nervous system network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system. Invertebrate animals show varying degrees of complexity in their nervous systems, but it is in the vertebrate animals (phylum Chordata , subphylum Vertebrata) that the system reaches its greatest complexity.

Anatomy and Function

In vertebrates the system has two main divisions, the central and the peripheral nervous systems. The central nervous system consists of the brain and spinal cord . Linked to these are the cranial, spinal, and autonomic nerves, which, with their branches, constitute the peripheral nervous system. The brain might be compared to a computer and its memory banks, the spinal cord to the conducting cable for the computer's input and output, and the nerves to a circuit supplying input information to the cable and transmitting the output to muscles and organs.

The nervous system is built up of nerve cells, called neurons, which are supported and protected by other cells. Of the 200 billion or so neurons making up the human nervous system, approximately half are found in the brain. From the cell body of a typical neuron extend one or more outgrowths (dendrites), threadlike structures that divide and subdivide into ever smaller branches. Another, usually longer structure called the axon also stretches from the cell body. It sometimes branches along its length but always branches at its microscopic tip. When the cell body of a neuron is chemically stimulated, it generates an impulse that passes from the axon of one neuron to the dendrite of another; the junction between axon and dendrite is called a synapse . Such impulses carry information throughout the nervous system. Electrical impulses may pass directly from axon to axon, from axon to dendrite, or from dendrite to dendrite.

So-called white matter in the central nervous system consists primarily of axons coated with light-colored myelin produced by certain neuroglial cells. Nerve cell bodies that are not coated with white matter are known as gray matter. Nonmyelinated axons that are outside the central nervous system are enclosed only in a tubelike neurilemma sheath composed of Schwann cells, which are necessary for nerve regeneration. There are regular intervals along peripheral axons where the myelin sheath is interrupted. These areas, called nodes of Ranvier, are the points between which nerve impulses, in myelinated fibers, jump, rather than pass, continuously along the fiber (as is the case in unmyelinated fibers). Transmission of impulses is faster in myelinated nerves, varying from about 3 to 300 ft (1-91 m) per sec.

Both myelinated and unmyelinated dendrites and axons are termed nerve fibers; a nerve is a bundle of nerve fibers; a cluster of nerve cell bodies (neurons) on a peripheral nerve is called a ganglion. Neurons are located either in the brain, in the spinal cord, or in peripheral ganglia. Grouped and interconnected ganglia form a plexus, or nerve center. Sensory (afferent) nerve fibers deliver impulses from receptor terminals in the skin and organs to the central nervous system via the peripheral nervous system. Motor (efferent) fibers carry impulses from the central nervous system to effector terminals in muscles and glands via the peripheral system.

The peripheral system has 12 pairs of cranial nerves: olfactory, optic, oculomotor, trochlear, trigeminal, abducent, facial, vestibulo-cochlear (formerly known as acoustic), glossopharyngeal, vagus, spinal accessory, and hypoglossal. These have their origin in the brain and primarily control the activities of structures in the head and neck. The spinal nerves arise in the spinal cord, 31 pairs radiating to either side of the body: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Autonomic Nervous System

The autonomic nerve fibers form a subsidiary system that regulates the iris of the eye and the smooth-muscle action of the heart, blood vessels, glands, lungs, stomach, colon, bladder, and other visceral organs not subject to willful control. Although the autonomic nervous system's impulses originate in the central nervous system, it performs the most basic human functions more or less automatically, without conscious intervention of higher brain centers. Because it is linked to those centers, however, the autonomic system is influenced by the emotions; for example, anger can increase the rate of heartbeat. All of the fibers of the autonomic nervous system are motor channels, and their impulses arise from the nerve tissue itself, so that the organs they innervate perform more or less involuntarily and do not require stimulation to function.

Autonomic nerve fibers exit from the central nervous system as part of other peripheral nerves but branch from them to form two more subsystems: the sympathetic and parasympathetic nervous systems, the actions of which usually oppose each other. For example, sympathetic nerves cause arteries to contract while parasympathetic nerves cause them to dilate. Sympathetic impulses are conducted to the organs by two or more neurons. The cell body of the first lies within the central nervous system and that of the second in an external ganglion. Eighteen pairs of such ganglia interconnect by nerve fibers to form a double chain just outside the spine and running parallel to it. Parasympathetic impulses are also relayed by at least two neurons, but the cell body of the second generally lies near or within the target organ.

The Nervous System and Reflexes

In general, nerve function is dependent on both sensory and motor fibers, sensory stimulation evoking motor response. Even the autonomic system is activated by sensory impulses from receptors in the organ or muscle. Where especially sensitive areas or powerful stimuli are concerned, it is not always necessary for a sensory impulse to reach the brain in order to trigger motor response. A sensory neuron may link directly to a motor neuron at a synapse in the spinal cord, forming a reflex arc that performs automatically. Thus, tapping the tendon below the kneecap causes the leg to jerk involuntarily because the impulse provoked by the tap, after traveling to the spinal cord, travels directly back to the leg muscle. Such a response is called an involuntary reflex action.

Commonly, the reflex arc includes one or more connector neurons that exert a modulating effect, allowing varying degrees of response, e.g., according to whether the stimulation is strong, weak, or prolonged. Reflex arcs are often linked with other arcs by nerve fibers in the spinal cord. Consequently, a number of reflex muscle responses may be triggered simultaneously, as when a person shudders and jerks away from the touch of an insect. Links between the reflex arcs and higher centers enable the brain to identify a sensory stimulus, such as pain; to note the reflex response, such as withdrawal; and to inhibit that response, as when the arm is held steady against the prick of a hypodermic needle.

Reflex patterns are inherited rather than learned, having evolved as involuntary survival mechanisms. But voluntary actions initiated in the brain may become reflex actions through continued association of a particular stimulus with a certain result. In such cases, an alteration of impulse routes occurs that permits responses without mediation by higher nerve centers. Such responses are called conditioned reflexes, the most famous example being one of the experiments Ivan Pavlov performed with dogs. After the dogs had learned to associate the provision of food with the sound of a bell, they salivated at the sound of the bell even when food was not offered. Habit formation and much of learning are dependent on conditioned reflexes. To illustrate, the brain of a student typist must coordinate sensory impulses from both the eyes and the muscles in order to direct the fingers to particular keys. After enough repetition the fingers automatically find and strike the proper keys even if the eyes are closed. The student has "learned" to type; that is, typing has become a conditioned reflex.

Disorders of the Nervous System

A number of diseases can significantly affect the proper functioning of the nervous system. Parkinson's disease, Huntington's disease , myasthenia gravis , and amyotrophic lateral sclerosis (commonly known as Lou Gehrig's disease) are some of the more severe diseases affecting the nervous system. Strokes, which are related to circulatory disorders, also may have permanent effects on the nervous system. Certain plant derivatives, such as belladonna , cocaine , and caffeine , have a variety of stimulatory, inhibitory, and hallucinatory effects on the nervous system.

Bibliography

See D. Ottoson, Physiology of the Nervous System (1982); G. Chapouthier and J. J. Matras, The Nervous System and How It Functions (1986); L. S. Kee, Introduction to the Human Nervous System (1987); P. Nathan, The Nervous System (3d ed. 1988); J. G. Panavelas et al., ed. The Making of the Nervous System (1988).

Nervous System

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 Brain

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

Bibliography

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.

nervous system Communications system consisting of interconnecting nerve cells or neurons that coordinate all life, growth, and physical and mental activity. The mammalian nervous system consists of the central nervous system (CNS) and the peripheral nervous system.