nervous system

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Related to Nervousystem: autonomic nervous system, central nervous system, peripheral nervous system, Nervous System Disorders, Human Nervous System, Nervous system diseases

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 vertebratevertebrate,
any animal having a backbone or spinal column. Verbrates can be traced back to the Silurian period. In the adults of nearly all forms the backbone consists of a series of vertebrae. All vertebrates belong to the subphylum Vertebrata of the phylum Chordata.
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 animals (phylum ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
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, 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 brainbrain,
the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function
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 and spinal cordspinal cord,
the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column.
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. 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 synapsesynapse
, junction between various signal-transmitter cells, either between two neurons or between a neuron and a muscle or gland. A nerve impulse reaches the synapse through the axon, or transmitting end, of a nerve cell, or neuron.
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. 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 PavlovPavlov, Ivan Petrovich
, 1849–1936, Russian physiologist and experimental psychologist. He was professor at the military medical academy and director of the physiology department at the Institute for Experimental Medicine, St. Petersburg, from 1890.
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 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 diseaseHuntington's disease,
hereditary, acute disturbance of the central nervous system usually beginning in middle age and characterized by involuntary muscular movements and progressive intellectual deterioration; formerly called Huntington's chorea.
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, myasthenia gravismyasthenia gravis
, chronic disorder of the muscles characterized by weakness and a tendency to tire easily. It is caused by an autoimmune attack on the acetylcholine receptor of the post synaptic neuromuscular junction.
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, and amyotrophic lateral sclerosisamyotrophic lateral sclerosis
(ALS) or motor neuron disease,
sometimes called Lou Gehrig's disease, degenerative disease that affects motor neurons in the brain and spinal cord, preventing them from sending impulses to the muscles.
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 (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 belladonnabelladonna
or deadly nightshade,
poisonous perennial plant, Atropa belladona, of the nightshade family. Native to Europe and now grown in the United States, the plant has reddish, bell-shaped flowers and shiny black berries.
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, cocainecocaine
, alkaloid drug derived from the leaves of the coca shrub. A commonly abused illegal drug, cocaine has limited medical uses, most often in surgical applications that take advantage of the fact that, in addition to its anesthetic effect, it constricts small arteries,
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, and caffeinecaffeine
, odorless, slightly bitter alkaloid found in coffee, tea, kola nuts (see cola), ilex plants (the source of the Latin American drink maté), and, in small amounts, in cocoa (see cacao). It can also be prepared synthetically from uric acid.
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, 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 (invertebrate)

All multicellular organisms have a nervous system, which may be defined as assemblages of cells specialized by their shape and function to act as the major coordinating organ of the body. Nervous tissue underlies the ability to sense the environment, to move and react to stimuli, and to generate and control all behavior of the organism. Compared to vertebrate nervous systems, invertebrate systems are somewhat simpler and can be more easily analyzed. Invertebrate nerve cells tend to be much larger and fewer in number than those of vertebrates. They are also easily accessible and less complexly organized; and they are hardy and amenable to revealing experimental manipulations. However, the rules governing the structure, chemistry, organization, and function of nervous tissue have been strongly conserved phylogenetically. Therefore, although humans and the higher vertebrates have unique behavioral and intellectual capabilities, the underlying physical-chemical principles of nerve cell activity and the strategies for organizing higher nervous systems are already present in the lower forms. Thus neuroscientists have taken advantage of the simpler nervous systems of invertebrates to acquire further understanding of those processes by which all brains function. See Nervous system (vertebrate)

Invertebrate and vertebrate nerve cells differ more in quantity, or degree, than in qualitative features. Aside from differences in size and numbers, the most striking difference is that invertebrate neurons have a unipolar shape, whereas most vertebrate neurons are multipolar. An additional general contrast between invertebrate and vertebrate nervous systems is that invertebrates tend to have more neurons displaced to the periphery (outside the central nervous system) and to perform more integrative and processing functions in the periphery. Vertebrates perform almost all their integration within the central nervous system, using interneurons. Invertebrate nervous systems also seem to have a greater potential for regrowth, regeneration, or repair after damage than do vertebrate nerve cells. Many invertebrates continue to add new nerve cells to their ganglia with age; vertebrates, in general, do not. Only vertebrate neurons have myelin sheaths, a specialized wrapping of glial membrane around axons, increasing their conduction speed. Invertebrates tend to enhance conduction velocity by using giant axons, particularly for certain escape responses.


Nervous system (vertebrate)

A coordinating and integrating system which functions in the adaptation of an organism to its environment. An environmental stimulus causes a response in an organism when specialized structures, receptors, are excited. Excitations are conducted by nerves to effectors which act to adapt the organism to the changed conditions of the environment.

Comparative morphology

The brain of all vertebrates, including humans, consists of three basic divisions: prosencephalon, mesencephalon, and rhombencephalon (Fig. 1). The individual divisions or patterns of the brain do not function separately to bring about a final response; rather, each pattern acts on a common set of connections in the spinal cord.

Lateral views of several vertebrate brains showing evolutionary relationshipsenlarge picture
Lateral views of several vertebrate brains showing evolutionary relationships

Spinal patterns are the final common patterns used by all higher brain pathways to influence all organs of the body. These reflexes are divided into two basic patterns: the monosynaptic arc and the multisynaptic arc. The monosynaptic arc, or myotatic reflex, maintains tonus and posture in vertebrates and consists of two neurons, a sensory and a motor neuron.

The multisynaptic arc, or flexor reflex, is the pattern by which an animal withdraws a part of its body from a noxious stimulus. Both sensory neurons and internuncial neurons send information to brain centers. Coordinated limb movement is based on a connective pattern of neurons at the spinal level.

The structure of the spinal cord and its connections are basically similar among all vertebrates. The major evolutionary changes in the spinal cord have been the increased segregation of cells and fibers of a common function from cells and fibers of other functions and the increase in the length of fibers which connect brain centers with spinal centers. See Postural equilibrium

The rhombencephalon of the brain is subdivided into a roof, or cerebellum, and a floor, or medulla oblongata. The medulla is similar to the spinal cord and is divided into a dorsal sensory region and a ventral motor region. It is an integrating and relay area between higher brain centers and the spinal cord. In addition to these nuclei and their connections, the medulla consists of both ascending and descending pathways to and from higher brain centers. The same basic connections occur throughout vertebrates.

In mammals, the cerebellum does not initiate movement; it only times the length of muscle contractions and orders the sequence in which muscles should contract to bring about a movement. The command to initiate a movement is received from the cerebral cortex (Fig. 2). Similarly, the cerebral cortex receives information regarding limb position and state of muscular contraction to ensure that its commands can be carried out by the cerebellum.

Mammalian brain in sagittal sectionenlarge picture
Mammalian brain in sagittal section

The mesencephalon is divided into a roof or optic tectum and a floor or tegmentum. The tegmentum contains the nuclei of the oculomotor and trochlear cranial nerves and a rostral continuation of the sensory nucleus of the trigeminal cranial nerve.

In the evolution of vertebrates, the prosencephalon develops as two major divisions, the diencephalon and the telencephalon. The diencephalon retains the tubular form and serves as a relay and integrating center for information passing to and from the telencephalon and lower centers. The telencephalon is divided into a pair of cerebral hemispheres and an unpaired tel­en­cephalon medium.

There are three divisions of the diencephalon in all vertebrates: an epithalamus which forms the roof of the neural tube, a thalamus which forms the walls of the neural tube, and a hypothalamus which forms the floor of the neural tube. The epithalamus and hypothalamus are primarily concerned with autonomic functions such as homeostasis. The thalamus is subdivided into dorsal and ventral regions. The dorsal region relays and integrates sensory information, and the ventral thalamus relays and integrates motor information. See Homeostasis, Instinctive behavior

The telencephalon is the most complex brain division in vertebrates. It is divided into a roof, or pallium, and a floor, or basal region. The pallium is divided into three primary divisions: a medial PI or hippocampal division, a dorsal PII or general pallial division, and a lateral PII division, often called the pyriform pallium.

The most striking change in the telencephalon of land vertebrates involves the PIIIa component. In mammals, it has proliferated with the PIIb component of the dorsal pallium to produce the mammalian neocortex. In all land vertebrates except amphibians, the PIIb and the PIIIa components, along with the corpus striatum (BI and BII), are the highest centers for the analysis of sensory information and motor coordination. The PI, PIIa, PIIIb, BIII, and posterior parts of BI and BII form part of the limbic system which is concerned with behavioral regulation.

Comparative histology

The nervous system is composed of several basic cell types, including nerve cells called neurons, interstitial cells called neurolemma (cells of Schwann), satellite cells, oligodendroglia, and astroglia; and several connective-tissue cell types, including fibroblasts and microglia, blood vessels, and extracellular fluids.

Each neuron possesses three fundamental properties, involving specialized capacity to react to stimuli, to transmit the resulting excitation rapidly to other portions of the cell, and to influence other neurons, muscle, or glandular cells. Each neuron consists of a cell body (soma), one to several cytoplasmic processes called dendrites, and one process called an axon. Cell bodies vary from about 7 to more than 70 micrometers in diameter; each contains a nucleus and several cytoplasmic structures, including Nissl (chromophil) granules, mitochondria, and neurofibrils. The cell body is continuously synthesizing new cytoplasm, especially protein, which flows down the cell processes. The dendrites range from a fraction of a millimeter to a few millimeters in length. An axon may range from about a millimeter up to many feet in length. The site where two neurons come into contact with each other and where influences of one neuron are transmitted to the other neuron is called a synapse. Neurotransmitters are secreted across the presynaptic membrane into the synaptic cleft where they may excite (excitatory synapse) or inhibit (inhibitory synapse) the postsynaptic membrane. See Biopotentials and ionic currents, Sensation, Synaptic transmission

There are three layers of connective tissue membranes, the meninges, covering the brain and spinal cord: the inner, pia mater; the middle layer, the arachnoid; and the outermost, the dura mater. Between the pia mater and the arachnoid is the subarachnoid space; this space and the ventricular cavities within the brain are filled with an extracellular fluid, the cerebrospinal fluid.

Comparative embryology

The anlage of the nervous system is formed in the outer germ layer, the ectoderm, although some later contributions are also obtained from the middle germ layer, the mesoderm. In most vertebrates a neural plate is formed, which later folds into a neural groove, then closes to form a neural tube. The formation of neural tissue within the ectoderm is due to inductive influences from underlying chordomesodermal structures. See Developmental biology, Embryonic induction, Neural crest

When the neural tube is developing, a segmentation of the central nervous system occurs by the formation of transverse bulges, neuromeres. At the time of neuromeric segmentation, the brain is subdivided into the so-called brain vesicles by local widenings of its lumen. In the rostral end more or less well-developed hemispheres are formed; in the middle of the brain anlage the mesencephalic bulge develops; and behind the latter the walls of the tube thicken into cerebellar folds. In this way the brain anlage is divided into five sections: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon, and its cavity is divided into the rudiments of the adult ventricles.

In spite of the extraordinary variation in adult morphology of the vertebrate brain in different species, the early phases of development are essentially similar. The spinal cord remains as a comparatively slightly differentiated tube.

The cranial or cerebral nerves are the peripheral nerves of the head that are related to the brain. Twelve pairs of cranial nerves have been distinguished in human anatomy and these nerves have been numbered rostrally to caudally as follows:

  •  I. Olfactory nerve, fila olfactoria
  •  II. Optic nerve, fasciculus opticus
  •  III. Oculomotor nerve
  •  IV. Trochlear nerve
  •  V. Trigeminal nerve, in most vertebrates divided into three branches: ophthalmic, maxillary, and mandibular
  •  VI. Abducens nerve
  •  VII. Facial nerve
  •  VIII. Statoacoustic nerve
  •  IX. Glossopharyngeal nerve
  •  X. Vagus nerve
  •  XI. Accessory nerve
  •  XII. Hypoglossal nerve

The spinal ganglia are formed from the neural crest which grows out like a continuous sheet from the dorsal margin of the neural tube and is secondarily split up into cell groups, the ganglia, by a segmentating influence from the somites. Fibers grow out from the ganglionic cells and form the sensory fibers of the spinal nerves. Motor nerve fibers emerge from cells situated in the ventral horns of the spinal cord. The ventral motor fibers and the dorsal sensory fibers fuse to form a common stem, which is again laterally divided into branches, innervating the corresponding segment of the body.

The ganglia of the sympathetic nervous system develop ventrolateral to the spinal cord as neural crest derivatives. At first a continual column of sympathetic nerve cells is formed; it later subdivides into segmental ganglia.

The parasympathetic system is made up of preganglionic fibers emanating as general visceromotor fibers from the brain and from the sacral cord segments. Cells migrate to form the peripheral ganglia along them. See Autonomic nervous system

Nervous System

 

the complex aggregate of structures in animals and man that coordinate the activity of all organs and systems of the body and enable the body to function as a unit during its constant interaction with the environment. The nervous system perceives external and internal stimuli; analyzes, selects, and processes this information; and regulates and coordinates the bodily functions accordingly. The system consists mainly of nerve tissue. The nerve cell, which is the principal unit of nerve tissue, is highly excitable and able to rapidly conduct excitation.

The development of the nervous system over the course of evolution is highly complex. Among the unicellular organisms, protozoans do not have a nervous system, but some infusorians have an intracellular reticulum that conducts excitation to other structures in the cell. The most primitive form of nervous system is the nerve net, which survives only in such lower coelenterates as members of the order Hydrida. In coelenterates the nerve cells and their outgrowths form a network in which impulses can be conducted in all directions. The nervous system becomes more complex over the course of subsequent evolution. Ganglia, or bundles of nerve cells, are formed deep within the body in free-living coelenterates. The ganglia are chiefly interconnected through long outgrowths from the nerve fibers and the nerves. The occurrence of this diffuse ganglionic type of nervous system is accompanied by the development of receptors—specialized nerve structures that differentiate according to the kind of stimulus they are to perceive. The direction in which a nerve impulse is propagated becomes fixed for a given nerve.

Animals in the next stage of evolution, including the annelids, arthropods, echinoderms, and mollusks, have a nervous system whose structure is transitional between the diffuse ganglionic and the ganglionic types. The nerve cells are concentrated in the ganglia, which are interconnected by nerve fibers and are also connected to the corresponding receptors and effector organs. Remote reception becomes possible. The ganglia that are located in the anterior end of the body of free-moving animals predominate over the other ganglia and become linked to the sense organs, the most important remote receptors. Since the cephalic ganglia in moving animals receive the most information, they become larger and more complex than the other ganglia. The trunk ganglia move close to the cephalic ganglia and merge with them to form complex brain structures that to some extent take over the activities of the other ganglia. The type of nervous system that is found in vertebrates differs markedly from the ganglionic type, usually found only in invertebrates. The central nervous system (CNS) develops from the neural tube, which is situated on the dorsal side of the body, and consists of the spinal cord and the brain. During the embryonic development of vertebrates, the nervous system is formed from the outermost germ layer, the ectoderm. First, the neural plate curls to form a groove; then, the groove is closed and the neural plate differentiates into the neural tube. The rudimentary ectodermal cells differentiate into neuroblasts (cells that become neurons) and spongioblasts (cells that become the cells of the neuroglia). In addition, migrating ectodermal cells form the peripheral ganglia. Groupings of outgrowths from certain neuroblasts become the cranial and spinal nerves that make up the peripheral nervous system.

Two flexures in the anterior end of the neural tube form three rudimentary brain vesicles, which later become the primary divisions of the brain. In the course of development, the anterior brain vesicle splits into two vesicles, one of which forms the telencephalon, including the cerebral hemispheres and the basal ganglia, while the other forms the diencephalon. The middle brain vesicle gives rise to the midbrain. From the midbrain evolve the cerebellum, pons Varolii, and medulla oblongata. The remaining part of the neural tube retains its tubular structure and forms the spinal cord, with thickenings in the lumbar and humeral regions. The spinal cord and the brain of vertebrates are covered by a series of membranes and encased in bony coverings, namely, the vertebral column and skull.

Over the course of evolution, the nervous system grows more complex and all of its interactions with the environment become possible. The anterior divisions of the brain become increasingly important as their development progresses. In fish the forebrain is barely differentiated, but the hindbrain and midbrain are well developed. The cerebellum is the most developed element in the nervous system of fish.

In amphibians and reptiles the hindbrain occupies relatively less space than in fish. The midbrain, which becomes more developed than the cerebellum, divides into the two inferior colliculi. The anterior brain vesicle becomes more complex structurally and functionally. It differentiates into the diencephalon and two hemispheres with well-developed nerve tissue that forms the primary cerebral cortex. The forebrain, initially associated only with olfaction, subsequently acquires even more complex functions.

The evolution of the nervous system in birds is unique. The structures of the brainstem dominate, that is, the midbrain and only the parts of the forebrain that are situated deep within the hemispheres, such as the basal ganglia and the diencephalon. The cerebellum in birds is highly developed, but the cerebral cortex remains only slightly differentiated.

The nervous system attains its highest degree of development in mammals. During embryogenesis the anterior end of the neural tube divides into five vesicles: the anterior vesicle gives rise to the cerebral hemispheres and the diencephalon; the middle vesicle, to the midbrain; and the posterior vesicle, to the hindbrain proper (comprising the pons Varolii and the cerebellum) and the medulla oblongata. The cerebral cortex forms numerous furrows and convolutions. The central lumen of the neural tube develops into the cerebral ventricles and the spinal canal. The neuronal organization of the brain grows greatly in complexity.

The nervous system of highly organized animals is subdivided into the somatic and the autonomic, or vegetative, nervous systems. The main structural feature of the autonomic nervous system is that its fibers do not directly proceed from the central nervous system to an effector organ but first enter the peripheral ganglia and only then terminate in cells whose axons are directly connected with the innervated organ. The autonomic nervous system is divided into two parts, parasympathetic and sympathetic, depending on the location of the ganglia and on some functional characteristics.

The structural and functional unit of the nervous system is the neuron, which consists of the nerve cell body and its outgrowths —the axon and dendrites. In addition to nerve cells, the nervous system is made up of glial cells. The neurons are independent units: their protoplasm does not pass from one cell to another. Neurons interact at their points of interconnection, the synapses. In most cases, a gap several hundred angstroms wide remains in the contact area between the ending of one neuron and the surface of another. This space is called the synaptic cleft.

The main functions of neurons are to perceive and analyze stimuli, to transmit this information, and to effect a response. Neurons are subdivided according to the type, function and rate of impulse propagation in the nerve fibers. Fibers of receptor, or afferent, neurons conduct nerve impulses from the receptors in the CNS; the cell bodies of afferent neurons are in the spinal ganglia or in the ganglia of the cranial nerves. Motor, or efferent, neurons link the CNS to the effectors; the bodies and dendrites of these neurons are in the CNS, but the axons extend beyond it. (An exception is the afferent neurons of the autonomic nervous system, whose bodies are in the peripheral ganglia.) Interneurons serve as links between the afferent and efferent neurons; interneuron cell bodies and outgrowths are located in the CNS.

The activity of the nervous system is based on two processes: excitation and inhibition. Excitation can be propagative or local. Local excitation, which is nonspreading, was discovered by N. E. Vvedenskii in 1901. Inhibition, which is brought about by a decrease in cell excitability, is closely related to excitation. Inhibition cannot be actively propagated through the nerve structures. The phenomenon of inhibition in nerve centers was discovered by I. M. Sechenov in 1863.

The cellular mechanisms of excitation and inhibition have been thoroughly studied. The body and the processes of the nerve cells are covered by a membrane with regularly spaced differences in potentials. These potentials are called membrane potentials. Stimulation of the sensory endings of a peripheral afferent neuron alters the differences between the membrane potentials. This gives rise to a nerve impulse that spreads along the nerve fiber and reaches the presynaptic membrane, where it induces the release into the synaptic cleft of a chemically highly active mediator. The surface of the postsynaptic membrane undergoes molecular reorganization under the influence of the mediator. As a result, the membrane becomes permeable to ions and is depolarized, causing an electrical reaction in the form of a local, excitatory postsynaptic potential. This local postsynaptic potential generates a new propagative impulse. The nerve impulses that originate during the excitation of the special inhibitory neurons induce hyperpolarization of the postsynaptic membrane and, consequently, inhibit the postsynaptic potential. Another type of inhibition has been observed: it is localized in the presynaptic structures and causes a prolonged decrease in the efficiency of synaptic transmission.

The activity of the nervous system is based on the reflex—the organism’s response to stimulation of the receptors as mediated by the nervous system. The term “reflex” was introduced in 1649 into the nascent science of physiology by R. Descartes, although no one at that time was concretely aware of the manner in which reflex activity operates. Such information did not become available until much later, when morphologists began to investigate the structure and function of nerve cells (R. Dutrochet, 1824; C. Ehrenberg, 1836; J. Purkinje, 1837; C. Golgi, 1873; S. Ramón y Cajal, 1909) and until physiologists studied the main properties of nerve tissue (L. Galvani, 1791; C. Matteucci, 1847; E. Du Bois-Reymond, 1848–49; N. E. Vvedenskii, 1901; A. F. Samoilov, 1924; D. S. Vorontsov, 1924). At the end of the 19th and beginning of the 20th centuries, maps of the nerve centers and nerve pathways in the brain were compiled for the first time, and information was obtained on the principal reflexes and on the localization of function in the brain (I. M. Sechenov, 1863; N. A. Mislavskii, 1885; V. M. Bekhterev, 1903; I. P. Pavlov, 1903; C. Sherrington, 1906; A. A. Ukhtomskii, 1911; I. S. Beritashvili, 1930; L. A. Orbeli, 1932; J. Fulton, 1932; E. Adrian, 1932; P. K. Anokhin, 1935; K. M. Bykov, 1941; H. Magoun, 1946).

Research on reflexes and localization of function has broadened steadily. All reflexes are propagated through a collection of nerve structures that form a reflex arc. The main elements of a reflex arc include the receptors, an afferent nerve pathway, CNS structures of varying complexity, the efferent nerve pathway, and the effector organ. Receptors can be excited only by stimuli to which they are sensitive. Exteroceptors perceive stimuli that originate in the external sensory organs, such as the eye and ear. Interoceptors are sensitive to mechanical, chemical, thermal, and other types of stimuli that affect the internal organs. All nerve signals that carry information to the CNS from the various receptors via the nerve fibers are identical (regardless of the nature of the stimulus) and are usually transmitted in a series of homogeneous impulses. Information about the characteristics of each specific stimulus is expressed by a code of impulse frequencies and by the restriction of nerve impulses to certain fibers. The impulse frequencies and the specificity of the fibers constitute the spatial-temporal coding of the stimuli that an animal must process.

In a given region of the body, the group of receptors whose stimulation elicits a certain type of reflex is called the receptive field of that reflex. Such fields can be superimposed. A group of nerve endings concentrated in the CNS and responsible for a given reflex is called a nerve center. Many fiber endings carrying impulses from various nerve cells can converge on a single neuron. The complex synaptic reprocessing of this flow of impulses ensures the further propagation of only one signal at any given moment. This phenomenon is called the convergence principle; it underlies activity at every level of the nervous system. The phenomenon, also referred to as Sherrington’s final common pathway principle, was further elaborated by Ukhtomskii and others.

A combination of several spatial and temporal features of synoptic activity is the basis for the various groupings of selectively functioning nerve cells. These groups of cells carry out the analysis of information that reaches the nervous system, and then they issue “commands” that control the body’s responses accordingly. These commands, like afferent signals, are transmitted from one cell to another and from the CNS to the effector organs in the form of a series of nerve impulses. These impulses arise in a cell when the summation of excitatory and inhibitory stimuli achieves a certain critical level called the excitation threshold, which is unique for a given cell.

The connections in the main reflex arcs are hereditarily determined. However, a reflex can be substantially altered by the condition of the nerve structures through which it is executed. Thus, a sharp increase or decrease in the excitability of the central structures in a reflex arc not only modifies the reflex quantitatively but also brings about certain qualitative changes in that reflex. The phenomenon of dominant nerve centers is an example of such a change.

Of great importance to normal reflex activity is reverse afference, the phenomenon by which information concerning the result of a reflex arrives from the effector organ by way of afferent pathways. Based on this information, if the result of the reflex is unsatisfactory, the activity of the appropriate nervous system structures can be altered until the result corresponds to the requirements of the organism. This phenomenon was investigated in 1935 by P. K. Anokhin.

All reflexes are divided into two main groups: unconditioned and conditioned. Unconditioned reflexes are innate and are executed over hereditarily determined pathways; conditioned reflexes are acquired during the lifetime of the organism through the formation of temporary connections in the CNS. For a given species of organism, such connections can be formed only in the nervous system structure that is most highly developed in that organism; for mammals and man this structure is the cerebral cortex. The formation of conditioned reflexes enables the organism to adapt in the best and most precise way possible to the constantly changing conditions of existence. Conditioned reflexes were discovered and studied by I. P. Pavlov at the end of the 19th and beginning of the 20th centuries.

Research on conditioned activity in animals and man led to the doctrine of higher nervous activity and to the doctrine of analyzers. Each analyzer comprises a perceptive apparatus, or receptor, and the conduction pathways and analyzing structures of the CNS, including the cortex. In higher animals the cerebral cortex is simply the aggregation of the cortical ends of the analyzers. The cortex carries out the highest forms of analytical and integrative activity, thereby ensuring the efficient and precise interaction of the organism with the environment.

The nervous system is not only capable of immediately analyzing incoming information with the help of interacting synaptic processes, but it can also store information about past activity through memory. The cellular mechanisms that account for the prolonged storage of information in the most highly developed divisions of the nervous system are now being intensively studied.

The nervous system is also responsible for regulating the metabolic processes in tissues. This was termed the adaptotrophic function of the CNS by Pavlov, Orbeli, and A. V. Tonkikh. Severance or other types of injury to the nerve fibers alter the properties of the cells that these fibers innervate. This cellular alteration affects both the physicochemical properties of the surface membrane and the biochemical processes occurring in the cytoplasm. These second-order alterations, in turn, cause serious disturbances in the organs and tissues. For example, neurotrophic ulcers arise in this manner. If innervation is restored by the regeneration of the nerve fibers, the disturbance corrects itself.

Neurology studies the structure, function, and development of the nervous system in man. Neuropathology and neurosurgery are concerned with neurological diseases.

REFERENCES

Orbeli, L. A. Lektsii po fiziologii nervnoi sistemy, 3rd ed. Moscow-Leningrad, 1938.
Orbeli, L. A. Izbr. trudy, vols. 1–5. Moscow-Leningrad, 1961–68.
Ukhtomskii, A. A. Sobr. soch., vols. 1–6. Leningrad, 1945–62.
Pavlov, I. P. Poln. sobr. soch., 2nd ed., vol. 2. Moscow, 1951.
Sechenov, I. M. Izbr. proizv., vol. 1. Moscow, 1952.
Koshtoiants, Kh. S. Osnovy sravnitel’noifiziologii, vol. 2. Moscow, 1957.
Beritashvili, I. S. Obshchaia fiziologiia myshechnoi i nervnoi sistemy, 3rd ed., vol. 1. Moscow, 1959.
Sepp, E. K. Istoriia razvitiia nervnoi sistemy pozvonochnykh, 2nd ed. Moscow, 1959.
Eccles, J. Fiziologiia nervnykh kletok. Moscow, 1959. (Translated from English.)
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vol. 2. Moscow, 1964.
Katz, B. Nerv, myshtsa i sinaps. Moscow, 1968. (Translated from English.)
Ochs, S. Osnovy neirofiziologii. Moscow, 1969. (Translated from English.)
Sherrington, C. Integrativnaia deiatel’nost’ nervnoi sistemy. Leningrad, 1969. (Translated from English.)
Kostiuk, P. G. Fiziologiia tsentral’noi nervnoi sistemy. Kiev, 1971.
Ariens Kappers, C. U., G. C. Huber, and E. C. Crosby. The Comparative Anatomy of the Nervous System of Vertebrates, Including Man, vols. 1–2. New York, 1936.
Bullock, T. H., and G. A. Horridge. Structure and Function in the Nervous Systems of Invertebrates, vols. 1–2. San Francisco-London, 1965.

P. G. KOSTIUK

nervous system

[′nər·vəs ‚sis·təm]
(anatomy)
A coordinating and integrating system which functions in the adaptation of an organism to its environment; in vertebrates, the system consists of the brain, brainstem, spinal cord, cranial and peripheral nerves, and ganglia.

nervous system

the sensory and control apparatus of all multicellular animals above the level of sponges, consisting of a network of nerve cells (see neurone)