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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).
A simple, unlearned, yet specific behavioral response to a specific stimulus. Reflexes are exhibited by virtually all animals from protozoa to primates. Along with other, more complex stimulus-bound responses such as fixed action patterns, they constitute much of the behavioral repertoire of invertebrates. In higher animals, such as primates, where learned behavior dominates, reflexes nevertheless persist as an important component of total behavior.
The simplest known reflexes require only one neuron or, in the strictest sense, none. For example, ciliated protozoa, which are single cells and have no neurons, nevertheless exhibit apparently reflexive behaviors. However, most reflexes require activity in a large sequence of neurons. The neurons involved in most reflexes are connected by specific synapses to form functional units in the nervous system. Such a sequence begins with sensory neurons and ends with effector cells such as skeletal muscles, smooth muscles, and glands, which are controlled by motor neurons. The central neurons which are often interposed between the sensory and motor neurons are called interneurons. The sensory side of the reflex arc conveys specificity as to which reflex will be activated. The remainder of the reflex response is governed by the specific synaptic connections that lead to the effector neurons. A familiar reflex is the knee-jerk or stretch reflex. It involves the patellar (kneecap) tendon and a group of upper leg muscles. Other muscle groups show similar reflexes.
in painting and, less commonly, in the other graphic arts, light and color represented as reflected from one object (or the sky) onto another object. The term “reflex” applies in this sense to both real objects and their depictions. Accurate and subtle depiction of reflex helps to convey the three-dimensionality and the wealth of colors of real objects.
a response of an organism mediated by the central nervous system after stimulation of receptors by internal or external environmental agents; it is manifested by the occurrence of or change in the functional activity of individual organs or the body as a whole. The term “reflex,” adopted from the physical sciences, emphasizes the fact that nervous activity is “reflected,” that is, it is a response to influences from the external or internal environment. The structural mechanism of a reflex is the reflex arc, which includes receptors, a sensory (afferent) nerve that conducts excitation from receptors to the brain or spinal cord, a nerve center located in the brain and spinal cord, and an efferent nerve, which conducts excitation from the brain or spinal cord to effector organs, that is, muscles, glands, and internal organs. The biological significance of reflexes consists in the regulation of the work of organs and their functional interactions to maintain the stability of the organism’s internal environment (homeostasis) while preserving its integrity and ability to adapt to the external environment. The reflex activity of the nervous system assures the organism’s functional integrity and controls the organism’s interaction with the external environment, that is, its behavior.
History of the study of reflexes. The concept of reflexes was first conceived by the French philosopher Descartes. The ancient physicians, for example, Galen in the second century, divided human motor actions into voluntary actions, which require the participation of consciousness in their execution, and involuntary actions, which are performed without the participation of consciousness. Descartes’s teaching on the reflex principle of nervous activity was based on the mechanism of involuntary movements. The entire process of nervous activity, characterized by automatism and involuntariness, consists in stimulation of the sensory apparatus and conduction of the apparatus’ impulses along peripheral nerves to the brain and from the brain to the muscles. As examples, Descartes cited blinking in response to the sudden appearance of an object before one’s eyes and withdrawal of a limb after the sudden application of a painful stimulus. He described the impulses conducted along peripheral nerves by the term “animal spirits,” which he borrowed from the ancient physicians. Despite the spiritual aura surrounding the term, Descartes attached to it actual and, for his time, completely scientific significance based on ideas from mechanics, kinematics, and hydraulics.
The studies of such 18th-century physiologists and anatomists as A. von Haller and G. Prochaska freed Descartes’s ideas from metaphysical terminology and mechanicism and applied them to the activity of the internal organs (several reflexes specific to various organs were found). C. Bell and F. Magendie made very important contributions to the understanding of reflexes and the reflex apparatus by showing that sensory (afferent) fibers enter the spinal cord as part of the posterior roots, while efferent fibers, such as motor ones, leave it as part of the anterior roots. This discovery enabled M. Hall, a British physician and physiologist, to advance clear-cut ideas on the reflex arc and make extensive clinical use of the theory of reflexes and the reflex arc.
Information was available by the second half of the 19th century on common elements in the mechanisms of both voluntary movements wholly related to manifestations of cerebral activity and involuntary automatic reflex actions, counterposed to cerebral activity. In his study Brain Reflexes (1863), I. M. Sechenov contended that all conscious and unconscious actions are reflex in origin. He substantiated the idea of the universal significance of the reflex principle in the functions of the spinal cord and brain for both involuntary and voluntary movements involving consciousness and cerebral activity. Sechenov’s concept enabled I. P. Pavlov to discover conditioned reflexes. Sechenov’s discovery of central inhibition is the most important aspect of the reflex theory. C. Sherrington, N. E. Vvedenskii, A. A. Ukhtomskii, and I. S. Beritashvili provided evidence that the reflexes of individual arcs are coordinated and integrated in the functional activity of organs based on the interaction of excitation and inhibition in the reflex centers.
The concept of the cellular organization of the nervous system plays an important role in elucidating the mechanisms of reflex action. The Spanish histologist S. Ramon y Cajal showed that the neuron is the structural and functional unit of the nervous system. This gave rise to the concept of the neuronal organization of reflex arcs and substantiated the concept of the synapse, the apparatus of interneuronal contact, and synaptic (that is, interneuronal) transmission of excitatory and inhibitory impulses in the reflex arcs (Sherrington, 1906).
Classification. The variety of reflexes led to the development of different classifications. Reflexes may be classified according to the anatomical arrangement of the central part of the reflex arcs, which are their nerve centers, as (1) spinal, involving neurons situated in the spinal cord, (2) bulbar, executed with the participation of medulla oblongata neurons, (3) mesencephalic, executed with the participation of midbrain neurons, or (4) cortical, executed with the participation of cerebrocortical neurons. According to the location of the reflexogenic zones, or receptive fields, reflexes are exteroceptive, proprioceptive, or interoceptive.
Reflexes can also be classified according to type and function of the effectors as motor reflexes (of skeletal muscles)—for example, flexor, extensor, locomotor, and statokinetic—or as autonomic reflexes of the internal organs—digestive, cardiovascular, excretory, and secretory. Depending on the degree of complexity of the neuronal organization of the reflex arcs, they can be subdivided into monosynaptic reflexes, whose arcs consist of an afferent neuron and an efferent neuron, such as the patellar reflex, or multisynaptic reflexes, whose arcs also contain one or more interneurons, such as the flexor reflex. With respect to their influence on effector activity, reflexes can be excitatory, that is, causing or intensifying (facilitating) effector activity, or inhibitory, that is, weakening and suppressing such activity, for example, the reflex acceleration of the heartbeat by the sympathetic nerve and retardation or cessation of the heartbeat by the vagus nerve.
Reflexes can also be classified according to their biological significance for the organism as a whole, for example, the defense (or protective), sexual, and orienting reflexes.
Pavlov justified dividing all reflexes according to origin, mechanism, and biological significance into unconditioned and conditioned reflexes. The former are hereditarily fixed and species-specific, which determines the constancy of the reflex connection between the afferent and efferent elements of their arcs. Conditioned reflexes are acquired during an individual’s lifetime as a result of a temporary connection (conditioned closure) between the various afferent and efferent apparatus of the organism. Since a conditioned temporary connection is formed in higher animals (vertebrates) with the necessary participation of the cerebral cortex, conditioned reflexes are also called cortical reflexes.
The biological function of unconditioned reflexes consists in regulating homeostasis and in preserving the integrity of the organism, whereas the function of conditioned reflexes is to ensure the most delicate adaptation possible to changing external conditions.
The term “reflex” is also applied to other reactions, even though the central nervous system is not involved, for example, axon reflexes and local reflexes executed by the peripheral nervous system.
Mechanism and properties. Reflexes are normally elicited by stimulation of the appropriate reflexogenic zones by external or internal agents, that is, by adequate stimuli of the receptors of these zones. The excitation that arises in the receptors—discharge of impulses—is conducted by afferent nerve conductors to the brain or spinal cord, where it is transmitted from an afferent neuron either directly to an efferent neuron (two-neuron arc) or through one or more interneurons (polyneuron arc). In the efferent neurons, excitation is transmitted by efferent nerve fibers in the reverse direction—from the brain or spinal cord to the various peripheral organs (effectors), for example, skeletal muscles, glands, and blood vessels—and a reflex response is induced, that is, a change in functional activity occurs.
The reflex response always lags behind the start of stimulation of the receptors. This lag time is called latency period. It varies, according to the complexity of the reflex, from a millisecond to several seconds.
Excitation is conducted in the reflex arcs in one direction, from the afferent neuron to the efferent one—never in the opposite direction. This property of reflex conduction is attributable to the chemical mechanism of interneuronal synaptic transmission, which consists basically in the formation and release by nerve endings of specific chemical mediators, for example, acetylcholine and epinephrine, that excite or inhibit the neurons with which the particular endings form synaptic contacts.
The properties of reflexes—intensity, duration, and dynamics—are determined both by the conditions of stimulation (adequacy, force, duration, location) and by the function state (background) of the reflex apparatus itself (excitability, impulses from other nerve centers, fatigue) and other internal factors.
Integration and coordination. Reflexes do not occur in isolation. They are combined (integrated) into complex reflex acts of definite functional and biological significance. For example, the very simple reflex response of an extremity to pain—the flexion reflex (flexing and withdrawal of an extremity)—is a complex multicomponent action involving the involuntary contraction of some muscles, inhibition of others, and changes in respiratory and cardiac activity. The organization of reflexes that control behavior, such as the orienting, food-procuring, defense, and sexual reflexes, is even more complex. Such reflexes include elements involving all the organs to some degree.
The processes responsible for the integration of reflexes are designated by the term “coordination.” Coordination entails essentially the combining of excitation and inhibition in the system of neurons that participate in the formation of reflexes of different complexities. The intimate nature of the mechanisms of these interactions is studied specifically by the technique of microelectrode intracellular recording of electrical reactions of neurons when the reflexes are elicited by stimulation of the receptors or afferent nerves. The synaptic apparatus of the neurons, which contains from a few hundred to 5,000 or 6,000 synaptic contacts per neuron, has both excitatory and inhibitory synapses. When the former are active due to the influx of impulses, a negative electrical reaction arises in the neuron and stimulates the discharge of other impulses. When the latter are active, a positive electrical reaction occurs that inhibits or blocks the transmission of excitation in the neuron. The quantitative relations of the activation of the synapses (number and intensity) determine the significance and extent of participation of the reflex center neurons in the execution of a particular reflex.
The coordination process that integrates reflexes of different complexities can be regarded as a distribution of excitation and inhibition in the neuronal systems involved in the execution of these reactions in accordance with a definite spatial and temporal program corresponding to these reactions. Biological cybernetics studies the factors that give rise to principles of shaping these programs. A high degree of coordination of movements is achieved by the feedback mechanism. The broad convergence in interneuronal relations characterized by hundreds and thousands of synaptic contacts of neurons with other neurons performing different functional roles is the basis for the assumption that the mechanisms of reflex action rest on a stochastic (probabilistic) principle rather than on a static, predetermined organization of reflex arcs.
P. A. KISELEV
Pathologic reflexes. Two types of pathologic reflexes are distinguished. The first type includes reflexes that are unusual in adults (they are sometimes peculiar to earlier stages of phylogeny or ontogeny) and that are manifested after structural or functional injury to different parts of the central nervous system. They are used in the diagnosis of neurological diseases (for example, Babinski’s reflex and the pathologic sucking reflex). The condition in which reflexes are of low intensity or absent is called hyporeflexia or areflexia, respectively. If reflexes are exaggerated or uneven, the condition is called hyperreflexia or anisoreflexia, respectively.
The second type of pathologic reflex includes inadequate and, from the biological standpoint, inappropriate responses to some, usually superstrong, internal or external stimulus.
A distinction is made between pathologic unconditioned and conditioned reflexes. Among the former are the pulmonocoronary reflex (cardiac arrest following irritation of some part of the tunica intima of the pulmonary artery by a foreign body), renorenal reflex (spasm of one ureter following irritation of the other by a calculus), and hepatocoronary reflex (spasm of coronary vessels during an attack of hepatic colic). The decisive factor in the formation of pathologic unconditioned reflexes is parabiosis, a phenomenon that develops in nerve structures as a result of superstrong stimulation and, as shown by N. E. Vvedenskii (1901) and I. P. Razenkov (1923–24), is responsible for the paradoxical nature of the responses.
Pathologic conditioned reflexes are induced by stimuli that are by nature indifferent as far as the body is concerned but are previously combined with superstrong unconditioned stimuli. For example, the coronary spasm that results from climbing a mountain in windy weather (stress stenocardia) may recur if the patient merely descends from the mountain in good weather. Pathologic conditioned reflexes differ from ordinary (physiological) conditioned reflexes in that they are formed after a single combination of stimuli and persist a long time without reinforcement. Pathologic reflexes may underlie some internal diseases.
V. A. FROLOV
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