Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
SYMPATHETIC SYSTEM
Online EncyclopediaEncyclopedia Home :: SUS-TAV
del.icio.us it!
|
SYMPATHETIC See also:SYSTEM , in See also:physiology. By the " sympathetic system " is understood a set of nerves and ganglia more or less sharply marked off from the cerebro-See also:spinal, both functionally and anatomically. (For See also:anatomy see See also:NERVOUS SYSTEM.) Formerly it was thought more See also:independent from the See also:rest of the See also:general nervous system than See also:recent discoveries have found it actually to be. It used to be supposed that the ganglia of the sympathetic system were analogous in See also:function to the See also:great central nervous masses forming the See also:brain and spinal See also:cord. These latter masses, as now becomes more and more evident, are the only structures in which occurs the See also:work of transmuting afferent-See also:nerve impulses into efferent-nerve impulses with all the accompanying changes in intensity, See also:rhythm; &c., which make up reflex See also:action. Such functions, it is now known, are not attributable to sympathetic ganglia. These last are structures in which one neurone makes communication with other neurones. To that extent, therefore, redistribution of nervous impulses does occur in them, impulses arriving by a few neurones being distributed so as to affect many. But the sympathetic ganglia are not the seat of reflex action. The sympathetic system is now known to consist entirely of conducting paths which, like the nerve-trunks of the cerebro-spinal system, merely conduct nerve impulses either toward the great nervous centres of the spinal cord and brain, or, on the other See also:hand, away from those great centres. In the cerebro-spinal nerves the preponderance of the See also:conduction is toward the centres, in the sympathetic system the preponderance of conduction is away from the centres. More is known of the sympathetic system from its efferent aspect than its afferent, and we shall consider the former first. One great difference between the efferent paths of the sympathetic and those of the See also:ordinary cerebro-spinal system is that the former carry nervous impulses not only to See also:muscular See also:tissue but to secreting glands, whereas the latter convey them to muscle only, indeed only to muscle of the striated See also:kind. Another difference is that the efferent path which the sympathetic affords from the great central nervous centres to its muscles and glands consists always of two nerve-cells or neurones, whereas the efferent path afforded by the cerebro-spinal motor nerves consists of one neurone only. The two neurones forming the sympathetic path are so arranged that one of them whose See also:cell-See also:body lies in the spinal cord has a See also:long axone-See also:process passing out from the cord in the motor spinal See also:root, and this extends to a See also:group of nerve-cells, a sympathetic ganglion, quite distant from the spinal cord and somewhere on the way to the distant See also:organ which is to be innervated. In this ganglion the first sympathetic neurone ends, forming functional connexion with ganglion cells there. These ganglion cells extend each of them an axone process which attains the organ (muscular cell or gland cell), which it is the See also:office of the sympathetic path to reach and See also:influence. The axone-process of the first nerve cell is a myelinated nerve-fibre extending from the spinal cord to the ganglion; it constitutes the pre-ganglionic fibre of the conduction See also:chain. The axone-process of the second nerve-cell, that is the neurone whose cell-body lies in the ganglion, is usually non-myelinate and constitutes the See also:post-ganglionic fibre of the chain. This construction, characteristic as it is of the sympathetic efferent path, has been found also in certain other efferent paths outside the sympathetic proper. And as these other efferent paths convey impulses to the same kind of See also:organs and tissues as do those of the sympathetic itself, it has been proposed to embrace them and the sympathetic under one name, the autonomic system. This See also:term includes all the efferent paths of the entire body excepting only those leading to the voluntary muscles. That the term " autonomic system " is not merely a convenience of nomenclature, but really represents a physiological entity, seems indicated by the action of nicotin. This See also:drug acts selectively on the autonomic ganglia and not on the cerebrospinal. In the former it paralyses the nexus between pre-ganglionic and post-ganglionic fibre. It is by taking See also:advantage of this See also:property that many of the recent researches which have done so much to elucidate the sympathetic have been executed. The term " autonomic system " must not be taken to imply that this system is independent of the central nervous system. As mentioned above in regard to the sympathetic, that is not the See also:case. The autonomic system is closely connected with the central nervous system through the ordinary channel of the nerve-roots, spinal and See also:cranial. It may, in fact, be regarded asan appendage of the cranial and spinal roots, or rather of certain of them, for with a considerable proportion of their number it is not connected. The sympathetic is that See also:part of the autonomic system which is connected with the spinal roots from the second thoracic to the second lumbar inclusive (See also:man). Its ganglia are divided by anatomists into the vertebral, those which See also:lie ,as a See also:double chain on the ventral See also:face of the vertebral See also:column, and those which lie scattered at various distances among the viscera, the pre-vertebral. See also:Langley has shown that there is no essential difference between these except that the vertebral send some of their post-ganglionic See also:fibres into the spinal nerves, whereas the latter send all their fibres to the viscera. The sympathetic sends its post-ganglionic fibres r. To the muscular coats of the whole of the alimentary See also:canal from the mouth to the rectum; to the glands opening into the canal from the salivary glands in front back to the intestinal glands; to the See also:blood vessels of the whole of the canal from mouth to anus inclusive. 2. To the generative organs, See also:external and See also:internal, and to the muscular coats of the urinary See also:bladder. 3. To the skin; (a) to its blood vessels, (b) to its cutaneous glands, (c) to unstriated muscle in the skin, e.g. the erectors of the hairs. 4. To the See also:iris muscles and blood vessels of the eyeball. The sympathetic nervous system is sometimes called the visceral. It will be seen from the above that this term is not well suited in some respects, because the sympathetic supplies many structures which are not visceral. Another objection is that a great See also:deal of important nerve-See also:supply to the viscera is furnished by parts of the autonomic system other than sympathetic. That the sympathetic does, however, of itself constitute a more or less homogeneous entity is indicated by a curious fact. The substance adrenalin, which is the active constituent of extracts of the See also:adrenal gland, has the property when introduced into the circulation of exciting all over the body just those actions which stimulation of the efferent fibres of the sympathetic causes, and no others. It is possible that when a nerve is stimulated some body at the nerve ending is set See also:free, and this by combining with another chemical substance induces activity in the end organ (gland or muscle). It may be that when a sympathetic nerve is excited adrenalin is set free and combines with some substance which induces activity. The rest of the autonomic system consists of two portions, a cranial and a sacral, so called from their proceeding from cranial and sacral nerve-roots respectively. The cranial portion is subdivided into a part belonging to the See also:mid-brain and a part belonging to the See also:hind-brain. The ciliary ganglion belonging to the eyeball is the ganglion of the former part, and its post-ganglionic fibres innervate the iris and the ciliary muscles. The hind-brain portion gives pre-ganglionic fibres to the facial (intermedius) glossopharyngeal and vagus nerves; its post-ganglionic See also:distribution is to the blood vessels of the mucous membrane of the mouth and See also:throat, to the musculature of the See also:digestive See also:tube from the See also:oesophagus to the See also:colon, to the See also:heart, and to the musculature of the See also:windpipe and lungs. The sacral part of the autonomic system issues from the spinal cord with the three foremost sacral nerves. Its ganglia are scattered in the neighbourhood of the pelvic organs, which they innervate. The distribution of its post-ganglionic fibres is to the See also:arteries of rectum, anus and external genitalia, to the musculature of colon, rectum, anus and the urinary bladder, and to that of the external genitalia. The part played by the sympathetic and the rest of the autonomic system in the See also:economy of the body is best considered by following broad divisions of organic functions. Movements of the Digestive Tube.—It is those movements of alimentation not usually within range of our consciousness which the. autonomic system regulates and controls. Nor is its See also:control over them apparently essential or very See also:complete. For instance, the pendular and peristaltic movements of the See also:intestine still go forward when all nerves reaching the viscus have been severed. Extirpation of the abdominal sympathetic has not led to obvious disturbance of digestion or See also:nutrition in the See also:dog. It is noteworthy that the sympathetic inhibits See also:con-See also:traction of the musculature of the See also:stomach and intestine, while the other, the vagus, portion of the autonomic system excites it. The actions of these two components of the system are, therefore, mutually opposed on the viscera innervated by both. Action on the Circulation.—The blood supply of most organs is under the control of vaso-constrictor nerves. All vaso-constrictor nerves are sympathetic. Organs to which vaso-constrictor nerves are supplied either poorly or not at all are the lungs, heart, See also:liver, brain and probably the skeletal muscles. The blood vessels of certain parts of the body have, in addition to vaso-constrictor nerves, nerves which relax their muscular See also:wall, vaso-dilatator nerves. The latter are never furnished by the sympathetic, they are in the mucous membranes and glands at the oral end of the body furnished by the cranial portion of the autonomic system. In regions at the aboral end of the body they are furnished by the sacral portion of the autonomic system. Elsewhere the vaso-dilatators when See also:present are derived from the nerve-cells of the spinal ganglia (Bayliss). The control of the calibre of the blood vessels by the autonomic system is of importance in several well-ascertained respects. By constricting the blood vessels of the viscera the system is able to favour an increase of blood supply to the brain. A noteworthy instance of such an action occurs when the erect attitude is assumed after a recumbent posture. Were it not for vaso-constriction in the abdominal organs the blood would then, under the action of gravity, sink into the more dependent parts of the body and the brain would be relatively emptied of its supply, and fainting and unconsciousness result. Again, it is essential to the normal functioning of the organs of warm-blooded animals that their temperature, except in the See also:surface layer of the skin, should be kept See also:constant. Part of the regulative mechanism for this lies in nervous control of the quantity of blood flowing through the surface See also:sheet of the skin. That sheet is a cool See also:zone through which a greater or smaller quantity of blood may, as required, be led and cooled. By the sympathetic vaso-constrictors the capacity of these vessels in the cool zone can be reduced, and thus the loss of See also:heat from the body through that channel lessened. In See also:cold See also:weather the vasoconstrictors See also:brace up these skin vessels and lessen the loss of heat from the body's surface. In hot weather the tonus of these nerves is relaxed and the skin vessels dilate; a greater proportion of the blood then circulates through the comparatively cool skin-zone. The heart itself is but a specialized part of the blood-vascular tubing, and its musculature, like that of the arteries, receives motor nerves from the sympathetic. These nerves to the heart from the sympathetic are known as the accelerators, since they quicken and See also:augment the beating of the cardiac muscle. The heart receives also nerves from the cranial part of the autonomic system, and the influence of these nerves is antagonistic to that of the sympathetic supply. The cranial autonomic nerves to the heart pass via the vagus nerves and lessen the beating of the heart both as to See also:rate and force. These inhibitory nerves of the heart are analogous to the dilatator nerves to the blood vessels, which, as mentioned above, come not from the sympathetic, but from the cranial and sacral portions of the autonomic system. Skin-glands.—In See also:close connexion with the temperature regulating function of the sympathetic stands its influence on the sweat secreting glands of the skin. Secretory nerves to the sweat glands are furnished apparently exclusively by the sympathetic. Pilomotor Nerves.—The skin in many places contains muscle of the unstriped kind. Contraction of this cutaneous muscular tissue causes knotting of the skin as in " See also:goose-skin," and erection of the hairs as in the See also:cat, or of the quills as in the See also:hedgehog and See also:porcupine. The efferent nerve-fibres to the unstriped muscles of the skin are always furnished by the sympathetic (pilomotor nerves, &c.). Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] SYMONS, GEORGE JAMES (1838-1900) |
[next] SYMPHONIA (Gr. o-vµcfrmvla) |