NERVOUS SYSTEM

in anatomy, an organized group of cells specialized for the conduction of a stimulus impulse from a sensory receptor through a nerve cell network to the site at which the response occurs. The nervous system enables a multicellular animal to respond to changes in its external and internal environment. Nerve tissue is composed primarily of cells called neurons, which are specialized to receive stimuli and conduct electrochemical impulses. There are three general types of neurons: sensory neurons, which relay information from the senses; motor neurons, which carry impulses to effectors; and interneurons, which transmit impulses between sensory and motor neurons. A typical vertebrate neuron consists of dendrites, a cell body (also called a soma), and an axon. The dendrites are fibres that receive stimulation from a sense receptor or nerve impulses from another neuron. The dendrites isolate the received stimulusin the form of a change in electrical potentialand in most cases conduct it toward the cell body and axon. The axon is the fibre that, upon stimulation to a certain threshold, generates a nerve impulse, transmitting it away from the cell body, to another neuron or to an effector muscle or gland. A synapse is the junction between two neurons. When a nerve impulse arrives at the terminals of an axon, a neurotransmitter chemical (e.g., acetylcholine or norepinephrine) is released in small spheres called synaptic vesicles. The neurotransmitter diffuses across the minute space, known as the synaptic cleft, and binds to receptor molecules on the receiving sites of the cell. Depending upon the amount of neurotransmitter and the type of receptor, a new nerve impulse is either excited or inhibited. The neurotransmitter is then destroyed by enzymes or taken up by the axon terminals, thereby limiting the duration of the response. In some cases, the transmission of nerve impulses is electrical; i.e., the arriving impulse passes directly from the presynaptic to the postsynaptic sites of the cell through open channels called gap junctions. Nervous systems are of two general types: diffuse and centralized. The diffuse nervous system is more primitive; it is found only in lower invertebrates, particularly among the coelenterates (e.g., jellyfishes and hydras), which are radially symmetrical. In a diffuse-type system, there is no brain and the nerve cells are distributed throughout the organism in a netlike pattern. Most other animals that have a nervous system show some degree of centralization, meaning that some portion of the system has a dominant role in coordinating information and directing responses. Even the radially symmetrical echinoderms (starfishes and their allies) have a dominant central nerve ring, from which extend radial nerves. Echinoderms, however, have no brain. A central nervous system composed of a brain and one or more major nerve cords seems to have been an evolutionary outgrowth of bilateral symmetry, the body plan that characterizes most higher invertebrates and all vertebrates. Certain of the flatworms are the most primitive animals to show such a central nervous system, though their brain is little more than a slight swelling of the nerve cords in the head region of the animal. A more complex central nervous system is found among the annelids (e.g., earthworms and leeches) and the arthropods (e.g., insects and crustaceans), which have a definite brain and ventral nerve cords. Centralization reaches its apogee in the vertebrates, which have a well-developed brain and a dorsal nerve cord (the spinal cord). The human nervous system consists of two main parts: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves that carry impulses to and from the central nervous system). It is distinguished from those of other higher vertebrates by the size and degree of specialization of the cerebral cortex, the part of the brain that analyzes the input of the senses, controls most voluntary muscular movement, and is the seat of reasoning, memory, and learning. The brainstem, which connects the brain to the spinal cord, plays a special role in controlling reflexes, conducting impulses to the viscera (internal organs), regulating the internal environment of the body, and maintaining an ideal state of activity within the nervous system itself. The spinal cord is a relay centre for impulses coming from and going to the body from the region of the neck down. A sensory neuron may synapse directly with a motor neuron in the spinal cord, producing a spinal reflex such as the familiar knee-jerk response. More commonly, however, the sensory neuron will synapse with interneurons in the spinal cord. If appropriate, one or more of these interneurons will excite motor neurons, initiating a reflex response. Other interneurons may carry the sensory information to the brain, which may direct further responses. These responses are transmitted through still other interneurons in the spinal cord to the appropriate motor neurons. The peripheral nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. All but one of the cranial nerves are concerned with the sensory or motor activities of the head and neck. (The one exception is the vagus nerve, which innervates a number of visceral organs.) Each spinal nerve is attached to the spinal cord by an afferent (sensory) root and an efferent (motor) root. These two roots merge just outside the cord to form a large mixed nerve (i.e., one that contains both sensory and motor neurons), which eventually divides into many branches that innervate a particular region of the body. The motor neurons of the peripheral nervous system compose two distinct subsystems: the somatic system, which innervates skeletal muscles, and the autonomic system, which controls the heart, the glands, and the smooth muscles that make up the walls of the blood vessels and the respiratory, digestive, excretory, and reproductive tracts. The actions of the somatic system are, by and large, subject to conscious control. By contrast, most actions of the autonomic system are involuntary responses. The pathways of nerve fibres in the autonomic system are classified as either sympathetic or parasympathetic. Most organs are supplied by both types, each controlling a different, usually antagonistic, response. The sympathetic system generally responds to stress situations, preparing the body for action. For example, the sympathetic system triggers an increase in blood pressure, blood sugar, and sweat; dilation of the pupils; and a diversion of blood flow to the muscles. The parasympathetic system tends to initiate and maintain routine functions of the viscerae.g., digestion or excretion. Nearly every mechanism of the nervous system is susceptible to damage or disease, the results of which range in severity from transient tics and minor personality changes to paralysis, major disruptions of the personality, and death. Infections, tumours, trauma, and congenital defects involving the spinal cord can result in paralysis. Diseases that attack motor neurons, such as Parkinson's disease and chorea, can cause such symptoms as uncontrollable shaking, difficulty in speaking, and stooped posture. For malformations of the nervous system, see neural tube defect. in anatomy, an organized group of cells specialized for the conduction of an electrochemical stimulus from a sensory receptor through a network to the site at which the response occurs. All living organisms are able to detect changes within themselves and in their environments. Changes in the external environment include those of light, temperature, sound, motion, and odour, while changes in the internal environment include those in the position of the head and limbs as well as in the internal organs. Once detected, these internal and external changes must be analyzed and acted upon in order to preserve the integrity, well-being, and status quo of the organism. As life on Earth evolved and the environment became more complex, the survival of organisms depended upon how well they could respond to changes in their surroundings. One factor necessary for survival was a speedy reaction or response. Since communication from one cell to another by chemical means was too slow to be adequate for survival, a system evolved that allowed for faster reaction. That system was the nervous system, which is based upon the almost instantaneous transmission of electrical impulses from one region of the body to another along specialized nerve cells. Nervous systems are of two general types, diffuse and centralized. In the diffuse type, found in lower invertebrate animals, there is no brain, and the nerve cells are distributed throughout the organism in a netlike pattern. In the centralized systems of the higher invertebrates and the vertebrate animals, some portion of the nervous system has a dominant role in coordinating information and directing responses. This centralization reaches its apogee in the vertebrates, which have a well-developed brain and spinal cord. Impulses are carried to and from the brain and spinal cord by nerve fibres that make up the peripheral nervous system. This article begins with a discussion of the general features of nervous systemsthat is, their function of responding to stimuli and the rather uniform electrochemical processes by which they carry out their response. Following is a discussion of the various types of nervous systems, from the simplest to the most complex. For detailed discussion of the biochemical and physiological processes of which the nervous system forms a part, see cell; regeneration; animal development; human development; and human aging. For description of receptors that initiate nerve impulses, see eye and ear. The organ systems that are supplied by nerves are described in muscle. Solomon D. Erulkar Additional reading General physiological features of the nervous system are presented in John P. Schad and Donald H. Ford, Basic Neurology: An Introduction to the Structure and Function of the Nervous System, 2nd rev. ed. (1973); Ernest Gardner, Fundamentals of Neurology: A Psychophysiological Approach, 6th ed. (1975); Abel Lajtha (ed.), Handbook of Neurochemistry, 2nd ed., 10 vol. (198285); David Ottoson, Physiology of the Nervous System (1983); Stephen W. Kuffler, John G. Nichols, and A. Robert Martin, From Neuron to Brain: A Cellular Approach to the Function of the Nervous System, 2nd ed. (1984); Melvin J. Swenson (ed.), Dukes' Physiology of Domestic Animals, 10th ed. (1984); Eric R. Kandel, James H. Schwartz and Thomas M. Jessell (eds.), Principles of Neural Science, 3rd ed. (1991); The Nervous System: Circuits of Communication (1985), part of the series The Human Body; Arthur C. Guyton, Textbook of Medical Physiology, 8th ed. (1991); and Gordon M. Shepherd, Neurobiology, 2nd ed. (1988). Handbook of Physiology, rev. ed., sect. 1, The Nervous System, 5 vol. in 9 (197787), examines the cellular biology of neurons, motor control, sensory processes, and regulatory systems and higher functions of the brain. See also D.M. Guthrie, Neuroethology: An Introduction (1980), a psychology of animal behaviour, including that of humans. Studies of the nerve cells include Alan Peters, Sanford L. Palay, and Henry DeF. Webster, The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed. (1991), a description of nerve cells based on numerous electron micrographs; and H. Hydn, The Neuron, ch. 5 in Jean Brachet and Alfred E. Mirsky (eds.), The Cell: Biochemistry, Physiology, Morphology, vol. 4, part 1, Specialized Cells (1960), pp. 215323. The effects of drugs on the central nervous system are detailed in Jack R. Cooper, Floyd E. Bloom, and Robert H. Roth, The Biochemical Basis of Neuropharmacology, 6th ed. (1991).The transmission of information in the nervous system via electrical signals is analyzed in three works of historical interest: Keith Lucas, The Conduction of the Nervous Impulse, rev. by E.D. Adrian (1917); E.D. Adrian, The Mechanism of Nervous Action: Electrical Studies of the Neurone (1932, reprinted 1959); and Joseph Erlanger and Herbert S. Gasser, Electrical Signs of Nervous Activity (1937, reprinted 1968), including bibliographies of the works of both authors. More recent works include Ichiji Tasaki, Nervous Transmission (1953), and Conduction of the Nerve Impulse, ch. 3 in Handbook of Physiology, sect. 1, Neurophysiology, vol. 1. (1959); John C. Eccles, The Physiology of Nerve Cells (1957, reprinted 1968), written by a co-recipient of the 1963 Nobel Prize for Physiology or Medicine; A.L. Hodgkin, The Conduction of the Nervous Impulse (1964, reissued 1971), a brief but lucid exposition; Mary A.B. Brazier, Electrical Activity of the Nervous System, 4th ed. (1977), a college-level survey; and Bertil Hille and William A. Catterall, Electrical Excitability and Ionic Channels, in George J. Siegel (ed.), Basic Neurochemistry, 4th ed. (1989), pp. 7190. Other methods of transmission are described by A.V. Hill, Chemical Wave Transmission in Nerve (1932); John C. Eccles, The Physiology of Synapses (1964); M. Gabe, Neurosecretion, trans. from French (1966), an account of neurosecretion in invertebrates and vertebrates; and Bernard Katz, The Release of Neural Transmitter Substances (1969).The evolution and development of the nervous system is explored by C.U. Arins Kappers, G. Carl Huber, and Elizabeth C. Crosby, The Comparative Anatomy of the Nervous System of Vertebrates, Including Man, 2 vol. (1936, reprinted in 3 vol., 1967); Theodore Holmes Bullock and G. Adrian Horridge, Structure and Function in the Nervous Systems of Invertebrates, 2 vol. (1965), a monumental, well-illustrated work containing comprehensive bibliographies that cover the available knowledge of the nervous systems of all the invertebrate groups; Hartwig Kuhlenbeck, The Central Nervous System of Vertebrates: A General Survey of Its Comparative Anatomy to the Pertinent Fundamental Biologic and Logical Concepts, 5 vol. in 7 (196778), including basic concepts and a general survey of the nervous systems of both invertebrates and vertebrates; Thomas L. Lentz, Primitive Nervous Systems (1968), an account of the neural organization of sponges, hydras, and planarias, including a hypothesis on the evolutionary origin of the nervous system; L.E. Bayliss, Principles of General Physiology, 5th ed., vol. 2, pp. 492533 (1960), a clear description, particularly concerned with the properties of the neuron, spinal reflexes, nerve excitation and inhibition, and reciprocal innervation; and Harvey B. Sarnat and Martin G. Netsky, Evolution of the Nervous System, 2nd ed. (1981), a comparative study of nervous systems. Thomas L. Lentz Charles R. Noback Solomon D. Erulkar