(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).