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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
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).
Central nervous system
That portion of the nervous system composed of the brain and spinal cord. The brain is enclosed in the skull, and the spinal cord within the spinal canal of the vertebral column. The brain and spinal cord are intimately covered by membranes called meninges and bathed in an extracellular fluid called cerebrospinal fluid. Approximately 90% of the cells of the central nervous system are glial cells which support, both physically and metabolically, the other cells, which are the nerve cells or neurons.
Functionally similar groups of neurons are clustered together in so-called nuclei of the central nervous system. When groups of neurons are organized in layers (called laminae) on the outer surface of the brain, the group is called a cortex, such as the cerebral cortex and cerebellar cortex. The long processes (axons) of neurons course in the central nervous system in functional groups called tracts. Since many of the axons have a layer of shiny fat (myelin) surrounding them, they appear white and are called the white matter of the central nervous system. The nuclei and cortex of the central nervous system have little myelin in them, appear gray, and are called the gray matter of the central nervous system. See Brain, Nervous system (vertebrate)
Central Nervous System
the principal part of the nervous system of animals and man, consisting of nerve cells (neurons) and their projecting parts (processes). It includes a system of closely interrelated groups of nerve cells (ganglia) in invertebrates and of the spinal cord and the brain in vertebrates, including humans. The main and specific function of the central nervous system (CNS) is to effect simple and complex highly differentiated reflexes. In higher animals and in man, the low and middle divisions of the CNS—the spinal cord, medulla oblongata, mesencephalon, diencephalon, and cerebellum—regulate the activity of the organs and systems of a highly developed organism, effect contact and interaction among them, and maintain the organism’s unity and the integrity of the organism’s activity. The highest divisions of the CNS—the cerebral cortex and contiguous subcortical structures—largely regulate the organism’s interrelationship with the external environment.
Structure and function. The CNS is connected to all the organs and tissues by the peripheral nervous system. In vertebrates this includes the cranial nerves, which arise from the brain; the spinal nerves, which arise from the spinal cord; the intervertebral nerve ganglia; and the peripheral part of the autonomic nervous system—that is, a system of nerve ganglia with efferent nerve fibers that enter (preganglionic) or leave (postganglionic) the ganglia. Sensory, or afferent, adductor fibers conduct excitation to the CNS from the peripheral receptors, and efferent (motor and autonomic) abductor fibers conduct excitation from the CNS to the cells of such effectors as the muscles, glands, and blood
vessels. All the divisions of the CNS have afferent neurons that receive stimuli from the periphery, as well as efferent neurons that transmit nerve impulses to various effector organs on the periphery. Afferent and efferent cells are able to interact by means of their processes and to form a two-neuron reflex arc that effects such elementary reflexes as the tendon reflexes of the spinal cord. In general, however, there are intercalary nerve cells, or interneurons, located in a reflex arc between the afferent and efferent neurons (see Figure 1). Contact among the different divisions of the CNS is also established by the numerous processes of their afferent, efferent, and intercalary neurons, which form short and long conducting pathways. The CNS also includes neuroglial cells that perform a supporting function and participate in the metabolism of the nerve cells.
The reflex principle of CNS activity. The reflex principle of CNS activity was established experimentally before the 19th century but was studied only in relation to the activity of those divisions situated below the level of the cerebral hemispheres. Scientists dealing with the CNS elucidated the general mechanisms and the adaptive nature of CNS reflex activity, as well as the funtional characteristics of its divisions. In the 19th century, I. M. Sechenov and several other progressive scientists advanced the bold idea that the activity of the higher divisions of the CNS, including mental activity, was reflex in nature. This progressive, materialist idea was the basis of the classic experimental and theoretical studies of I. P. Pavlov that led to his theory of higher nervous activity. Pavlov established that the reflexes effected in highly developed organisms by the cerebral cortex, unlike the reflexes of the lower divisions of the CNS, are not innate but develop during the life of the organism as it interacts with the environment; the reflexes of the cerebral cortex enable the organism to adapt with maximum efficiency to its environment. Pavlov called this new class of reflexes conditioned reflexes, in contrast to innate, or unconditioned, reflexes.
Rejecting the original, rudimentary view of the reflex as a stereotyped, mechanical, and passive response of the CNS, the classic studies of the neurophysiologists E. Pflüger, Sechenov, Pavlov, and C. Sherrington established that unconditioned and in particular conditioned reflexes are highly dynamic and changeable: responses to the same stimuli vary with external and internal environmental conditions and with the functional state of the CNS itself.
The principal patterns of CNS activity are closely related to the characteristics of the reflex arc, which is the structural basis of every reflex action. The reflex arc conducts excitation in only one direction, from the receptor ending to the effector, owing to the structural and functional polarization inherent in all nerve cells. Microstructures called synapses, which are located on the terminal branches of each neuron’s axon, enable the neuron to come in contact with the bodies or the dendrites of other neurons and to transmit its activity to them unilaterally (the Bell-Magendie law). The diverse external and internal receptors have become specialized over the course of evolution in their ability to respond with precision to qualitatively distinct types of energy—luminous, acoustic, thermal, mechanical, and chemical. The receptors transform this energy during the process of nerve excitation, which is successively transmitted from some elements of the reflex arc to others in the form of rhythmic impulses. The excitation undergoes significant changes in rhythm, intensity, velocity, and character in its multistage journey to the final element. Reflex excitation may produce in the effectors a variety of effects owing to the structural and functional characteristics of the effectors (muscles, glands, or blood vessels) themselves.
The functions of the central divisions of the reflex arc, in contrast to those of simple nerve conductors, are marked by a relatively slow onset and by the presence of excitation and of the phasic oscillations of the level of excitability that are produced by waves of excitation. This unique functional inertness—a relatively lengthy persistence of a state of high excitability—causes the phenomenon of summation (weak, ineffective stimuli become effective when repeated) as well as the related phenomenon of attenuation.
The central elements of the reflex arc can change or transform the rhythm of stimulation. The resulting excitation usually occurs with its own inherent rhythm of waves, a rhythm that is sometimes faster and sometimes slower than the rhythm of initial stimulation. Between the force of stimulation and the intensity of the reflex response there is only a relative correspondence, which is ordinarily observed after moderate stimulation. If stimulation is strong and prolonged, the nerve centers become inhibited, in which case weak stimulation begins to elicit a greater reflex effect than does strong stimulation, a phenomenon known as the paradoxical reflex. A relatively high susceptibility to fatigue and pronounced sensitivity to biochemical changes in the organism’s internal environment, especially to an insufficiency of oxygen in the blood and to certain toxins, are also characteristic of the central elements of the reflex arc. All these traits are caused mainly by the properties of the synapses, dendrites, and bodies of the neurons themselves.
A reflex arc is usually depicted in a simplified form, as a chain of individual receptor, afferent, intercalary, efferent, and effector cells. In reality, however, a reflex arc is composed of many such chains, whose links are not individual cells of various kinds but an aggregate of homogeneous interrelated cells. An aggregate of receptor cells forms the reflexogenic zone of a reflex, a group of effector cells constitutes the reflex’s operating mechanism, and an aggregate of neurons in the CNS forms a corresponding nerve center.
Pavlov defined a nerve center as an aggregate of nerve elements situated in different parts of the CNS, closely interrelated, constituting a unified system, and regulating a specific function of the organism. Developing and concretizing this concept, E. A. Asratian suggested that the central part of an unconditioned reflex arc be regarded as a multistage structure consisting of several branches; each branch passes through one of the divisions of the CNS participating in the given reflex and is characterized by specificity (see Figure 2). These branches are of unequal importance in effecting reflexes: the branches situated at some levels are used for certain reflexes, and branches at other levels are used for other reflexes. For example, the main branch of the arc associated with cardiovascular and respiratory reflexes passes through the medulla oblongata, and the main branch of the arc associated with digestive, defensive, and sexual reflexes passes through the diencephalon. The branch of a multistage arc that passes through the cerebral cortex corresponds to what Pavlov called the cortical representation of the unconditioned reflex, which is the basis for the formation of the corresponding conditioned reflexes.
Coordination of reflexes. The CNS effects many diverse reflexes differing in their receptor, central, and effector mechanisms as well as in type, character, direction, origin, and degree of complexity. Nevertheless, these reflexes do not originate or occur in an uncoordinated or chaotic manner but have a specific composition, order, and sequence according to the organism’s current needs for unified, integrated, and adaptive activity. The coordinating activity of the CNS is determined by its structural and functional characteristics. The extent of genetic kinship among the different structural elements of the CNS and the nature of their interrelationship are important prerequisites for their interaction and for the coordination of reflexes. Of particular importance in this connection are the structural characteristics known as divergent and convergent pathways. Through the divergent pathways, the numerous terminal branches of the axons bring each afferent neuron in contact with many efferent neurons, either directly or through an intermediate neuron (see Figure 3,a). This makes it possible for a single afferent neuron to activate many nearby and remote efferent neurons and associated reflexes in a definite sequence. For example, moderate stimulation of the pad of the hind paw of a decerebrated cat initially elicits the reflex of flexing the talocalcaneal joint. Gradual intensification of the stimulating current elicits in succession the reflexes of flexing the knee joint and hip joint of the same extremity, the extension reflex of the opposite hind leg, the extension reflex of the homonymous front leg, and, finally, the reflex of flexing the opposite front leg. These reactions result from the gradual dispersion (irradiation) of the excitation arising in the afferent element of the primordial reflex to the related structures of the nearby and remote reflexes of the spinal cord by means of the short and long intraspinal conducting pathways.
By means of the convergent pathways a number of afferent neurons are brought into contact with an efferent neuron by the terminal branches of the axons, either directly or through intermediate neurons (see Figure 3,b). In the latter case the branches of several afferent neurons converge on a single interneuron, which serves as a unique common pathway. The branches of several such interneurons in turn converge on a single efferent neuron or an aggregate of them as on a common terminal pathway, which is blocked to other currents of excitation. The convergence of pathways in the CNS makes it possible for a single efferent neuron to be utilized by many homogeneous and heterogeneous afferent neurons, which are several times more numerous than efferent neurons. For example, separate, moderate stimulation of two different regions of the receptive field of the scratch reflex does not elicit the appropriate reflexes. However, simultaneous stimulation of both regions by a current of the same force provokes a distinct reflex owing to the summation of subthreshold excitations. Mutual intensification of reflexes is also achieved by simultaneous stimulation of the receptive fields of two different reflexes that have a common terminal pathway and that produce the same effect.
The phenomena of divergence and convergence were initially discovered and studied in relation to the activity of the spinal cord but were later found to exist in other divisions of the CNS as well, especially the cerebral cortex. Since afferent nerve elements greatly exceed efferent elements in the higher divisions of the CNS, the principle of the convergence of multiple pathways to the sensorimotor region, the main cortical common pathway, is quite pronounced. This makes it possible for afferent nerves to help effect various unconditioned motor reflexes and for diverse conditioned reflexes to develop from these unconditioned reflexes.
Recently developed electrophysiological techniques have enabled such contemporary neurophysiologists as the Italian G. Moruzzi and P. G. Kostiuk to discover and study the micro-structural and microfunctional bases of individual neurons. These scientists have also identified and studied the mechanisms of divergence and convergence, particularly in neurons of the spinal cord, reticular formation, thalamus, cerebral cortex, and other divisions of the CNS. The body and dendrites of every neuron in the reticular formation and cerebral cortex were found to have synaptic contacts with numerous other neurons that are activated by stimuli of different modalities and that exert both excitatory and inhibitory influence on the common pathway.
Such factors as the original functional condition of the nerve structures involved in a reflex, the force, biological role, and duration of the external stimulus, and the effect of internal neural and humoral factors are of considerable importance in the coordination of reflexes and in CNS activity. However, the main functional basis for the coordination of reflexes is the ability of the CNS to activate through excitation certain synaptic contacts and pathways and, in particular, simultaneously to block the inhibition of other synaptic contacts and pathways. These processes occur in a great variety of combinations and spatial and temporal relations according to the current needs of the organism and the existing conditions of the external environment.
The key role of inhibition in the coordinating activity of the CNS is clearly manifested in the antagonistic interactions of reflexes, particularly when different receptors or receptive fields having a common effector but evoking different kinds of reflexes are stimulated at the same time. Such mutually inhibitory relations exist among locomotor, defensive, and delayed reflexes in which a role is played by identical extensor motoneurons and the muscles of the extremities innervated by them. Each of these reflexes may occur separately and without hindrance if only its receptive field is stimulated. But if the receptive field of another reflex is stimulated during one of the reflexes, or if the receptive fields of both reflexes are stimulated, a conflict ensues over the control of the common terminal pathway. The reflex that predominates is the one whose receptive field at the given moment is stimulated more strongly and the one that is more essential to the organism under the given conditions. The other reflexes are blocked by the inhibitory process, thereby creating favorable conditions for the unimpeded effecting of the overcoming reflex.
Another example of such coordination of reflexes is the reciprocal innervation of the antagonistic muscles of the extremities and of the respiratory and other systems. Sherrington and the Russian physiologists P. A. Spiro and N. E. Vvedenskii demonstrated that reflex excitation of motoneurons of the extremities’ flexor muscles is accompanied by inhibition of the motoneurons of these muscles’ anatomical antagonists, the extensor muscles, and vice versa. Antagonistic interrelations may also be manifested by reflexes belonging to different functional systems; for example, respiratory movements are briefly halted during swallowing. The great importance of inhibition in the coordinating function of the CNS may be clearly shown by injecting an experimental animal with strychnine, which temporarily deprives the CNS of its ability to develop inhibition. The coordination of reflexes disappears almost entirely in the animal: any local stimulation elicits a simultaneous motor reaction by all the muscles of the body.
Sechenov and the Dutch physiologist R. Magnus discovered the importance of the original functional condition of the CNS with regard to its coordinating activity. Stimulation of a given receptive field of an animal’s hind leg after transection of its spinal cord induces contrary effects, depending on the original condition of the stimulated extremity. If the extremity is extended, stimulation elicits the flexor reflex, and if the extremity is flexed, stimulation elicits the extensor reflex. The original position of the extremity is reflected in the corresponding nerve center through the excitation of appropriate skin and muscle neural pathways; as a result, the functional condition of the center changes.
The receptors embedded in the effectors, and particularly in the motor apparatus, inform the adequate structures of the CNS about the original condition of the effector at rest and also about the nature, intensity, duration, and dynamics of the effector’s activity. The continuous flow of information from the receptors of functioning effectors plays an important part in the adjustment and self-regulation of activity according to the current needs of the organism and the given situation confronting it. Physiologists have long been aware of the important role played in the CNS by this important principle of reflex self-regulation of functions. Designated by the term “feedback,” it eventually became a fundamental principle of cybernetics.
The unique circular interaction between the central and peripheral elements of a reflex arc is sometimes manifested by mutual activation and sometimes by a circular rotation of excitation within the elements of the arc. This circular interaction is also manifested by the formation of a special chain of reflexes: reflex contraction of a muscle stimulates its receptors, which in turn cause new reflex contraction of the same muscle (C. Bell, A. F. Samoilov). Circular interaction also occurs among the neurons of the central element of a reflex arc and is manifested in different forms. An example is the phenomenon known as Renshaw inhibition, named after the American neurophysiologist B. Renshaw. In this phenomenon, the axon of a spinal motoneuron sends a recurrent collateral to the spinal cord that is brought into contact with the same motoneuron by an inhibitory interneuron and that inhibits this motoneuron if it becomes excessively excited. This phenomenon of negative feedback was described in relation to the pyramidal nerve cells of the cerebral cortex and to the cells of other CNS structures.
An example of positive feedback is the phenomenon of steadily intensifying nerve excitation, described by the Spanish physiologist R. Lorente de No. This phenomenon is caused by the prolonged circulation of excitation in a multibranched closed circuit of neurons in the reflex center: the recurrent collateral of one of these neurons comes into contact with one or more interneurons. These interneurons, when they come into contact with the original neuron, close the circuit and form the structural foundation for repeated circulation of excitation and for a unique type of self-intensification.
The coordinated and the antagonistic interactions of reflexes are inseparably connected and constitute different aspects of a single coordinating process. When there is an inherent common terminal pathway, the coordinated reflexes are cumulative and intensify one another. The antagonistic reflexes, on the other hand, engage in conflict over this pathway, and the overcoming reflex inhibits its rivals for a certain period of time. In certain situations, for example, under the influence of such external or internal factors as local mechanical pressure, a hormone, a toxin, or a polarizing current, the excitability of the central mechanism of a reflex increases substantially and evenly, and this mechanism temporarily becomes a unique common terminal pathway.
When the receptive fields of heterogeneous reflexes are stimulated, rather than the reflexes specific to these fields, the result is the onset or intensification of a reflex whose center had previously experienced intensified excitation. This type of coordination of reflexes was identified and studied in several modifications that are referred to in modern neurophysiology as the facilitation of pathways, the summation reflex, attenuation, and the dominant. It is believed that the neural center, when in a state of increased excitability or stimulation, inhibits certain elements of the central structures of different reflexes. It also diverts excitation traveling along the initial elements of the reflex arc from the usual route, as though attracting them to itself. Finally, it comes to a culmination with its own excitation and initiates or intensifies a reflex that is of importance for the organism at the moment.
The coordination of reflexes also depends on the functional condition of their central mechanism. For example, a decrease in this mechanism’s excitability owing to fatigue, neurohumoral factors, or toxins causes even previously dominant reflexes readily to yield the common terminal pathway to other reflexes. Thus, the diverse forms of coordination of reflexes, like the coordinating activity of the CNS in general, are based on active neural processes that have opposing effects. These include excitation and inhibition, different combinations and variations of the distribution of excitation and inhibition in highly complex macrostruc-tural and microstructural elements of the CNS, complex dynamics, mutual penetration, and the conflict and interaction among these processes.
Subordination, specialization, and localization of functions. CNS activity is characterized by functional subordination, that is, a hierarchical ranking of the system’s divisions that has evolved over a long period of time. This hierarchical ranking, as well as the CNS’s structural heterogeneity and functional inequality, are manifested at relatively early stages in the historical development of organisms. The central neural formations and the receptors of the head develop earlier than those in other parts of the body. The main division of the CNS develops by means of the enlargement of its mass, by continuous structural differentiation, and by functional specialization of the system’s existing divisions. The CNS’s main division also develops by means of the constant emergence within it of new central formations whose reflex activity is at a continuously higher level and of continuously greater significance; these formations direct and regulate the activity of all the lower divisions of the CNS. This process of the continuous development, specialization, localization, and subordination of the CNS functions reaches its most complex level in higher mammals and particularly in anthropoid apes and man.
The CNS is in a state of tonus even when there are no visible external signs of its activity. Tonic excitation of the CNS is a manifestation of a general functional readiness for the initiation and continuation of activity. The subordination of structures, particularly in the medulla oblongata, mesencephalon, and diencephalon, may be clearly detected in the tonic excitation of the CNS. In higher animals, for example, surgical transection or cryogenic block of the pathways between the medulla oblongata and the spinal cord (that is, the halting of the subordinating influence of the medulla oblongata on the spinal cord) results in spinal shock—a deep and prolonged inhibition of spinal reflexes. A consequence of such transection at the level of the superior colliculi is the phenomenon of decerebrate rigidity, that is, a strong tonic contraction of all the extensor muscles and loss of the animal’s ability to assume or maintain its normal posture.
Subordination among the divisions of the CNS is even more evident during their activity. Each higher element of the CNS effects reflexes that are increasingly complex in terms of structure and composition and integrates them more completely, while also involving in its activity the reflexes that are regulated by the CNS’s lower elements. The characteristics of the reflex activity of the main divisions of the CNS may be described as follows. Reflexes of segments of the spinal cord involve only individual parts of the body, such as the extremities. The more complex reflexes of the medulla oblongata extend to the digestive, respiratory, cardiovascular, and motor systems. Mesencephalic reflexes involve the body’s entire skeletal musculature and coordinate such complex motor functions as standing and walking. The reflexes of the diencephalic structures regulate and coordinate the activity of the internal organs of all the body’s systems in every possible combination and in harmony with their vital unconditioned reflexes, for example, the food-grasping, defensive, and sexual reflexes. The cerebral hemispheres are capable of improving all these reflexes, combining them into complexes of reflexes, and creating qualitatively new types of reflexes—conditioned reflexes. Thus, the higher the level of an animal’s development and the higher the level of its CNS’s organization, the greater the dominance of the higher divisions over the lower ones and the more significant their participation in the regulation of the organism’s functions.
The increasing importance of the higher CNS divisions over the course of evolution in the organism’s life processes is called the cerebralization, encephalization, or corticalization of functions. However, the lower divisions of the CNS also influence the higher ones, and all information from the external and internal organs is gradually transmitted upward. Therefore, the phenomenon of subordination of the CNS must be regarded simply as an expression of the prevalent direction in the complex and varied interaction among the neural structures of different levels. For example, the reticular formation strongly stimulates and inhibits the functional condition of almost all parts of the CNS, including the cerebral cortex. In turn, the cerebral cortex influences the functional condition and the activity of the reticular formation and other deep-lying brain structures, including the intermediate ganglia of the ascending tracts, thereby regulating the flow of information they transmit. The circular interaction between the CNS structures as well as the self-regulation of their functions confirms the soundness of Pavlov’s view that the cerebral cortex plays a major role in the combined and integrative activity of the entire CNS.
The structural and functional characteristics of the CNS are responsible for the variety and efficiency of the activities it performs to fulfill the organism’s normal needs as well as its requirements for new forms of coordination. The abundant reserve capabilities and compensatory adaptations of the CNS are biologically significant both during the organism’s normal existence and after injury to the peripheral sensory and effector organs, the afferent and efferent nerve structures, and the organs of the CNS itself.
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