SYMPATHETIC SYSTEM, in physiology. By the "sym pathetic system" is understood a set of nerves and ganglia more or less sharply marked off from the cerebro-spinal, both func tionally and anatomically. (For anatomy see NERVOUS SYSTEM.) Formerly it was thought more independent from the rest of the general nervous system than recent discoveries have found it actually to be. 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 hand, away from those great centres. In the cerebro-spinal nerves the preponderance of the conduction is toward the centres, in the sympathetic system the preponder ance 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 ordinary cerebro-spinal system is that the former carry nervous impulses not only to muscular tissue but to secret ing glands, whereas the latter convey them to muscle only, indeed only to muscle of the striated 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 cell-body lies in the spinal cord has a long axon-process passing out from the cord in the motor spinal root, and this extends to a group of nerve-cells, a sympathetic ganglion, quite distant from the spinal cord and somewhere on the way to the distant organ which is to be innervated. In this ganglion the first sympathetic neurone ends, forming functional connection with ganglion cells there. These ganglion cells extend
each of them an axon-process which attains the organ (muscular cell or gland cell), which it is the office of the sympathetic path to reach and influence. The axon-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 con duction chain. The axon-process of the second nerve-cell, that is the neurone whose bell-body lies in the ganglion, is usually non-myelinate and constitutes the 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 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 sys tem. This 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 conven ience of nomenclature, but really represents a physiological en tity, seems indicated by the action of nicotine. This drug acts selectively on the autonomic ganglia and not on the cerebro spinal. In the former it paralyses the nexus between pre-gang lionic and post-ganglionic fibre. It is by taking advantage of this 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 case. The autonomic system is closely connected with the cen tral nervous system through the ordinary channel of the nerve roots, spinal and cranial. It may, in fact, be regarded as an appendage of certain of the cranial and spinal roots.