerves to muscle cells. In this way, flatworms can make specific responses to stimuli, such as turning away from light, or curling up when touched. Higher invertebrates, such as annelids, arthropods, and mollusks, have a more complex nervous system with more highly developed sensory structures that allow the animals to receive, process, and respond to stimuli in a greater variety of ways. An example of this is the compound eyes of insects, which send sensory information through nerve fibers to the ganglia in the head that serve as the brain. The information is then relayed to the other parts of the body through a nerve cord found on the ventral (lower) surface of the animal. The effectiveness of this arrangement is demonstrated by the rapid escape response of flies when attempts are made to kill them. In the octopus, a mollusk with well developed eyes and a central concentration of nerve cells, responses are highly specific, and the ability to learn how to perform complex tasks is evident.
The nervous system shows the greatest development in vertebrates. There is an increase in centralization with increasing elaboration of the brain with areas with specific functions. The central nervous system includes the brain and a dorsal (upper) spinal cord encased and protected by the skeletal system. The central nervous system is connected to the rest of the body through a peripheral nervous system that includes the nerves connecting the brain and spinal cord with receptors such as the ear and eyes and effectors such as the muscles in the body.
In the evolution of vertebrates from fish to mammals, the most significant changes have occurred in the structure of the brain. Even in the earliest vertebrates, the brain had three divisions: the hindbrain, midbrain, and forebrain. In fish, the hindbrain is dominant and concerned mainly with motor reflexes. The largest section of the fish brain is the optic lobes in the mid-brain, with the anterior of the brain (forebrain) composed of the olfactory lobes and the cerebrum. In the progression from fish to mammals, the hindbrain becomes less and less prominent, and the area of the brain used for receiving and integrating information becomes greater and greater as shown by an increase in the size and development of the cerebrum. The cerebrum is the part of the brain involved in learning voluntary movement as well as the interpretation of sensation. Birds and mammals have the largest brain mass relative to body size with the largest ratio found in humans and porpoises. In humans, the brain weighs approximately 3 lb (1.4 kg) with the cerebrum making up 80% of the total brain mass.
In humans, centralization has reached the greatest degree of specialization. The brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface that eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neurons found in the brain are produced—the sum total of all the neurons that the brain will ever contain in an individual’s lifetime. The brain is one of the largest organs in the body and consists of three main regions: the forebrain, midbrain, and hindbrain. The cerebrum, which is the most
important area for neural processing, together with the thalamus and hypothalamus, forms the forebrain. In the midbrain are centers for the receipt and integration of several types of sensory information, such as seeing and hearing. The information is then sent on to specific areas in the cerebrum to be processed.
The hindbrain consists of three parts: the medulla oblongata, the pons, and cerebellum, and it functions in maintaining homeostasis and coordinating movement. The pons and medulla of the hindbrain, together with the midbrain, form the brainstem, which is the location of reflex centers such as those that control heart beat rate and breathing rate. The other part of the central nervous system, the spinal cord, serves as a pathway for nerve tracts carrying impulses to and from the brain. It acts as the site for simple reflexes such as the familiar knee jerk. If a slice were made into the spinal cord, it would show a cord with a small central canal surrounded by an area of gray matter shaped like a butterfly surrounded by white matter. The gray matter is composed of large masses of cell bodies, dendrites and unmyelinated axons; the white matter is composed of bundles of axons that are called tracts, which send information to the brain or send information away from the brain.
The central nervous system operates through the peripheral nervous system, which is the roadway that links the central nervous system to the rest of the body. The nerves that carry information to the central nervous system from sensory receptors such as the eye are called sensory nerves or afferent nerves; those that carry impulses away from the central nervous system to effector organs such as the muscles are called motor nerves or efferent nerves. Commonly the fibers of sensory and motor neurons are bundled together to form mixed nerves. There are 12 pairs of cranial nerves that run to or from the brain, such as the optic and vagus nerves. There are 31 pairs of nerves called spinal nerves that originate from the spinal cord, such as the sciatic nerve and ulnar nerve, the nerve that is stimulated when a person hits their elbow. Specific areas of the body are served by each of the spinal nerves. All sensory nerves enter the cord through a dorsal root, and all motor nerves exit through a ventral root. If the dorsal section of a root is destroyed, sensation from that area is also destroyed, but the muscles are still able to function. In the opposite situation, damage to the ventral root destroys muscle function, but sensory information is still processed.
There are two main divisions to the peripheral nervous system, the somatic and the autonomic. The somatic system involves the skeletal muscles. It is considered voluntary since there is control over movement such as writing or throwing a ball. The cell bodies of the somatic system are in the central nervous system (CNS) with the axons running all the way to the skeletal muscles. The autonomic nervous system (ANS) affects internal organs. It is considered involuntary since the processes such as heart beat rate and glandular secretions occur with usually little control on the part of the individual.
The autonomic nervous system, in turn, is divided into two divisions, the parasympathetic and sympathetic. The parasympathetic system is most active in normal, restful situations and is dominant during quiet, relaxed periods. It acts to decrease the heartbeat and to stimulate the motility and secretions necessary for digestion. The sympathetic nervous system is most active during times of stress and arousal and is dominant when energy is required, when it increases the rate and strength of the contractions of the heart and inhibits the motility of the intestine.
In addition to their effects, the two divisions of the ANS differ anatomically. The nerves of the parasympathetic system originate at the top and bottom of the central nervous system, while those of the sympathetic system emerge from the upper and central spinal cord. At the site of the effector organs, axons in the parasympathetic system release acetylcholine, while those in the sympathetic system release norepinephrine. Together with hormones, the autonomic system maintains homeostasis, the internal balance of the body.
The functional unit of the nervous system is the neuron, a cell specialized to receive and transmit impulses. The types and functions of neurons found in organisms seem to be directed by several regulatory genes and by certain cues that occur during development. Once neurons mature, they lose the ability to divide. An exception is the olfactory neurons that are replaced every 60 days from stem cells resting beneath them, which ensures a supply as they wear out. Even though there are a variety of neurons, the essential structures are the same in each: a cell body containing the nucleus and two kinds of processes extending from it, the axon and dendrite. Axons transmit impulses away from the cell body to the dendrites of adjoining neurons. Some axons may be over 3 ft (1 m) in length, such as the sciatic nerve that runs from the spinal cord to the lower leg.
The axons of the peripheral nerves are enclosed in a fatty (myelin) sheath formed from specialized cells called Schwann cells. The myelin sheath acts to insulate the axon, which helps to accelerate the transmission of a nerve impulse. Gaps along the sheath expose the axon fiber and are important in allowing nerve impulses to jump from one section of the axon to another. The speed at which nerve impulses travel depends on the diameter of the axon and the presence of the myelin sheath with some impulses from the large
Action potential— A transient change in the electrical potential across a membrane that results in the generation of a nerve impulse.
Axon— The threadlike projection of a neuron that carries an impulse away from the cell body of the neuron.
Dendrites— Branched structures of nerve cell bodies that receive impulses from axons and carry them to the nerve cell body.
Depolarization— A tendency of a cell membrane when stimulated to allow charged (ionic) chemical particles to enter or leave the cell. This favors the neutralization of excess positive or negative particles within the cell.
Ganglion— Cluster of nerve cell bodies. In vertebrates, found outside of the central nervous system and act as relay stations for impulses. In invertebrates, act as a central control.
Homeostasis— The internal stability of an organism.
Myelin— A multilayered membrane system of a Schwann cell that wraps around an axon. It is made of lipoproteins that act as insulators in speeding up the transmission of nerve impulses.
Nerve— Bundles of axons in a connective tissue sheath which follow a specific path.
Nerve impulse— A transient change in the electrochemical nature of a neuron.
Neuron— Cell specialized to receive and transmit impulses; functional unit of the nervous system.
Neurotransmitter— A chemical released at the end of an axon which is picked up by receptors such as dendrites, muscles, or secretory cells.
Polarized— Two different charges on either side of a membrane caused by a difference in the distribution of charges; in resting nerve cells maintained by the sodiumpotassium pump.
Reflex— A rapid response to a stimulus that involves a sensory and motor neuron and may involve an interneuron.
Refractory period— Recovery period for the neuron in which no new impulse can be generated; it cannot respond to a stimulus until it is repolarized.
Sodium-potassium pump— A special transport protein in the membrane of cells that moves sodium ions out and potassium ions into the cell against their concentration gradients.
Synapse— Junction between cells where the exchange of electrical or chemical information takes place.
motor nerves to the leg muscles traveling as fast as 394 ft (120 m) per second. Damage to the sheath in multiple sclerosis patients causes impaired muscle control and other symptoms, an indication of the importance of the myelin sheath in the transmission of nerve impulses. Axons are bundled together and enclosed by connective tissue to form nerves. Dendrites are usually highly branched extensions of the cell body that receive impulses from axons. In some cases they may be very small, as seen in some of the neurons of the brain, or long, as is the sensory dendrite which runs from the foot to the spinal cord.
When a stimulus is strong enough, a nerve impulse is generated in an all-or-none response, which means that a stimulus strong enough to generate a nerve impulse has been given. The stimulus triggers chemical and electrical changes in the neuron. Before an impulse is received, a resting neuron is polarized with different charges on either side of the cell membrane.
The exterior of the cell is positively charged with a larger number of sodium ions present compared to the interior of the cell. The interior of the cell is negatively charged since it contains more potassium ions than the exterior of the cell. As a result, of the differences in charges, an electrochemical difference of about -70 millivolts occurs. The sodium-potassium pump, a system that removes sodium ions from inside the cell and draws potassium ions back in, maintains the electrical balance of the resting cell. Since the cell has to do work to maintain the ion concentration, ATP molecules are used to provide the necessary energy. Once a nerve impulse is generated, the permeability of the cell membrane changes, sodium ions flow into, and potassium ions flow out of, the cell. The flow of ions causes a reversal in charges, with a positive charge now occurring on the interior of the cell and a negative charge on the exterior. The cell is said to be depolarized, resulting in an action potential causing the nerve impulse to move along the axon. As depolarization of the membrane proceeds along the nerve, a series of reactions start with the opening and closing of ion gates, which allow the potassium ions to flow back into the cell and sodium ions to move out of the cell. The nerve becomes polarized again since the charges are restored. Until a nerve becomes re-polarized it cannot respond to a new stimulus; the time for recovery is called the refractory period and takes about four-thousandth (0.0004th) of a second. The more intense the stimulus results in more frequent firing of the neuron. When the impulse reaches the end of the axon, it causes the release of chemicals from small vesicles called neurotransmitters that diffuse across the synaptic gap, the small space between the axon and receptors in the dendrites. There is no physical contact between axons and dendrites (except in electrical transmission, usually found in invertebrates) that takes place through gap junctions.
The type of response by the receiving cell may be excitatory or inhibitory depending upon a number of factors including the type of neurotransmitters involved. All nerve impulses are the same whether they originate from the ear, heart, or stomach. How the impulse is interpreted is the job of the central nervous system. A blow to the head near the optic center of the brain produces the same results as though the impulse had originated in the eyes. The neurons are the functional units of the nervous system through which coordination and control in organisms is executed.
Al-Chalabi, Ammar. The Brain: A Beginner’s Guide. Oxford, UK: Oneworld, 2006.
Bear, Mark F. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2007.
Campbell, Neil A. Essential Biology with Physiology. San Francisco, CA: Pearson/Benjamin Cummings, 2004.
Evans-Martin, F. Fay. The Nervous System. Philadelphia, PA: Chelsea House, 2005.
Krogh, David. Biology: A Guide to the Natural World. Upper Saddle River, NJ: Pearson Education, 2005.
McDowell, Julie. The Nervous System and Sense Organs. Westport, CT: Greenwood Press, 2004.
Starr, Cecie. Basic Concepts in Biology. Belmont, CA: Thomson, Brooks/Cole, 2006.
Chudler, Eric H., University of Washington. “Neuroscience for Kids.” <http://faculty.washington.edu/chudler/introb.html> (accessed October 19, 2006).
The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or as complex as the centralized system of mammals . In all nervous systems the functional unit is the nerve cell or neuron , a cell specialized to transmit and receive a stimulus .
Evolution of invertebrate nervous systems
To survive, animals have to respond to changes in their internal and external environment. General responses are found in animals that have a simple nervous system and can only process information in a limited way. An example of this type of nervous system is found in the common freshwater Hydra, a cylinder-shaped inverebrate. It has a nerve net of neurons between the outer and inner layers of a sac-like body. The nerve net transmits impulses in all directions with no means of processing the information to make a specific response. In flatworms , such as planaria, there is a simple centralized nervous system. Here, neurons are organized into structures called ganglia which act to receive stimuli from the sensory structures and transmit them by way of a ladder-like arrangement of nerves to muscle cells. In this way, flatworms can make specific responses to stimuli, such as turning away from light , or curling up when touched. Higher invertebrates , such as annelids, arthropods , and mollusks , have a more complex nervous system with more highly developed sensory structures that allow the animals to receive, process, and respond to stimuli in a greater variety of ways. An example of this is the compound eyes of insects which send sensory information through nerve fibers to the ganglia in the head that serve as the brain . The information is then relayed to the other parts of the body through a nerve cord found on the ventral (lower) surface of the animal . The effectiveness of this arrangement is demonstrated by the rapid escape response of flies when attempts are made to kill them. In the octopus , a mollusk with well developed eyes and a central concentration of nerve cells, responses are highly specific, and the ability to learn how to perform complex tasks is evident.
Evolution of the vertebrate nervous system
The nervous system shows the greatest development in vertebrates . There is an increase in centralization with increasing elaboration of the brain with areas with specific functions. The central nervous system includes the brain and a dorsal (upper) spinal cord encased and protected by the skeletal system . The central nervous system is connected to the rest of the body through a peripheral nervous system that includes the nerves connecting the brain and spinal cord with receptors such as the ear and eyes and effectors such as the muscles in the body. In the evolution of vertebrates from fish to mammals, the most significant changes have occurred in the structure of the brain. Even in the earliest vertebrates, the brain had three divisions: the hindbrain, midbrain, and forebrain. In fish, the hindbrain is dominant and concerned mainly with motor reflexes. The largest section of the fish brain is the optic lobes in the midbrain, with the anterior of the brain (forebrain) composed of the olfactory lobes and the cerebrum. In the progression from fish to mammals, the hindbrain becomes less and less prominent, and the area of the brain used for receiving and integrating information becomes greater and greater as shown by an increase in the size and development of the cerebrum. The cerebrum is the part of the brain involved in learning voluntary movement as well as the interpretation of sensation. Birds and mammals have the largest brain mass relative to body size with the largest ratio found in man and porpoises. In man the brain weighs approximately 3 lb (1.4 kg) with the cerebrum making up 80% of the total brain mass.
Central nervous system
In humans, centralization has reached the greatest degree of specialization. The brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface which eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neurons found in the brain are produced-the sum total of all the neurons that the brain will ever contain in an individual's lifetime. The brain is one of the largest organs in the body and consists of three main regions: the forebrain, midbrain, and hindbrain. The cerebrum, which is the most important area for neural processing, together with the thalamus and hypothalamus, forms the forebrain. In the midbrain are centers for the receipt and integration of several types of sensory information, such as seeing and hearing . The information is then sent on to specific areas in the cerebrum to be processed. The hindbrain consists of three parts: the medulla oblongata, the pons, and cerebellum, and it functions in maintaining homeostasis and coordinating movement. The pons and medulla of the hindbrain, together with the midbrain, form the brainstem, which is the location of reflex centers such as those that control heart beat rate and breathing rate. The other part of the central nervous system, the spinal cord, serves as a pathway for nerve tracts carrying impulses to and from the brain. It acts as the site for simple reflexes such as the familiar knee jerk. If a slice were made into the spinal cord, it would show a cord with a small central canal surrounded by an area of gray matter shaped like a butterfly surrounded by white matter. The gray matter is composed of large masses of cell bodies, dendrites and unmyelinated axons; the white matter is composed of bundles of axons that are called tracts, which send information to the brain or send information away from the brain.
Peripheral nervous system
The central nervous system operates through the peripheral nervous system which is the "roadway" that links the central nervous system to the rest of the body. The nerves that carry information to the central nervous system from sensory receptors such as the eye are called sensory nerves or afferent nerves; those that carry impulses away from the central nervous system to effector organs such as the muscles are called motor nerves or efferent nerves. Commonly the fibers of sensory and motor neurons are bundled together to form mixed nerves. There are 12 pairs of cranial nerves that run to or from the brain, such as the optic and vagus nerves. There are 31 pairs of nerves called spinal nerves that originate from the spinal cord, such as the sciatic nerve and ulnar nerve, the nerve that is stimulated when you hit your elbow. Specific areas of the body are served by each of the spinal nerves. All sensory nerves enter the cord through a dorsal root, and all motor nerves exit through a ventral root. If the dorsal section of a root is destroyed, sensation from that area is also destroyed, but the muscles are still able to function. In the opposite situation, damage to the ventral root destroys muscle function, but sensory information is still processed.
There are two main divisions to the peripheral nervous system, the somatic and the autonomic. The somatic system involves the skeletal muscles and is considered voluntary since there is control over movement such as writing or throwing a ball. The cell bodies of the somatic system are in the central nervous system (CNS) with the axons running all the way to the skeletal muscles. The autonomic nervous system (ANS) affects internal organs and is considered involuntary since the processes such as heart beat rate and glandular secretions occur with usually little control on the part of the individual. The autonomic nervous system, in turn, is divided into two divisions, the parasympathetic and sympathetic. The parasympathetic system is most active in normal, restful situations and is dominant during quiet, relaxed periods. It acts to decrease the heartbeat and to stimulate the motility and secretions necessary for digestion. The sympathetic nervous system is most active during times of stress and arousal and is dominant when energy is required, when it increases the rate and strength of the contractions of the heart and inhibits the motility of the intestine. In addition to their effects, the two divisions of the ANS differ anatomically. The nerves of the parasympathetic system originate at the top and bottom of the central nervous system, while those of the sympathetic system emerge from the upper and central spinal cord. At the site of the effector organs, axons in the parasympathetic system release acetylcholine , while those in the sympathetic system release norepinephrine. Together with hormones , the autonomic system maintains homeostasis, the internal balance of the body.
The functional unit of the nervous system is the neuron, a cell specialized to receive and transmit impulses. The types and functions of neurons found in organisms seem to be directed by several regulatory genes and by certain cues that occur during development. Once neurons mature, they lose the ability to divide. An exception is the olfactory neurons which are replaced every 60 days from stem cells resting beneath them, which ensures a supply as they wear out. Even though there are a variety of neurons, the essential structures are the same in each: a cell body containing the nucleus and two kinds of processes extending from it, the axon and dendrite. Axons transmit impulses away from the cell body to the dendrites of adjoining neurons. Some axons may be over 3 ft (1 m) in length, such as the sciatic nerve which runs from the spinal cord to the lower leg. The axons of the peripheral nerves are enclosed in a fatty (myelin) sheath formed from specialized cells called Schwann cells. The myelin sheath acts to insulate the axon, which helps to accelerate the transmission of a nerve impulse. Gaps along the sheath expose the axon fiber and are important in allowing nerve impulses to jump from one section of the axon to another. The speed at which nerve impulses travel depends on the diameter of the axon and the presence of the myelin sheath with some impulses from the large motor nerves to the leg muscles traveling as fast as 394 ft (120 m) per second. Damage to the sheath in multiple sclerosis patients causes impaired muscle control and other symptoms, an indication of the importance of the myelin sheath in the transmission of nerve impulses. Axons are bundled together and enclosed by connective tissue to form nerves. Dendrites are usually highly branched extensions of the cell body which receive impulses from axons. In some cases they may be very small, as seen in some of the neurons of the brain, or long, as is the sensory dendrite which runs from the foot to the spinal cord.
When a stimulus is strong enough, a nerve impulse is generated in an "all or none" response which means that a stimulus strong enough to generate a nerve impulse has been given. The stimulus triggers chemical and electrical changes in the neuron. Before an impulse is received, a resting neuron is polarized with different charges on either side of the cell membrane . The exterior of the cell is positively charged with a larger number of sodium ions present compared to the interior of the cell. The interior of the cell is negatively charged since it contains more potassium ions than the exterior of the cell. As a result of the differences in charges, an electro-chemical difference of about -70 millivolts occurs. The sodium-potassium pump, a system which removes sodium ions from inside the cell and draws potassium ions back in, maintains the electrical balance of the resting cell. Since the cell has to do work to maintain the ion concentration, ATP molecules are used to provide the necessary energy. Once a nerve impulse is generated, the permeability of the cell membrane changes, sodium ions flow into, and potassium ions flow out of, the cell. The flow of ions causes a reversal in charges, with a positive charge now occurring on the interior of the cell and a negative charge on the exterior. The cell is said to be depolarized, resulting in an action potential causing the nerve impulse to move along the axon. As depolarization of the membrane proceeds along the nerve, a series of reactions start with the opening and closing of ion gates, which allow the potassium ions to flow back into the cell and sodium ions to move out of the cell. The nerve becomes polarized again since the charges are restored. Until a nerve becomes repolarized it cannot respond to a new stimulus; the time for recovery is called the refractory period and takes about 0.0004 of a second. The more intense the stimulus, the more frequent the firing of the neuron. When the impulse reaches the end of the axon, it causes the release of chemicals from small vesicles called neurotransmitters which diffuse across the synaptic gap, the small space between the axon and receptors in the dendrites. There is no physical contact between axons and dendrites (except in electrical transmission, usually found in invertebrates) which takes place through gap junctions.
The type of response by the receiving cell may be excitatory or inhibitory depending upon a number of factors including the type of neurotransmitters involved. All nerve impulses are the same whether they originate from the ear, heart, or stomach. How the impulse is interpreted is the job of the central nervous system. A blow to the head near the optic center of the brain produces the same results as though the impulse had originated in the eyes. The neurons are the functional units of the nervous system through which coordination and control in organisms is executed.
BSCS Revision Team. Biological Science: A Molecular Approach. Lexington, MA: D.C. Heath & Co., 1990.
Campbell, Neil A. Biology. Menlo Park, CA: Benjamin/Cummings, Publishing Company, 1987.
Carey, Joseph, ed. Brain Facts: A Primer on the Brain and the Nervous System. Washington, DC: Society for Neuroscience, 1993.
Curtis, Helena, and N. Sue Barnes. Biology. 5th ed. New York: Worth Publishers, 1989.
Holtzman, Eric, and Alex B. Novikoff. Cells and Organelles. Philadelphia: Saunders College Publishing, 1984.
Kuffler, Stephen W., and John G. Nicholls. From Neuron to Brain. Sunderland, MA: Sinauer Associates, 1976.
Purves, Dale. Body and Brain: A Trophic Theory of Neural Connections. Cambridge: Harvard University Press, 1988.
Raven, Peter H., and George B. Johnson. Biology. 3rd ed. St. Louis: Mosby Year Book, 1992.
Towle, Albert. Modern Biology. Austin, TX: Holt, Reinhart, and Winston, 1991.
- Action potential
—A transient change in the electrical potential across a membrane which results in the generation of a nerve impulse.
—The threadlike projection of a neuron that carries an impulse away from the cell body of the neuron.
—Branched structures of nerve cell bodies which receive impulses from axons and carry them to the nerve cell body.
—A tendency of a cell membrane when stimulated to allow charged (ionic) chemical particles to enter or leave the cell. This favors the neutralization of excess positive or negative particles within the cell.
—Cluster of nerve cell bodies. In vertebrates, found outside of the central nervous system and act as relay stations for impulses. In invertebrates, act as a "central" control.
—The internal stability of an organism.
—A multilayered membrane system of a Schwann cell that wraps around an axon. Made up of lipoproteins that act as insulators in speeding up the transmission of nerve impulses.
—Bundles of axons in a connective tissue sheath which follow a specific path.
- Nerve impulse
—A transient change in the electro-chemical nature of a neuron.
—Cell specialized to receive and transmit impulses; functional unit of the nervous system.
—A chemical released at the end of an axon which is picked up by receptors such as dendrites, muscles, or secretory cells.
—Two different charges on either side of a membrane caused by a difference in the distribution of charges; in resting nerve cells maintained by the sodiumpotassium pump.
—A rapid response to a stimulus that involves a sensory and motor neuron and may involve an interneuron.
- Refractory period
—Recovery period for the neuron in which no new impulse can be generated; it cannot respond to a stimulus until it is repolarized.
- Sodium-potassium pump
—A special transport protein in the membrane of cells that moves sodium ions out and potassium ions into the cell against their concentration gradients.
—Junction between cells where the exchange of electrical or chemical information takes place.
The nervous system is a highly precise and complex system of cells that allows animals to sense, process, and react to cues from the physical environment. The fundamental duty of the nervous system is to transfer information at relatively high speed from one part of the animal to another. Every animal has at least a rudimentary nervous system. Although plants and fungi are able to sense and respond to aspects of their environment, they do this based solely on chemical physiological responses and not because of the combined activity of specialized cells. The means by which a nervous system transfers information is through electrochemical signal transmission. Single nerve cells, neurons, can receive information in the form of a chemical and electrical signal, and transfer this information to other neurons , as well as to somatic cells, non-neurons.
The reason and the means by which animals originally developed a nervous system are very difficult to ascertain. Certainly, ancestral animals gained an advantage by being able to sense their environment, and as multicellular organisms became very large, a fast efficient system of communication was needed. However, the function and identity of the first neuron remains a mystery.
Components of the Nervous System
The nervous system of vertebrates is functionally divided between the central nervous system, consisting of the brain and spinal cord , and the peripheral nervous system , including all neurons that do not have their cell bodies within the brain or spinal cord. Primarily, the nervous system is composed of four cell types: neurons, Schwann cells, oligodendrocytes, and astrocytes.
Neurons are the information transfer cells that perform the primary activity of the nervous system. Schwann cells, oligodendrocytes, and astrocytes are support cells for neurons. Schwann cells are located only in the peripheral nervous system, but they have the same function as oligodendrocytes, which are located solely within the central nervous system. Both cell types wrap a fatty myelin sheath around the axon, the electrical signal, to insulate it and thereby increase the speed of conduction. This axonal covering is white, whereas the neuron is gray, so that nerves composed primarily of axons look white because of the myelin, and regions formed mostly by cell bodies look gray.
Support cells can also absorb excess neurotransmitter and provide certain precursor molecules that the neurons will use to construct essential proteins and metabolites. Astrocytes appear only in the central nervous system, and their function is to absorb nutrients from the bloodstream and conduct them to the neurons. Data suggests that support cells are also instrumental in directing immature neurons into their correct location during development, as well as ensuring the integrity of synapses and guiding regrowth of axons after injury.
The brain and nervous system are composed of grouped functional systems. This means that neurons can be categorized based on what kind of information they convey. These like-neurons are organized into pathways of conduction punctuated by processing nodes. The conduction pathways are formed of nerves or fibers containing primarily neuronal axons. The nodes, called ganglia or brain nuclei, are mostly composed of neuronal cell bodies and dendrites . Because information is transferred along pathways, and each node processes the information in a characteristic manner, the nervous system is referred to as a labeled-line system.
The nervous system is also called a parallel pathway system, because sensations such as sounds and visual inputs are transferred to the brain in an organized manner within separate nerves. For instance, sounds are divided up into their respective frequencies, and each frequency travels in its own fiber, parallel to the other frequency fibers grouped together within the auditory nervous system. The different sounds thus remain segregated until they are processed in the cortex. Finally, although distinct regions of the brain perform unique tasks, many overall concepts that are important psychologically to humans are not located in any one region of the brain. Memory, emotion, intelligence, and personality are all examples of emergent properties, meaning that they result from the coordinated activities of many brain regions.
Neurons carry information in the form of an action potential , which is a rapid (several milliseconds long) change in the electrical conduction of the cell membrane. When a neuron produces an action potential, it is described as firing, and a single action potential is called a spike. Action potentials are the primary form of communication between neurons, and the entire nervous system is mediated by this signal.
One may then wonder how perception can be so complex. This is because many factors contribute to the information encoded by the action potentials, including the frequency of action potentials, the probability of an action potential in any particular cell, the morphology (shape) of the neuron, the number and location of neurons that contribute the information, the number and location of neurons that receive the information, the type of neurotransmitter it uses, and the contributions of support cells. Furthermore, although each individual neuron can only produce an action potential for communication, this signal can have a different shape and character for different neurons.
The opposite of an action potential is a hyperpolarizing potential . This is instigated by inhibitory neurons, which release a neurotransmitter that decreases the probability that the neuron will fire. There may be thousands of inputs to a single neuron, or just one, and the contributions of all the factors listed above allow the combinatory activity of all the neurons in the ordered nervous system to produce consciousness, cognition (knowing), behavior, sensation, and homeostasis (maintenance of an organism's general health) in animals.
Peripheral nervous system.
In vertebrates the peripheral nervous system is composed of both motor neurons, which instigate muscle movement and activity, and sensory neurons, which convey information about the external and internal state of the organism. Furthermore, interneurons are important intermediates in both sensory and motor pathways, because they connect different circuits and can modify a signal as it follows a particular course. All subdivisions of the peripheral nervous system are comprised of these three neuronal types. The peripheral nervous system can be further divided into the autonomic nervous system and the somatic nervous system . Because it mediates the activity of heart muscle, smooth muscle, and exocrine glands, the autonomic nervous system is also referred to as the involuntary nervous system. The somatic nervous system is called voluntary because it controls the skeletal muscles.
Autonomic nervous system.
The autonomic nervous system is made up of the sympathetic, parasympathetic, and enteric divisions. The enteric system is a subsection of the peripheral nervous system located in the gastrointestinal tract of the gut and is responsible for mediating digestive reflexes. The high number and dense compaction of neurons in this system, and its autonomy with respect to the brain, cause some scientists to qualify it as a primitive "second brain."
The sympathetic and parasympathetic divisions of the peripheral nervous system are functional opposites. Whereas the parasympathetic division is responsible for homeostatic activities, such as maintaining a basal respiratory pattern, heartbeat, and normal metabolism, the sympathetic division governs the body's reaction to extreme situations. It instigates emergency measures in response to stress from strong emotions, athletic exertions, battle, severe temperature change, and blood loss. The sympathetic division thus increases activity in the heart and other organs, the sweat glands, the vascular system, and certain smooth muscle groups. Because the autonomic division controls day-to-day bodily functions, it has been characterized as controlling "rest and digest" activities, whereas the sympathetic division is responsible for "fight or flight" reactions.
Somatic nervous system.
The somatic nervous system allows vertebrates to monitor and control skeletal muscle output and to consciously sense aspects of the environment. Sensations originating at the skin or muscles of the trunk and limbs of an animal are called somatosensory information . Neurons located in the skin, muscle, joints, and ligaments of the body are specialized for transmitting somatosensory information to the central nervous system. Conveying the position of the limbs, muscle exertion, joint stress, temperature, tickle, pain, and tactile information, these sensory neurons enter the spinal cord via the dorsal root ganglia . The term "dorsal" means "toward the back of the body," whereas "ventral" means "toward the front of the body." Ganglia are congregations of neuronal cell bodies located outside of the brain. All sensory information enters the spinal cord from the dorsal side and then travels up to the brain.
Motor nerves controlling muscle movement descend from the brain and send axons out of the ventral side of the spinal cord. There are ventral roots that contain motor axons, but, unlike the somatosensory nerves, there are no motor ganglia. Motor neurons in the somatic nervous system innervate (connect with) skeletal muscles and can be controlled by a mix of voluntary and involuntary impulses.
Sensation and motor control of the face, head, and neck do not enter the brain through the spinal cord, but instead are transmitted through cranial nerves that pass through holes in the skull. When animals plan to make a movement, the cerebral cortex sends a message down through the brain to the spinal cord and out to that muscle. Sensory neurons located within the muscle sense its movement and send that information back up to the brain through the somatosensory pathway.
The vertebrate brain is housed in the skull, at the rostral end of the organism, whereas the tail end of the animal is the caudal end. Quadrupedal, fourlegged, animals have distinct rostral (head), caudal (tail), dorsal (back), and ventral (stomach) poles. At a point in human development, the brain bends 90° so that humans may stand vertically with face pointing forward, whereas the quadruped's head and brain remain in the straight axis of the spinal cord. Thus, below this bend, dorsal refers to the back and ventral to the chest sides of the body; but, above the bend, dorsal refers to the upward direction and ventral points downward.
All vertebrates have a bilaterally symmetrical brain, meaning that specialized regions on one side of the brain are mirrored on the other side. Although animals with more complex brains contain several specialized structures and pathways that differ from one hemisphere to the other, for the most part this mirror image organization is conserved.
The brain is divided into three basic regions, the hindbrain, midbrain, and forebrain. The hindbrain contains the pons, cerebellum, and medulla oblongata. At the top of the spinal cord is the medulla oblongata, a thickened region of neural tissue responsible for basic life processes such as breathing, digestion, and control of heart rate. Directly above (rostral to) the medulla is the pons, which conducts information relating to movement, gustation (taste), respiration, and sleep. The cerebellum, a large, highly folded structure composed of six tissue layers, lies dorsal to the pons and medulla. The cerebellum smoothens and coordinates muscle movements and is responsible for learned motor patterns, such as riding a bicycle.
The midbrain lies rostral to the hindbrain, and between these regions is the cephalic flexure, the bend that disrupts the longitudinal axis of the human central nervous system. The midbrain, primarily a relay site for motor and sensory neurons, is the focus of clinical research for its involvement in motor dysfunction diseases such as Parkinson's. Additionally, it is becoming increasingly clear that complex signal properties for sensory systems are established in the midbrain, rather than higher up, in the cortex.
The forebrain can be subdivided into the diencephalon and the telencephalon. The diencephalon is situated directly rostral to the midbrain. It contains the thalamus, which is a nexus for all information destined for the cerebral cortex, and the hypothalamus. The hypothalamus serves to integrate autonomic signals and endocrine activity with the organism's behavior. It regulates body temperature, eating and digestion rates, hormonal control of mating and pregnancy, and the sympathetic division of the autonomic nervous system.
The telencephalon houses the basal ganglia, hippocampus, amygdaloid nuclei, and cerebral cortex. The first three of these structures are buried in the center of the brain, surrounded by the cerebral cortex, cerebellum, and midbrain. The basal ganglia are essential for regulating motor performance. The hippocampus is implicated in short-term memory, and with aspects of long-term memory storage. The amygdala and its associated nuclei coordinate emotion and the effect of emotional state on autonomic and endocrine functions.
The cerebral cortex is involved with higher functioning, association formation, conscious perception, thought, memory, and emotion. The two hemispheres are divided but are interconnected by a bridge called the corpus callosum. Each hemisphere is divided anatomically into four lobes that are separated by prominent folds in the tissue: the occipital, parietal, temporal, and frontal lobes. The occipital lobe is the most dorso-caudal, located at the back of the skull. It contains primary processing centers for vision. The parietal lobe, centrally located on the dorsal cortex, processes sensory and motor information from the body. A distinct fold in the cortex called the central sulcus separates the primary motor cortex (just rostral of the sulcus) from the primary sensory cortex (just caudal of the sulcus). These thin strips of cortex extending from the top of the brain around the lateral sides encode sensation and motor input to every body region in a highly predictable manner. The amount of cortex dedicated to a particular body region is in direct proportion to the amount of motor control or sensory input from that region. The temporal lobe angles down ventrally on the lateral sides of the brain. It contains higher processing centers for audition, vision, and memory. The frontal lobe is the most rostral, and it contains association areas that may be a site for the storage of long-term memories.
Specialized Systems in Animals
The nervous systems of particular animals are specialized to the life habits of those animals. For example, some migratory animals may rely on detection of electromagnetic cues from Earth's crust to guide them over great distances. Weakly electric fish sense their environment and communicate with each other through emission of electrical impulses. These specialized senses require a specialized nervous system to collect and interpret information from the environment. Marine invertebrates , such as the giant squid, have very different neurons from those of vertebrates. The nerve cells are unmyelinated (not myelin-containing support cells), and thus the diameter of the axon must be very large sufficiently to increase conduction speed of the neuron.
Most invertebrates, including insects, have a centralized brain, but the most primitive animals instead have a diffuse distribution of distinct ganglia within each of their segments. These ganglia interact to control organismal activities, but there is no central processing center, as in vertebrates. Studying the simpler nervous systems of invertebrates aids in the understanding of their biological processes.
see also Growth and Differentiation of the Nervous System; Neuron; Sense Organs.
Rebecca M. Steinberg
Allman, John. Evolving Brains. New York: Scientific American Library, distributed by W. H. Freeman & Co., 2000.
Cooper, Leon N., ed. How We Learn, How We Remember: Toward an Understanding of Brain and Neural Systems. Selected Papers of Leon N. Cooper. River Edge, NJ: World Scientific, 1995.
Kandel, Eric R., James H. Schwartz, et al. Principles of Neural Science. New York: McGraw-Hill, 2000.
Kotulask, Ronald. Inside the Brain: Revolutionary Discoveries of How the Mind Works. Kansas City, MO: Andrews McMeel, 1996.
LeDoux, Joseph E. The Emotional Brain: The Mysterious Underpinnings of Emotional Life. London: Phoenix, 1999.
Purves, Dale, William S. Mark, et al. Neuroscience. Sunderland, MA: Sinauer Associates, 2001.
The nervous system is a network of nerve cells that allows an animal to collect, process, and respond to information. As an internal communications system, the nervous system enables an animal to react and adjust to changes in its environment. Almost all animals have some type of nervous system, but the human nervous system allows us to speak, solve problems, and have creative ideas—activities that make humans different from all other animals.
Animals have a nervous system but plants do not. Plants react and respond to changes in their environment by slowly altering their growth patterns using different hormones. When a plant inclines itself toward a light source or drops its leaves, it is doing so on command from a naturally occurring chemical called a hormone that it produces as a response to something outside itself. Such a slow system of internal communication could only be practical for an organism that can make its own food and does not move. For animals, however, their very existence and reproductive ability often rests on being able to react immediately to something in their environment. Often they are either trying to catch something to eat or they are trying to escape being caught. Movement is, therefore, essential to animals, and their nervous systems must always allow them to act appropriately, efficiently, and most important of all, rapidly.
EVOLUTIONARY DEVELOPMENT OF THE NERVOUS SYSTEM
Even the simplest multicellular organism has to constantly gather and analyze information about its environment if it is to maintain its inner balance (known as homeostasis) and survive in a constantly changing habitat. The single-celled amoeba does not have a real nervous system, but it still responds appropriately to stimuli like light or food. However, more complex organisms need to do more than simply react to stimuli and therefore need a more complex communications system. Probably the simplest nervous system is the one used by the class of animals called Scyphozoa (phylum Cnidaria), better known as jellyfish, hydra, and sea anemones. These animals use a system described as a nerve net that directly connects the receiving cell to the cell that does the responding. Flatworms are more complex than jellyfish and concentrate both their receiving and sending sensors in their forward end (like a head). This means that their front part is the first to meet the stimuli, and this permits them to react more rapidly. Flatworms also have bilateral symmetry (a body that is basically the same on both halves). Bilateral symmetry includes a typical vertebrate system with pairing of nerves down either side of a central column.
Continuing up the ladder past the jellyfish and the flatworm, the beginnings of the vertebrate (an animal with a backbone) model can be seen inside an earthworm or a grasshopper. Besides having bilateral symmetry, they also demonstrate what is called segmentation. They have identical segments (each of which has a pair of nerves) linked together and connected to a central organ that could be called a primitive brain. Once this model was established and seemed to work well, it eventually evolved into the sophisticated nervous system that is basic to humans and all other animals with backbones. The backbone or the vertebral column evolved into its present form because it proved, first of all, to be an ideal up-and-down framework to support the body and make it both strong and flexible. Its hollow center was eventually expanded and modified so that it could hold a spinal cord connected to a brain. Finally, the skull that held the brain then slowly developed a range of sensory equipment (ears, eyes, nose) that gathered information about the outside world and fed it into the brain.
THE CENTRAL NERVOUS SYSTEMS
SANTIAGO RAMON Y CAJAL
Spanish histologist (a person specializing in the study of tissues and organs) Santiago Ramon y Cajal (1852–1934) laid the foundations of modern neurology, which is the scientific study of the nervous system. He established that the nervous system is made up of independent units of nerve cells called neurons, and also made important discoveries relating to the transmission of nerve impulses and the cellular structures of the brain.
Santiago Ramon y Cajal was born in the remote village of Petilla de Aragon, Spain. His father was a self-trained country doctor who wanted his son to have a real medical education. Therefore, the family moved to the university city of Zaragoza, and young Santiago studied medicine there. Santiago was a rebellious youth who was more interested in drawing than studying, and his father finally forced him to work for a barber and a shoemaker as a way of disciplining him and making him appreciate school. This apparently worked, and he earned his medical degree from the University of Zaragoza in 1873. He then joined Spain's army medical service and served as army surgeon in Cuba for a year. However, while in Cuba he caught malaria and returned to Spain to recuperate. Going back to school, he earned his doctorate in medicine in 1879 and began a teaching career. Preferring to do research and to teach rather than practice medicine, he turned to the field of anatomy, which had always been his favorite subject. Anatomy studies the structure of living things and figures out how the different parts of an organism are shaped and how they fit together. His favorite branch of anatomy was histology.
Working with an old, abandoned microscope he found at the University of Zaragoza, Ramon y Cajal eventually turned toward the study of the most complex tissues in the body, which are those of the nervous system. These can only be studied under the microscope if they are stained (dyed), and Ramon y Cajal was able to improve upon some of the stains in use at this time. When he began his research on the nervous system, little or nothing was known about what might be called the path that a nervous impulse takes. Most scientists thought it traveled over something like a connected grid, or network of wires, but no one really knew for sure since they had never been able to examine nervous tissue closely. Using his own improved stain techniques on brain tissue, Ramon y Cajal was able, by 1889, to demonstrate that the nervous system was by far much more complex than anyone had imagined. He then went on to show that the neuron, or nerve cell, was the basic unit of the nervous system, and that it was very different from the other ordinary cells of the body. Ramon y Cajal then offered a controversial "neuron theory" that few accepted. He stated that the nervous system consists of a network of separate nerve fibers whose ends never actually "touch" or are not actually connected with the surrounding nerve cells. Today it is known that neurons are indeed not "hard-wired" together but instead have a synapse or space between them, across which the electrical charge or nervous impulse "jumps." He also stated that nerve impulses travel in only one direction, and that the brain's neurons had different structural patterns in different areas, suggesting that one part might have a different job than another. We now call this "localization," meaning that a certain part of the brain controls memory and another intelligence. Ramon y Cajal also conducted important research on the tissues of the inner ear and eye. Overall, modern neurology really began with his work, since he established the correct role played by the neuron and the nervous impulse.
The central nervous system consists of the brain and spinal cord and acts as network central, or the "main switchboard" for the entire system. The cord itself lies within and is protected by a central canal that is made up of stacks of bony vertebrae. Like an electrical cable, it has pathways that lead to and from the brain, and its nerve cells are lined up in a columnar way. Inside the backbone, the spinal column is bathed in cerebrospinal fluid that circulates around both it and the brain and acts as a liquid shock absorber. At the top of the column sits the brain—the system's real control center. It is actually a continuation of the spinal cord, which extends upwards and expands into segments called ventricles and aqueducts. The brain itself has three parts: the hind-brain, the midbrain, and the forebrain. The hindbrain occupies the rear of the skull and connects with the cord. It contains the medulla oblongata and the cerebellum. The medulla controls the body's involuntary processes like breathing and the heartbeat. The cerebellum coordinates the body's many muscles and allows it to move properly. The midbrain is sort of a coordinating center for information collected by sight and hearing, and it also relays information to higher centers of the brain. The forebrain contains the higher brain centers such as the cerebrum. The cerebrum is the part of the brain involved in voluntary actions and with functions related to memory, learning, and sensing things. In humans, the cerebrum is larger than the hindbrain and midbrain put together, and is the place that governs thinking, reasoning, and the use of complex language.
THE PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system can be described as a branching network of nerves that carry signals to and from the central nervous system. This system runs throughout the body of a vertebrate and has two different types of nerves: afferent nerves (or sensory neurons) bring input into the central nervous system, while efferent nerves (or motor neurons) take output away and communicate it to the muscles and glands. Some nerves in the peripheral nervous system can be controlled voluntarily, such as those that allow us to walk or run, but those that are not under our control (like the production of saliva or our beating heart) are considered a division of the peripheral nervous system called the autonomic nervous system. This system regulates functions like digestion that we cannot control. These nerves keep the body running smoothly by automatically adjusting its many systems.
The entire system works as it does because it is based on the neuron or nerve cell, the fundamental unit of communication in all nervous systems. Neurons never act alone. Rather, they transmit impulses to one another in the form of electrical signals and link together the entire nervous system. In many ways, neurons are the body's electrical highway. The neuron is a specialized cell and consists of three parts: the soma, the dendrites, and the axons. The soma is the cell body with its cytoplasm and nucleus. Dendrites and axons are hairlike arms or branches that extend from the body and channel information in one direction. Dendrites form the input part of the system and carry information toward the cell body. Axons are for output and therefore carry information away from the cell body. A typical neuron has several dendrites and one axon. Finally, neurons pass impulses to one another in a one-way direction across a space called a synapse. When a nerve impulse reaches the end of an axon and arrives at the synapse, a transmitter substance is released from the axon into the synapse. This chemical neurotransmitter goes across and binds to a receptor in the adjoining dendrite and triggers an impulse in it. The brain of the average adult contains about 1,000,000,000 neurons. Since the functioning of the entire system is dependent on the precise and proper functioning of the different types of neurons, anything that interrupts or disturbs that synaptic function can cause a problem with the organism. Today, scientists know of many genetic and infectious diseases that can severely interfere with this function. Many drugs have also been developed that can positively or negatively affect it.
The nervous system is a collection of cells, tissues, and organs through which an organism receives information from its surroundings and then directs the organism as to how to respond to that information. As an example, imagine that a child accidentally touches a very hot piece of metal. The cells in the child's hand that detect heat send a message to the child's brain. The brain receives and analyzes that message and sends back a message to the child's hand. The message tells the muscles of the hand to pull itself away from the heat.
The basic unit of the nervous system is a neuron. A neuron is a nerve cell capable of passing messages from one end to the other. In the example above, the "hot" message was passed from one neuron to the next along a path that runs from the child's hand to its brain. The "move your hand" message then passed from one neuron to the next along another path running from the child's brain back to its hand.
Types of nervous systems
The complexity of nervous systems differs from organism to organism. In the simplest of organisms, the nervous system may consist of little more than a random collection of neurons. Such systems are known as a nerve net. An example of an animal with a nerve net is the hydra, a cylinder-shaped freshwater polyp. Hydra respond to stimuli such as heat, light, and touch, but their nerve net is not a very effective way to transmit messages. Their responses tend to be weak and localized.
In other organisms, neurons are bunched together in structures known as ganglia (single: ganglion). Flatworms, for example, have a pair of ganglia that function like a simple brain. The ganglia are attached to two nerve cords that run the length of the worm's body. These two cords are attached to each other by other nerves. This kind of nervous system is sometimes described as a ladder-type nervous system.
Words to Know
Autonomic nervous system: A collection of neurons that carry messages from the central nervous system to the heart, smooth muscles, and glands generally not as a result of conscious action on the part of the brain.
Central nervous system: The portion of the nervous system in a higher organism that consists of the brain and spinal cord.
Ganglion: A bundle of neurons that acts something like a primitive brain.
Motor neutrons: Neurons that carry messages from the central nervous system to muscle cells.
Nerve net: A simple type of nervous system consisting of a random collection of neurons.
Neuron: A nerve cell.
Parasympathetic nervous system: A collection of neurons that control a variety of internal functions of the body under normal conditions.
Peripheral nervous system: The portion of the nervous system in an organism that consists of all the neurons outside the central nervous system.
Sensory neurons: Neurons that respond to stimuli from an organism's surroundings.
Somatic nervous system: A collection of neurons that carries messages from the central nervous system to muscle cells.
Stimuli: Something that causes a response.
Sympathetic nervous system: A collection of neurons that control a variety of internal functions when the body is exposed to stressful conditions.
The human nervous system. The most complex nervous systems are found in the vertebrates (animals with backbones), including humans. These nervous systems consist of two major divisions: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord, and the peripheral system of all neurons outside the central nervous system. The brains of different vertebrate species differ from each other in their size and complexity, but all contain three general areas, known as the forebrain, midbrain, and hind-brain. These areas look different, however, and have somewhat different functions in various species.
The peripheral nervous system consists of two kinds of neurons known as sensory neurons and motor neurons. Sensory neurons are located in the sensory organs, such as the eye and ear. They are able to detect stimuli from outside the organism, such as light or sound. They then pass that information through the peripheral nervous system to the spinal cord and then on to the brain. Motor neurons carry messages from the brain, through the spinal cord, and to the muscles. They tell certain muscles to contract in order to respond to stimuli in some way or another.
The peripheral nervous system can be subdivided into two parts: the somatic system and the autonomic system. The somatic system involves the skeletal muscles. It is considered to be a voluntary system since the brain exerts control over movements such as writing or throwing a ball. The autonomic nervous system affects internal organs, such as the heart, lungs, stomach, and liver. It is considered to be an involuntary system since the processes it controls occur without conscious effort on the part
of an individual. For example, we do not need to think about digesting our food in order for that event to take place.
Where would humans be without pain? We feel pain when we put a finger into a flame or touch a sharp object. What would happen if our body did not recognize what had happened? What would happen if we left our finger in the flame or did not pull away from the sharp object? Pain is obviously a way that organisms have evolved for protecting themselves from dangerous situations.
Although the reality of pain is well known to everyone, scientists still know relatively little as to how pain actually occurs. Current theories suggest that a "painful" event results in the release of certain "pain message" chemicals. These chemicals travel through the peripheral nervous system and into the central nervous system. Within the spinal cord and the brain, those pain messages are analyzed and an appropriate response is prepared. For example, the arrival of a pain message in the spinal cord is thought to result in the release of chemicals known as endorphins and enkephalins. These compounds are then thought to travel back to the sensory neurons and prevent the release of any additional pain message chemicals.
An interesting feature of pain is the role that individual psychology plays. For example, some people seem to be more sensitive to pain than others. This difference may reflect factors such as fear, expectation, upbringing, and emotions as much as physical factors. Another interesting phenomenon is phantom pain. Phantom pain is the pain a person feels in an amputated limb. How can pain continue when the site of that pain is no longer there? These and many other questions remain in the search to discover those bodily changes that occur during pain, the reasons that pain occurs, and the ways in which pain can be eliminated.
The autonomic nervous system is itself divided into two parts: the parasympathetic and sympathetic systems. The parasympathetic system is active primarily in normal, restful situations. It acts to decrease heartbeat and to stimulate the movement of food and the secretions necessary for digestion. The sympathetic nervous system is most active during times of stress and becomes dominant when the body needs energy. It increases the rate and strength of heart contractions and slows down the process of digestion. The sympathetic and parasympathetic nervous systems are said to operate antagonistically. In other words, when one system is dominant, the other is quiet.
Nerves and muscles usually work together so smoothly that we don't even realize what is happening. Messages from the brain carry instructions to motor neurons, telling them to move in one way or another. Whenever we walk, talk, smile, turn our head, or pick up a pencil, our nervous and muscular systems are working in perfect harmony.
But this smooth combination can break down. Nerve messages do not reach motor neurons properly, or those neurons do not respond as they have been told to respond. The result of such break downs is a neuromuscular disease. Perhaps the best known example of such disorders is muscular dystrophy (MD). The term muscular dystrophy actually applies to a variety of closely related conditions. The most common form of muscular dystrophy is progressive (or Duchenne) muscular dystrophy.
Progressive muscular dystrophy is an inherited disorder that affects males about five times as often as females. It occurs in approximately 1 out of every 3,600 newborn males. The condition is characterized by weakness in the pelvis, shoulders, and spine and is usually observed by the age of five. The condition becomes more serious with age, and those who inherit MD seldom live to maturity.
The causes of other forms of muscular dystrophy and other neuromuscular disorders are not well known. They continue to be, however, the subject of intense research by medical scientists.
[See also Brain; Muscular system; Neuron ]
The nervous system is subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). Basically, the brain and spinal cord form the CNS, while the rest is PNS. The CNS is well protected inside the skull and vertebral column. The PNS is essentially the nerves, which run through most of the tissues of the body. The function of the nervous system is to collect information from the body and the outside world, through the sense organs, to process it in the CNS, and to distribute relevant commands to the muscles and glands throughout the body.
Like other body tissues, the nervous system is composed of cells, similar in general form to other cells in the body, but with some important modifications. One might imagine that nervous tissue consists of nerve cells and very little else. In fact a multitude of other components are essential to proper functioning of the nervous system, and form an integral part of it. Still, the most important cells of the nervous system are the nerve cells (neurons). Their most distinctive feature is their thin processes, called fibres or axons, which transmit impulses (action potentials) and which contact muscles or glands, or, in most cases, other nerve cells. So the nervous system can be looked upon as an enormous series of ‘chains’ or circuits of neurons, each receiving excitatory and inhibitory messages from other neurons, and each sending impulses along its axon if the balance of incoming signals is in favour of excitation. A typical neuron in the brain may receive 10 000 terminals from incoming axons.
Many other cell types are necessary to support the neurons. Blood vessels supply blood to the nervous tissue and drain it away into the major veins. A large percentage of the human race will die from diseases associated with cerebral blood vessels, while many more people will be permanently handicapped, especially by stroke (blockage or rupture of blood vessels).
The most important — certainly the most numerous — other supporting cells in the nervous system are the glial cells, or glia (from the Greek for glue). Amazingly, there are about 10 times as many glia as neurons in the nervous system. The most distinctive glial cells in the PNS are the Schwann cells, which wrap themselves around peripheral nerves to produce the fatty, insulating sheath called myelin. In the CNS various types of glial cells are involved in myelination, the transfer of nutrients from capillaries to neurons, and are also components of the defence system of the CNS, protecting against infection and helping remove degenerated neurons.
In the PNS, groups of, usually, a few hundred axons form bundles, several of which are united into a nerve trunk. Individual axons are well protected and peripheral nerves are fairly flexible. They even stretch somewhat, which is necessary if they run near a limb joint, or when a surgeon wishes to suture together two divided nerve stumps. Larger nerves have their own tiny blood vessels.
The nervous system also includes the special sense organs (eyes, ears, etc.) and sensory axons throughout the body. The essential feature of a sense organ is the specialized neurons, called receptor cells, whose membranes include molecular mechanisms for detecting particular events outside the cell (such as the presence of particular chemicals, or light, or pressure on the membrane). Receptor cells ‘transduce’ the energy of these events into electrical changes inside the cell, which eventually produce a set of nerve impulses that race along axons towards the CNS. In the skin there are free nerve endings, specialized to signal touch, pain, and temperature. Muscle spindles and Golgi tendon organs are receptor organs found inside skeletal muscle, which are stimulated by stretch of the muscle or tension on the tendon. They help inform the CNS about the state of activity of the muscles and therefore the position and balance of the body. Such information can either be conscious (involving signals reaching the cerebral cortex of the brain) or unconscious (being used for example in spinal reflexes).
In the special sense organs, such as the eye and the ear, highly specialized receptors respond to light and sound. Sensory information also comes from the viscera and blood vessels. Although viscera can produce conscious sensation, such as pain when they are distended, visceral sensation is mainly used unconsciously by the autonomic nervous system.
The autonomic nervous system has both central and peripheral components. It is concerned with the automatic control of bodily function. It is subdivided into sympathetic and parasympathetic portions. To some extent, these two systems have opposing actions. For instance, sympathetic activity classically prepares for ‘fight or flight’, raising blood pressure and heart rate, facilitating breathing, dilating the pupils, and deviating blood from the skin and gastrointestinal tract to skeletal muscles. Parasympathetic activity, in contrast, adapts the body for rest and digestion. Cell bodies of sympathetic neurons are in the middle levels of the spinal cord. Their axons leave the cord and end on nerve cells in the sympathetic trunk, a long nerve tract beside the vertebral column. Thence the axons of these relaying nerve cells join, and are distributed with, other nerves of the peripheral system, to reach glands and blood vessels in all parts of the body (except the CNS itself). Others run to the eyes, to the heart and lungs, and to the abdominal and pelvic organs. Parasympathetic cell bodies are in the brain stem, with axons running in the tenth cranial (vagus) nerves, reaching glands around the mouth and throat, and extending down to the heart and lungs and to most of the abdominal organs. There is a second set of parasympathetic nerve cells in the lowest segments of the spinal cord, that send out fibres to the pelvic organs.
Thus the nervous system is responsible for rapid conduction of information throughout the body. Neurons are highly differentiated and, except in early fetal life, are generally incapable of division or mitosis to reproduce themselves. This means that if lost through disease or injury they cannot be replaced. On the other hand, axons regenerate readily in the PNS (as anyone who has cut a cutaneous nerve knows). One of the most important goals of neuroscience research in the years to come will be to understand why this is, and whether damaged neurons in the CNS can be persuaded to repair themselves.
See also autonomic nervous system; brain; central nervous system; nerves; neurotransmitter; synapse.
An electrochemical conducting network that transmits messages from the brain through the nerves to locations throughout the body.
The nervous system is responsible for the perception of external and internal conditions and the body's response to them. It has two major divisions: the central and peripheral nervous systems. The central nervous system (CNS), consisting of the brain and the spinal cord, is that part of the nervous system that is encased in bone; the brain is located in the cranial cavity of the skull, and the spinal cord in the spinal column, or backbone. Both are protected by cerebrospinal fluid and a series of three membranes called meninges. The CNS receives information from the skin and muscles and sends out motor commands as well.
The brain functions as the center of instinctive, emotional, and cognitive processes. It is composed of three primary divisions, the forebrain, midbrain, and hind-brain, and divided into the left and right hemispheres. The first division, the forebrain, is the largest and most complicated of the brain structures and is responsible for most types of complex mental activity and behavior. The forebrain consists of two main divisions: the diencephalon and the cerebrum. The thalamus and hypothalamus make up the diencephalon. The parts of the cerebrum—the larger part of the forebrain—include the corpus callosum, striatum, septum, hippocampus, and amygdala, all covered by the cerebral cortex.
The midbrain, or mesencephalon, is the small area near the lower middle of the brain. Portions of the mid-brain have been shown to control smooth and reflexive movements and it is important in the regulation of attention , sleep , and arousal. The hindbrain (rhombencephalon), which is basically a continuation of the spinal cord, is the part of the brain that receives incoming messages first. Lying beneath the cerebral hemispheres, it consists of three structures: the cerebellum, the medulla, and the pons, which control such vital functions of the autonomic nervous system as breathing, blood pressure, and heart rate.
The spinal cord is a long bundle of neural tissue continuous with the brain that occupies the interior canal of the spinal column and functions as the primary communication link between the brain and the body. It is the origin of 31 bilateral pairs of spinal nerves which radiate outward from the central nervous system through openings between adjacent vertebrae. The spinal cord receives signals from the peripheral senses and relays them to the brain.
The peripheral nervous system (PNS) includes all parts of the nervous system not covered by bone and carries out sensory and motor functions. It is composed of 12 pairs of cranial and 31 pairs of spinal nerves which lead to the left and right sides of the body. The PNS is divided into two subsystems: the somatic and autonomic nervous systems. The somatic nervous system senses and acts upon the external world. Its sensory neurons transmit signals from receptor cells located in sense organs, such as the skin and eye, to the CNS. Motor neurons carry outgoing messages from the CNS to neuromuscular cells (effectors) found in muscles, joints, glands, and organs, which facilitate action. The skeletal muscles, which are responsible for bodily movement, are controlled by the somatic nervous system.
The autonomic nervous system (ANS) relays messages between the CNS and the heart, lungs, and other glands and organs. These messages increase or decrease their activity in accordance with demands placed on the body. The ANS affects activities that are basically outside of conscious control, such as respiration and digestion. The autonomic nervous system is further subdivided into two branches. The sympathetic system speeds up muscles and mobilizes the body for action. This is the system responsible for the reaction to danger known as the "fight or flight" response. In contrast, the parasympathetic system, which slows down muscles, regulates bodily functions to conserve energy. For example, it is this system that slows heart rate and blood flow after a large meal is eaten to conserve energy for digestion. Disorders of the autonomic nervous system involve reactions such as fainting, uncontrollable sweating, and sexual dysfunction .
The nervous system is composed of two types of cells: neurons, which transmit information through electrochemical impulses, and glial cells, which hold the neurons together and help them communicate with each other. There are three kinds of neurons. Receptor neurons register stimulation from the environment (such as cells in the eye responding to light or skin cells responding to pressure). When they are stimulated, they send signals to the brain, which are then converted into various types of information. Motor, or effector neurons transmit messages from the brain and spinal cord that provide for muscular contraction, which results in movement. Finally, interneurons transmit signals between different parts of the nervous system. Most neurons are composed of five parts: the cell body, which contains the nucleus; dendrites, short fibers that usually receive signals from other neurons; the axon, a long fiber leading away from the cell body that transmits signals to other neurons, muscles, or glands; the myelin sheath, a fatty substance that insulates the axon; and synapses , minute gaps through which signals are transmitted between neurons. The many axon and dendrite fibers radiating from neurons permit each one to be in contact with many thousands of other neurons.
Communication at the synapses between neurons relies on chemicals called neurotransmitters. More than 50 different neurotransmitters have been identified, and more are constantly being discovered. Recently, it was found that the gases nitric oxide and carbon monoxide are neurotransmitters. Different transmitters predominate in different parts of the nervous system, and a particular neurotransmitter may perform different functions in different locations. Researchers have proposed that almost all drugs work through interaction with neurotransmitters. Important neurotransmitters include acetylcholine (ACh), which is used by motor neurons in the spinal cord; the catecholamines (including norepinephrine and dopamine), which are important in the arousal of the sympathetic nervous system; serotonin, which affects body temperature, sensory perception, and the onset of sleep; and a group of transmitters called endorphins, which are involved in the relief of pain .
Among the major functions of the central nervous system is that of the reflex arc, which provides immediate, involuntary reaction to potentially harmful stimulireactions commonly referred to as reflexes (such as drawing one's hand back from a hot stove). The reflex arc is a circuit of neurons by which signals travel from a sensory receptor to a motor neuron , rapidly turning sensory input into action. The complexity of the nervous system makes it a challenge to study—millions of neurons may lie beneath a single square centimeter of brain surface, each synapsing with as many as 600 other neurons, and many different parts of the brain may be involved in a single task.
The Mind and Beyond. Alexandria, VA: Time-Life Books, 1991.
The sophistication and the incredible dimensions of the human nervous system are the basis for the infinite range and subtlety of human movement. The nervous system, centered by the brain, generates every impulse that is directed into the musculoskeletal system for the stimulation of both muscular movement and reaction. The brain is the organ that operates the body; the human mind is the more intangible concept, connected to the physical organ and the nervous system, but extending into the aspects of intelligence, reasoning abilities, and human perception.
The brain is the most far-reaching organ in the body, with its influence and its control over every aspect of human function extended by way of the network that is the nervous system. Physical abilities that are at the essence of athletic ability, including muscular control, hand-eye coordination, reaction time, and the utilization of the body's composition of fast-twitch versus slow-twitch fibers, are all determined by the brain.
As the chief component of the nervous system generally, the brain is positioned at the top of the first branch of the nervous system, familiar as the central nervous system (CNS). The CNS has two parts, the brain and the spinal cord, that extend from the region of the brain known as the brain stem, located at the base of the skull. The spinal cord runs through the spinal column, a bony protective structure, to the base of the spine at the pelvis. The spinal cord is primarily composed of nerve cells, known as neurons.
The brain is divided into a series of regions, each of which has a distinct responsibility for a function of the body. The cerebellum, located near the base of the brain, is the learning center of the brain. The hypothalamus, connected to the pituitary gland, regulates body temperature and other functions that are a response to external stimuli. Whenever any of these control centers seek to transmit a message to another part of the body for action, the message begins it travel along the spinal cord.
The central nervous system is connected to a far more extensive nerve network. This structure is the peripheral nervous system. Unlike the central nervous system components, which are protected by the bone of the skull and the spine, the elements of the peripheral nervous system are not protected, extending through the tissue in pathways. The peripheral nervous system is a highly complex series of nerves and neurons that extend to every part of the body. The peripheral nervous system is itself subdivided into two major operational systems: the somatic (or voluntary) nervous system and the autonomic nervous system. The somatic system directs movement and the control of the skeletal muscles. The nerves that extend into the muscles ultimately terminate in a motor neuron, the device that transmits the particular instruction to the adjacent muscle fibers. The speed with which the particular neuron is designed to direct its impulses into the muscle fiber will dictate whether the fiber is a fast-twitch or slow-twitch fiber.
The other branch of the peripheral nervous system, the autonomic system, has three further subdivisions. As the name implies, the autonomic system is responsible for the regulation of a number of bodily functions that are either involuntary, or where the body generates an initial response that may be the subject of further voluntary action. The sympathetic nervous system includes the management of the body's "fight or flight" response, triggered when the brain, after receiving stimulation of a threat or other challenge, directs the production of adrenaline, the hormone that stimulates heart rate, respiratory function and the expansion of blood vessel capacity.
The counterpoint to the sympathetic nervous system is the parasympathetic nervous system, whose function is often summarized as the "rest and digest" response. After stimulation, the parasympathetic system acts to calm the body, through stimulation of the salivary gland to encourage eating and slowing the heart rate.
The third aspect of the autonomic system is sometime regarded as a separate nervous system, due to the nature of the organs that it controls. The enteric nervous system regulates the stomach and colon.
The nervous system functions as an entity, with the brain providing the ultimate direction. Although many nervous system functions are involuntary, damage to the central nervous system in particular can dramatically alter nervous system function as a whole. Much of the response of the nervous system to injury or impairment is one directional, for if the body senses damage to a lesser aspect of one of the subordinate components to the nervous system, it will endeavor to compensate; brain or spinal cord injuries do not permit alternate paths or compensatory routes.