Autonomic Nervous System(redirected from Autonomic agents)
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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).
Autonomic nervous system
The part of the nervous system that innervates smooth and cardiac muscle and the glands, and regulates visceral processes including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation. The autonomic nervous system functions primarily at a subconscious level. It is traditionally partitioned into the sympathetic system and the parasympathetic system, based on the region of the brain or spinal cord in which the autonomic nerves have their origin. The sympathetic system is defined by the autonomic fibers that exit thoracic and lumbar segments of the spinal cord. The parasympathetic system is defined by the autonomic fibers that either exit the brainstem via the cranial nerves or exit the sacral segments of the spinal cord. See Parasympathetic nervous system, Sympathetic nervous system
The defining features of the autonomic nervous system were initially limited to motor fibers innervating glands and smooth and cardiac muscle. This definition limited the autonomic nervous system to visceral efferent fibers and excluded the sensory fibers that accompany most visceral motor fibers. Although the definition is often expanded to include both peripheral and central structures (such as the hypothalamus), contemporary literature continues to define the autonomic nervous system solely as a motor system. However, from a functional perspective, the autonomic nervous system includes afferent pathways conveying information regarding the visceral organs and the brain areas (such as the medulla and the hypothalamus) that interpret the afferent feedback and exert control over the motor output back to the visceral organs. See Homeostasis
Autonomic Nervous System
(also vegetative nervous system), the part of the nervous system that regulates the organs of blood circulation, respiration, digestion, excretion, and reproduction, as well as metabolism, thereby regulating the functional state of all the tissues of vertebrate animals and man.
The term “vegetative nervous system” was introduced by the French biologist M. Bichat (1800), who divided the nervous system into animal (somatic), that is, regulating the functions peculiar to animals alone and responsible for sensations and body movements, and vegetative, regulating the main vital processes—nutrition, respiration, reproduction, and growth (peculiar not only to animals but to plants as well). The functions regulated by the vegetative nervous system may not be performed or halted voluntarily. Hence the English physiologist J. Langley called it autonomic. However, the “autonomy” of the autonomic nervous system with respect to the higher divisions of the brain is extremely relative because impulses traveling from the cortex of the cerebral hemispheres to the centers of the autonomic nervous system may alter the functioning of the internal organs. Each complex reaction of the organism, any behavioral act, voluntary or involuntary, includes the sensing of stimuli, sensations, body movements, and the functional changes in the organs innervated by the autonomic nervous system.
The autonomic nervous system is divided into two parts according to anatomical and physiological features: sympathetic and parasympathetic. The centers of the sympathetic nervous system (SNS) are located in the thoracic and lumbar segments of the spinal cord. The centers of the parasympathetic nervous system (PNS) are located in the midbrain and medulla oblongata and in the sacral segments of the spinal cord. The main nerve of the PNS—the one that transmits the influence of the PNS to many organs of the body—is the vagus nerve. The sympathetic and parasympathetic centers are subordinated to the centers of the autonomic nervous system located in the diencephalon—in the hypothalamus, which coordinates the functions of both parts of the autonomic nervous system and regulates metabolism and the functions of many organs and systems. The highest control over the autonomic nervous system is exerted by the centers of the cerebral hemispheres that ensure the integrated reaction of the body and maintain through the autonomic nervous system the necessary correspondence of intensity between the vital processes—metabolism, blood circulation, respiration, and so on—and the requirements of the body.
All the sympathetic and parasympathetic nervous pathways to the periphery are formed by two successively connected nerve cells (neurons). The cell body of the first neuron is in the midbrain, medulla oblongata, or spinal cord. The long process (axon) of the first neuron ends in nerve cells on the periphery that form ganglia. Within the ganglion is the cell body of the second neuron, whose process transmits impulses to the organ it innervates. (The fibers of the first neuron are called preganglionic; the fibers of the second are called postganglionic.) Thus, the nerves of the autonomic nervous system, unlike the motor nerves of the striated muscles which are continuous after emerging from the central nervous system, have a break in their fibers. The peripheral neurons of the SNS form ganglia on both sides of the spinal cord (sympathetic trunks) and in the neck and abdominal cavity. The peripheral neurons of the PNS are located directly in the organs they innervate. Every preganglionic fiber ends in many neurons in the ganglia, which greatly widens the zone of influence of the preganglionic neurons. Every postganglionic neuron has endings formed by various processes of preganglionic neurons. Hence the impulses reaching the nerve cell via different nerve fibers may be summated.
The preganglionic nerve fibers have a thin medullated, or myelin, sheath and are 2-3½microns in diameter—that is, they are far thinner than the motor fibers innervating the striated muscles. Most of the postganglionic fibers lack a myelin sheath and are even thinner. The nerve fibers of the autonomic nervous system are characterized by low excitability and a low rate of conduction of excitation. The endings of the parasympathetic and sympathetic fibers differ with respect to the chemical transmitters of nervous impulses (mediators) formed in them. The mediator acetylcholine is formed in the endings of all the parasympathetic nerve fibers and the preganglionic sympathetic nerve fibers, as well as the postganglionic sympathetic fibers innervating the sweat glands. The mediator norepinephrine is formed in the endings of the postganglionic sympathetic fibers, except those innervating the sweat glands. The English physiologist H. Dale suggested dividing the nerve fibers into cholinergic and adrenergic ones, depending on the chemical nature of the mediators formed in their endings. After transection and degeneration of sympathetic or parasympathetic nerves, the sensitivity of the denervated organs to the corresponding mediators increases sharply. The organ deprived of sympathetic innervation is particularly sensitive to norepinephrine and epinephrine, while the organ deprived of parasympathetic innervation is particularly sensitive to acetylcholine.
Excitation of the SNS stimulates the body; excitation of the PNS helps to restore the resources used up by the body. The SNS and PNS are functional antagonists and therefore exert opposite influences on many organs. Thus, under the influence of impulses traveling along the sympathetic nerves, heart contractions accelerate and intensify, blood pressure in the arteries rises, glycogen is broken down in the liver and muscles, blood glucose increases, the pupils dilate, sensitivity of the sensory organs and efficiency of the central nervous system increase, the bronchi constrict, stomach and intestinal contractions are inhibited, the secretion of gastric and pancreatic juices diminishes, and the urinary bladder relaxes and evacuation is inhibited. Under the influence of impulses arriving via the parasympathetic nerves, heart contractions slow and weaken, arterial pressure drops, blood glucose decreases, stomach and intestinal contractions are stimulated, the secretion of gastric and pancreatic juices is intensified, and so on. The activity and state of certain organs are controlled solely by the sympathetic nerves—for example, the sweat glands, most blood vessels (except those of the tongue, salivary glands, and genitalia, the blood vessels of which are constricted by sympathetic nerves and dilated by parasympathetic nerves), adrenal glands, and uterus.
The autonomic nervous system has a threefold effect on organs: activating, correcting, and adaptotrophic. The activating effect is manifested by impulses stimulating the activity of an organ that functions periodically (for example, stimulation of secretion by the sweat glands under the influence of sympathetic nerves). The correcting effect is manifested by an intensification or weakening of the activity and state of excitation (tonus) of organs that possess automatism and that function continuously or are in a constant state of excitation (for example, the effect of the autonomic nervous system on heart action and the state of the blood vessels). The adaptotrophic function of the autonomic nervous system, mainly the SNS, consists of regulating metabolism and the functional state (excitability, efficiency) of organs and tissues; it prepares the body for activity and adjusts the work of the organs to external conditions and current needs of the body.
The role of the SNS in adapting the body to various situations requiring physical exertion was demonstrated in experiments on animals from whom both sympathetic trunks and all the sympathetic ganglia were removed (sympathectomy). While resting, such animals scarcely differ from normal ones. But during vigorous muscular exertion, overheating, chilling, loss of blood, or emotional stress, animals whose organs have been deprived of sympathetic influences have little endurance. Because of impairment of thermoregulation, they tolerate abrupt fluctuations in external temperature less easily than do normal animals. They become chilled sooner when exposed to cold and become overheated sooner when exposed to heat.
The autonomic nervous system (mainly its sympathetic division) becomes excited in extreme, life-threatening situations that require maximum exertion of the body’s forces—for example, suffocation, loss of blood, attack by an enemy, and trauma:—and in emotional reactions. This accounts for the acceleration and intensification of heart contractions, dilatation of the skin blood vessels, and reddening of the face in joy; for the pallor, sweating, gooseflesh, inhibition of gastric secretion, and change in intestinal peristalsis in fear; for pupil dilatation in anger, pain; and so on.
The physiological manifestations of emotions are caused mainly by excitation of the SNS. For example, in emotional stress and excitation of the central nervous system provoked by pain, impulses reaching certain endocrine glands along the fibers of the autonomic nervous system intensify the secretion of hormones into the blood. The American physiologist W. Cannon showed that emotional reactions increase the entry into the blood of epinephrine from the adrenal glands under the influence of impulses reaching them from the sympathetic nerves. Excitation of the autonomic centers of the hypothalamus resulting from pain stimulates the entry of various hormones from the pituitary, thyroid, and other glands into the blood. Release into the blood of epinephine (which affects many organs like the sympathetic nerves), vasopressin (which constricts the blood vessels and halts urination), and other hormones under the influence of the autonomic nervous system supplements and intensifies its direct action on the functions of various organs stimulated by the entry of nerve impulses. This is how neurohormonal regulation of the body’s activity is effected. Thus, the activity of the autonomic nervous system consists of the complementary interaction of its sympathetic and parasympathetic divisions. The SNS largely stimulates the processes associated with the release of energy (dissimilation), with activity, whereas the PNS stimulates the processes associated with the accumulation of energy and matter (assimilation).
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Speranskaia, E. N. Voprosy fiziologii vegetativnogo otdela nervnoi sistemy. Moscow-Leningrad, 1961.
Rosin, Ia. A. Fiziologiia vegetativnoi nervnoi sistemy. Moscow, 1965.
Gellhorn, E., and G. Loofbourrow. Emotsii i emotsional’nye rasstroistva. Moscow, 1966. (Translated from English.)
Burn, J. H. The Autonomic Nervous System, 2nd ed. Oxford, 1965.
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E. B. BABSKII