sympathetic nervous system(redirected from Sympathetic tone)
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sympathetic nervous system:see nervous systemnervous 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.
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Sympathetic nervous system
The portion of the autonomic nervous system concerned with nonvolitional preparation of the organism for emergency situations. See Autonomic nervous system
The sympathetic nervous system is best understood in mammals. It consists of two neuron chains from the thoracic and lumbar regions of the spinal cord to viscera and blood vessels. The first or preganglionic neuron has its cell body in the spinal cord and sends its axon to synapse with a postganglionic sympathetic neuron, which lies either in a chain of sympathetic ganglia paralleling the spinal cord or in a sympathetic ganglion near the base of the large blood vessels vascularizing the alimentary viscera. The postganglionic axons are longer than the preganglionic axons and extend to glands or smooth muscles of viscera and blood vessels. Sensory visceral nerve fibers innervate blood vessels and viscera and carry sensory information to the spinal cord, thus providing a visceral reflex (see illustration). See Nervous system (vertebrate) See Parasympathetic nervous system
Sympathetic Nervous System
in vertebrates, including man, a part of the autonomic nervous system consisting of sympathetic nerve centers, the right and left sympathetic trunks (which extend along the spine), ganglia, and nerve branches that unite the ganglia with each other and with the effectors and spinal cord.
The sympathetic trunk is a chain of ganglia joined by inter-ganglial commissures that lies (on the right or left) on the bodies of the vertebrae. Each ganglion is also joined to one of the spinal nerves. Fibers of the sympathetic nervous system innervate all organs and tissues of the body without exception. The centers of the sympathetic nervous system are located in the thoracic and lumbar segments of the spinal cord. Sympathetic nuclei, which form the lateral gray horns of the spinal cord, are found only in the 15th or 16th segments, from the last cervical or first thoracic segments to the third lumbar segment. The nuclei are regarded as a working apparatus subordinate to the supraseg-mental formations localized in the medulla oblongata and the hypothalamus, which are both controlled by the cerebral cortex. The cerebellum is especially important in the physiology of the sympathetic nervous system and in the coordination of the processes governed by the system.
The sympathetic nervous system is an efferent system that conducts impulses to various internal organs. Most scientists deny the existence of sympathetic afferent fibers, although many have produced proof of their existence. Sympathetic fibers pass through the abdominal cavity as part of the greater, lesser, and lumbar splanchnic nerves. Afferent nerves, which conduct impulses from internal organs, are represented in the cortex of the large cerebral hemispheres and in the subcortical ganglia.
Sympathetic nerve impulses follow a bineural path from the central nervous system to the organs performing the particular function. The first neuron is located in the lateral horns of the spinal cord. The axons, or processes, of the first neuron (preganglionic fibers) run from the spinal cord, through the ventral roots of the corresponding segments, and into mixed spinal nerves. As constituents of white fibrous branches, the axons extend from the spinal nerves to the corresponding ganglion of the sympathetic trunk, where some of the fibers terminate in synapses on effector neurons; when this happens, each preganglionic fiber is in contact with as many as 30 nerve cells. The other preganglionic fibers pass through the ganglia of the sympathetic trunk without terminating on the cells of the trunk. Together with other fibers these preganglionic fibers form the greater, lesser, and lumbar splanchnic nerves, which enter the prevertebral sympathetic ganglia. Certain preganglionic fibers pass straight through even these ganglia and, reaching the working organ, make a break in the nerve ganglia of the organ’s walls.
The second effector neuron is located in the peripheral sympathetic ganglia. Its processes, which are called postganglionic fibers, enter the innervated organ. The second neuron is located in the paravertebral ganglia or the prevertebral ganglia; the latter include the celiac ganglia and the inferior mesenteric ganglion, which are located at a distance from the central nervous system, near the internal organs. Postganglionic fibers enter a spinal nerve through gray fibrous branches and reach the innervated organ as a constituent of the nerve. Consequently, a break in each efferent sympthetic pathway in the arc closed in the spinal cord happens only once—either in the ganglion of the sympathetic trunk or in ganglia that are at a distance from the spine. In addition to the sympathetic arc closed in the spinal cord, there are also short sympathetic reflex arcs that are closed in the peripheral sympathetic ganglia (of the solar plexus, by the inferior mesenteric).
The speed of the conduction of excitation in the sympathetic preganglionic fibers and especially in the sympathetic postganglionic fibers is many times less than in somatic fibers, averaging approximately 1–3 m/sec. A strong stimulus is needed to produce effects in sympathetic fibers. As a rule, an excitation that arises in the sympathetic nervous system involves a large number of neurons, and as a result, the effects of the stimulus are not localized in a definite organ but encompass extensive areas. The stimulation of sympathetic fibers produces reactions characterized by their comparatively slow and prolonged nature and by slow and prolonged extinction of their elapsing processes. Many substances, including ganglionic blocks and ergot preparations, suppress the effects of the excitation of the sympathetic nervous system.
Certain chemical preparations have the same effect on organs and tissues as does the stimulation of the sympathetic nerves. When sympathetic nerves are stimulated, substances whose action is similar to that of certain chemical preparations are secreted by the terminal formations of postganglionic sympathetic fibers. The mediator acetylcholine is formed in the endings of all preganglionic fibers and in the endings of postganglionic fibers that innervate the sweat glands, and norepinephrine is formed in the endings of all postganglionic fibers except those that innervate the sweat glands.
The influences of the sympathetic and parasympathetic nervous systems on the activity of an organ are often directly opposed to one another. The stimulation of the sympathetic fibers that innervate various organs causes such standard reactions as the acceleration and intensification of the heartbeat, the dilatation of the pupil, mild lacrimation, the contraction of the smooth-muscle fibers (pilomotors) that raise the hairs, the secretion of the sweat glands, the sparse secretion of viscous saliva and gastric juice, and the inhibition of contractions and attenuation of the tonus of the smooth musculature of the stomach and intestine, except the region of the ileocecal sphincter. The stimulation of sympathetic fibers also results in the relaxation of the musculature of the urinary bladder and the inhibition of the contractions of its obturator sphincter, the dilatation of the coronary vessels of the heart, and the stenosis of the small arteries of the abdominal organs, skin, lungs, and brain. Also possible are changes in the excitability of receptors and various sections of the central nervous system and increases in the excitability and the force of contraction and changes in mechanical properties of fatigued skeletal muscle.
Neurons of the sympathetic nervous system that act on functioning organs are in a state of constant tonic excitation as a result of the interaction of unconditioned and conditioned reflexes, which are effected by higher sections of the central nervous system. Tonic impulses of the sympathetic nervous system are extremely important in maintaining the stability of the internal environment of the body, or homeostasis. The reflex correlation between all internal organs is ensured through the sympathetic fibers and centers. Reflexes that activate the sympathetic nervous system may arise from the stimulation of visceral or somatic nerves. Thus, with viscero-visceral reflexes, excitation develops and terminates in the internal organs; for example, the stimulation of the peritoneum causes a deceleration of heart activity. With visceromotor reflexes, excitation passes from internal organs to the skeletal musculature (stimulation of the peritoneum increases the tonus of the abdominal muscles).
Animals whose sympathetic trunks and ganglia have been completely removed (sympathectomized) are outwardly only slightly different from normal animals, although under certain loads, for example, muscular work or chilling, they are less hardy. Thus, the sympathetic nervous system regulates the functional state of tissues and adapts them to perform their functions under any conditions. The sympathetic nervous system chiefly stimulates processes associated with the discharge of energy in the body and with high levels of activity. The physiological manifestations of emotions are associated predominantly with the excitation of the sympathetic nervous system.
REFERENCESOrbeli, L. A. Lektsii po fiziologii nervnoi sistemy. Moscow-Leningrad, 1938.
Chernigovskii, V. N. Interotseptory. Moscow, 1960.
Bulygin, I. A. Zamykatel’naia i retseptornaia funktsii vegetativnykh gangliev. Minsk, 1964.
Baklavadzhian, O. G. Vegetativnaia reguliatsiia elektricheskoi aktivnosti mozga. Leningrad, 1967.
Nozdrachev, A. D. Kortikosteroidy i simpaticheskaia nervnaia sistema. Leningrad, 1969.
Skok, V. I. Fiziologiia vegetativnykh gangliev. Leningrad, 1970.
See also references under VEGETATIVE NERVOUS SYSTEM.
A. D. NOZDRACHEV