Nervous System
Nervous System
Evolution of invertebrate nervous systems
Evolution of the vertebrate nervous system
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 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.
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 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.
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 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.
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 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.
Neuron
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
KEY TERMS
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.
Nerve impulse
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.
See also Neuron; Neurosurgery; Neurotransmitter.
Resources
BOOKS
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.
OTHER
Chudler, Eric H., University of Washington. “Neuroscience for Kids.” <http://faculty.washington.edu/chudler/introb.html> (accessed October 19, 2006).
Mary Finley