Book - The Nervous System of Vertebrates (1907) 13
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Chapter XIII. The Sympathetic System
To understand the sympathetic sys.tem it is necessary to begin with the study of its development. At a time when the spinal ganglia and the dorsal and ventral nerve roots are formed there is noticed on the mesial side of the composite ventral ramus of each nerve a collection of ganglion cells which later forms the sympathetic ganglion. A stage a little earlier than this has been recognized in mammals. The beginning of the development of the sympathetic is the outgrowth of fibers from the ventral root and also from the dorsal root ganglion, in the direction of the aorta. Then cells from the spinal ganglion are seen to migrate along these fibers. These constitute the group of cells first mentioned. As development proceeds the group of cells moves away from the spinal ganglion toward the aorta, but remains connected with the spinal nerve by the strand of fibers which grew out first. There are thus formed a pair of ganglia in each segment, lying below the notochord and lateral to the aorta, and connected with the spinal nerves by the rami communicantes. The ganglia are those known as the ganglia of the chain but at this stage they are not yet connected longitudinally into a chain.
The process of growth of fibers and the migration of cells along them continues beyond these chain ganglia and results on the one hand in joining the ganglia together by longitudinal cords, and on the other hand in the formation of additional ganglia. The cells which migrate from the chain ganglia form first certain median prevertebral ganglia or plexuses. These are in man the cardiac, solar and hypogastric plexuses. Further migration of cells carries them to or even among the tissues of several of the organs innervated by the sympathetic, where peripheral ganglia are formed. Examples of these are the small ganglia in the heart, the plexuses of Auerbach and Meissner in the wall of the digestive canal, etc. While the sympathetic elements are migrating in this way to their definitive positions, some of the cells of the chain ganglia and perhaps of other ganglia send fibers back along the rami communicantes into the spinal ganglia or the rami of the spinal nerves. The fibers which grow out from the spinal nerves acquire myelin sheaths and so become white fibers, and the part of each ramus communicans formed by them is known as the white ramus communicans. The fibers which grow from the sympathetic cells back into the spinal ganglion or nerve form the gray ramus communicans.
In selachians a pair of chain ganglia is formed in each segment in the trunk and for some distance into the tail. The anterior six pairs of trunk ganglia disappear during development. This Hoffmann attributes to the shifting backward of the heart and other organs with reference to the spinal column and nerves. In man the three cervical ganglia which are found in the adult are supposed to be formed by the fusion of a larger number of primary segmental ganglia of the chain. It is important to notice that there is not a complete segmental series of white rami communicantes hi mammals. In the cervical segments from which the spinal accessory nerve roots take their origin there are few or no myelinated fibers in the rami communicantes, while in the segments immediately following the last root of the accessory nerve there appears suddenly a great increase in the number of such fibers. A second increase in these fibers begins at the caudal border of the brachial plexus and extends to the beginning of the lumbar plexus. The fibers of the white rami are small myelinated fibers which are excito-motor or excito-glandular in function and are in a broad way serially homologous with the efferent fibers of the vagus and spinal accessory nerves. The question of this homology will be taken up again a little later (p. 215).
The development of the sympathetic system in the head has not been well studied. Some indications as to its source can be obtained by considering the character of the cranial nerves from which it is derived, and from its structure in the most primitive vertebrates. In selachians the development of the ciliary ganglion has been repeatedly described but it is still imperfectly understood.
It seems to be formed in part from the ganglionic anlage known as the Nervus thalamicus, and early comes into relation with the ophthalmicus and oculomotorius nerves. In the chick it is derived in part from the neural tube and hi part from the most anterior portion of the ganglion of the ophthalmicus profundus nerve. The development of the other sympathetic ganglia of the head has not been directly followed. Ganglia are present in bony fishes in connection with each of the dorsal cranial nerves, from which presumably they are derived during development. As these nerves have no somatic motor component it is evident that this 'component does not enter into the head sympathetic. It is therefore probable that the fibers which grow out from the ventral spinal nerves to help form the sympathetic are not somatic motor fibers. Indeed, it is known from the adult structure in mammals that the fibers which go by the ventral spinal roots to the sympathetic take their origin from cells in the visceral motor column of the cord. In cyclostomes the only connections of the sympathetic with the cranial nerves are with the visceral portions of the facialis and vagus. Indeed, from what we know of the development and structure of the sympathetic system in lower vertebrates, the general conclusion must be drawn that it is an outgrowth from the visceral nerves, including efferent fibers and ganglion cells. The formation of a relatively distinct system is due primarily to the migration of ganglion cells along the primitive nerves of the viscera toward the areas supplied by them.
In describing the structure of the sympathetic system four types of nerve elements must be considered: (i) sensory fibers whose ganglion cells are in the spinal ganglia; (2) efferent fibers whose cells of origin are in the spinal cord or brain; (3) sympathetic excitatory cells; and (4) sympathetic sensory cells. The description of these elements will be more clear if Fig. 108 is consulted in connection with the text.
Fig. 108. A diagram of the sympathetic system and the arrangement of its neurones in a mammal. On the left are shown the typical elements of a trunk segment including the sympathetic system. On the right are shown only the somatic afferent and efferent neurones of the spinal nerve. Of the sympathetic system are shown the white and gray rami, three ganglia of the chain, one prevertebral ganglion and one peripheral ganglion. The symbols used are explained in the figure. In most respects the diagram follows one of Huber's figures.
(1) Sensory fibers whose ganglion cells are in the spinal ganglia. These fibers are the visceral sensory fibers already described in a previous chapter (p. 156). They are the largest of the myelinated fibers running in the sympathetic nerves and may be seen to pass through one or more of the sympathetic ganglia without forming any connection with the sympathetic cells. The fibers are finally distributed by widespread branches to the mucosa of the viscera. Some of these fibers also enter the Pacinian corpuscles as the fibers of those sense organs. The distribution of these fibers and their lack of histological connection with the sympathetic ganglia show that they do not belong properly to the sympathetic system. The truth is rather that they are older than the sympathetic system and that the sympathetic ganglia are placed along the course of these primitive visceral sensory fibers. Centrally, these fibers enter the visceral sensory column of the cord or brain as already described.
Fig. 109. Diagram illustrating the spinal representation of the sympathetic nerves in a mammal. From Onuf and Collins. CL Clarke's column; intm. Z., intermediate zone; Becht. N., Bechterew's nucleus; lat., lateral horn cell-group; pare., paracentral cell-group.
(2) Efferent fibers whose cells of origin are in the spinal cord or brain. The location of the cells of origin of these fibers has been determined with accuracy in the spinal cord of the cat. As indicated in Fig. 109 they are located in a zone of the gray matter between the dorsal and ventral horns and extending from the central canal to the lateral horn and the base of the dorsal horn. This is the portion of the gray matter which has previously been called the visceral efferent column (p. 200). As already stated, the fibers from this column in lower vertebrates pass out through the dorsal roots, but' in the mammals which have been most used for the study of the sympathetic they pass out by way of the ventral roots.
The fibers are small myelinated fibers, usually less than 4 p in diameter, which enter the ganglia of the chain and find endings in relation with the cells of these or other sympathetic ganglia. They may (a) end in the chain ganglion first entered, (b) run through it to end in another ganglion of the chain, (c) end in one of the prevertebral ganglia, or (d) in one of the peripheral ganglia. During their course the fibers may give collaterals to one ganglion and pass on to end in another. The method of ending of these fibers is important. They pierce the capsule of the sympathetic cells and their branches interlace to form more or less complex plexuses or baskets immediately in contact with the sympathetic cells. By means of these pericellular baskets the impulses sent out from the central nervous system are transferred to the sympathetic excitatory cells. Such endings are found in the prevertebral and peripheral ganglia as well as in the ganglia of the chain, and it is believed that the great majority if not all of the excitatory cells of the sympathetic are thus brought under the direct influence of the central nervous system.
(3) Sympathetic excitatory cells. The sympathetic cells have in general the same forms as cells in other parts of the nervous system. They may have a single process which is a neurite, or a neurite and one dendrite, or a neurite and several dendrites. The last is the rule -for the great majority of cells, at least in mammals. The cell-body is surrounded by a nucleated capsule which is pierced by the dendrites. Outside the capsule the dendrites divide and subdivide into very delicate branches which interlace with those of other cells to form a rich plexus. In most cases the dendrites end within the ganglion in which the cell lies, but it has been shown that occasional dendrites pass along a sympathetic nerve and reach another ganglion, in which they break up into end-branches. It appears that the dendrites of sympathetic cells play a minor part hi the reception of impulses and it is not clear that either the pericellular plexuses of dendrites or the passage of dendrites from one ganglion to another has any functional significance. The neurite of the sympathetic cell arises either from the cell-body or from a dendrite and may or may not become myelinated. When it is myelinated it is so fine as still to be distinguished from the smallest fibers of cerebrospinal origin and the myelin sheath may extend for a longer or shorter part of the course of the fiber. The neurites, after a longer or shorter course in the splanchnic nerves or by way of the gray rami communicantes and one of the peripheral rami of the spinal or cranial nerves, end in (a) involuntary muscle, (b) heart muscle, (c) glands, or (d) other sympathetic ganglia. All smooth muscle, whether in the wall of the alimentary canal, in the ducts of glands, in the urinogenital system, in blood vessels, the skin or the eye is innervated by neurites from sympathetic cells. The ending is by means of simple branches often with small knobs or enlargements. The heart muscle is innervated by neurites from the cells in the intrinsic sympathetic ganglia of the heart. The endings may be more complex, somewhat like those in striated muscle. The secreting cells of glands are innervated by simple endings of sympathetic neurites which enter the glands along the ducts or blood vessels or which come from neurones situated in the glands themselves. The ending of the neurites of sympathetic cells in other sympathetic ganglia is still a matter of dispute. Histologists have described the endings of neurites upon the dendrites of sympathetic cells, but physiologists have obtained no functional evidence to corroborate the supposition that these are the endings of neurites arising from sympathetic neurones. Inasmuch as the efferent cerebro-spinal fibers are universally believed to end in the pericellular baskets within the capsules of sympathetic cells, it is probable that the endings in connection with the dendrites come from sympathetic cells and that suitable forms of experiment for determining their functions have not yet been devised. It is also uncertain as yet whether these fibers run from one of the chain ganglia to a more peripheral ganglion or from a more peripheral to a more proximal ganglion. The important question regarding these endings is whether the visceral excitatory chain consists of more than two links. The physiologists claim that only one sympathetic cell intervenes in any case between the efferent cerebro-spinal fiber and the muscle or gland innervated. The existence of sympathetic endings in sympathetic ganglia, if clearly established, would seem to show that in some cases two such neurones enter into the excitatory chain.
(4) Sympathetic sensory cells. Certain cells in the peripheral ganglia, as in Auerbach's plexus, have longer dendrites than those of ordinary sympathetic cells and these dendrites are supposed to be distributed to the mucosae and to serve as sensory fibers. The neurites of these cells pass through one or more sympathetic ganglia to which they give branches. These branches enter into the plexus of dendrites in the ganglion and may serve to arouse peripheral reflexes by stimulating the excitatory cells in the ganglion. It is stated as probable (Dogiel) that these neurites run on through the gray rami communicantes and form the pericellular endings which are known to occur in the spinal ganglia of several classes of vertebrates. These are complex endings immediately around the bodies of certain spinal ganglion cells which are described as cells of type II. These second type cells have neurites which break up into branches within the spinal ganglion and form pericellular baskets about the bodies of ordinary spinal ganglion cells. The functions and the structural arrangement of the sensory sympathetic cells and their supposed connection with the spinal ganglion cells require further study.
Fig. no. A diagram to illustrate Langley's "axone reflex". After Langley. Preg. fiber, preganglionic fiber; Postg. fiber, postganglionic fibers; Inf. mes. gang., inferior mesenteric ganglion; b, bladder.
The statement is made (Onuf) that fibers which enter the visceral sensory column of the spinal cord are caused to degenerate by cutting the rami communicantes of the sympathetic. This would indicate that sensory cells situated in the sympathetic system send their neurites directly into the spinal cord. Such cells and fibers are not shown in Figure 108.
Another form of peripheral reflex has been suggested in which the branches of a single neurone only would be involved. It is supposed that an impulse may travel from a peripheral ganglion back along an efferent fiber and go out from it along a collateral to stimulate a sympathetic excitatory cell. This form of reflex is illustrated in Figure no. It must be said that this hypothesis seems very improbable in view of what we know of the polarity of neurones in other parts of the nervous system, and that there is little direct evidence in its support.
The essential feature of the sympathetic system is that in the visceral reflexes governing smooth muscle, heart muscle and glands, there is interpolated in the efferent limb of the reflex chain a peripheral neurone between the cerebro- spinal fiber and the organ innervated. There may be two such neurones interpolated and the sympathetic may carry out peripheral reflexes without the aid of cerebro-spinal elements, but these things are still uncertain, as are also the sensory sympathetic neurones. The sympathetic system does not to any great extent carry on independent or automatic functions. The great majority of its actions are directly aroused by efferent impulses coming from the brain or spinal cord and in response to the stimulation of visceral sensory fibers which run through, but have no connection with, the sympathetic ganglia. In a strict sense the sympathetic consists solely of the neurones whose cell-bodies lie in the various ganglia, the excitatory and sensory sympathetic cells above described. The afferent and efferent neurones whose cell-bodies lie respectively in the cranial or spinal ganglia and in the brain or cord belong properly to the cerebro-spinal system and not to the sympathetic.
The efferent cerebro-spinal fibers which end in sympathetic ganglia were given in the last chapter (p. 200) as the visceral efferent fibers of the spinal nerves. Similar fibers occur hi the cranial nerves in all vertebrates in which the sympathetic is developed in the head region. Where the head sympathetic is not developed the functions of the sympathetic, if performed at all, are probably performed by efferent visceral fibers in the cranial nerves without the intervention of peripheral neurones. There is, of course, the further alternative that there may exist peripheral ganglia which are not developed from the cranial ganglia in the typical manner. If in the lower fishes the head sympathetic is wanting there can scarcely be any sharp distinction drawn between the fibers which innervate smooth muscle and glands and those which innervate the striated muscles of the gill arches. Wherever the sympathetic is developed, however, such a distinction is very clear, since in the one case a peripheral neurone is interpolated in the reflex chain, in the other case not. The two sorts of fibers take their origin from the same zone in the brain and spinal cord but it happens that all the fibers which innervate striated muscle are confined to the cranial nerves, including the spinal accessory, while the sympathetic system extends through both head and trunk. The nuclei of origin for fibers which innervate striated muscle have probably become distinct from the nuclei of origin of sympathetic efferent fibers (see Figure 107). An explanation of this condition may be offered as follows. In primitive vertebrates all visceral muscles were presumably non-striated and the visceral reflex chain consisted of simple afferent and efferent limbs without peripheral neurones. The more active branchial muscles became striated and retained their direct innervation. For the innervation of the non-striated muscle and glands neurones migrated from the cranial and spinal ganglia and came to be interpolated in the efferent pathway. As to the morphological status of these migrated neurones of the sympathetic ganglia no sufficient explanation can at present be given. As to the sensory neurones, it would be not at all surprising that ganglion cells should migrate toward their innervation territory, from the spinal ganglia into the viscera, but it is just these sensory cells whose existence and arrangement are in dispute. The excitatory neurones in the sympathetic system belong to a special category. They seem to have had their origin from neural crest material, but whether as modified spinal ganglion cells or directly from indifferent ectodermal cells there is no evidence. The double origin of the ciliary ganglion in the chick is of interest in this connection.
Regardless of these unsettled theoretical questions it should be held clearly in mind that the sympathetic system is an offshoot or subsidiary portion of the visceral afferent and efferent divisions of the nervous system which has come to have a special structure and arrangement owing to the conditions of visceral activities.
Demonstration or Laboratory Work
- Dissect the sympathetic system of a frog and a mammal.
- Study sections of sympathetic ganglia prepared by the Golgi or methylene blue method.
Balfour, F. M.: Monograph on the Development of Elasmobranch Fishes. 1878.
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Dogiel, A. S.: Zur Frage iiber den feineren Bau des sympathischen Nervensystems bei den Saugethieren. Arch.f.mik.Anat., Bd. 46. 1895.
Gaskell, W. H.: On the Structure, Distribution and Function of the Nerves which innervate the Visceral and Vascular Systems. Jour, of Physiol., Vol. 7. 1886.
Van Gehuchten, A. : Les cellules nerveuses du sympathique. La Cellule, Tome 8. 1892.
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Hoffmann, C. K.: Zur Entwickelungsgeschichte des Sympathicus. i. Die Entwickelung des Sympathicus bei den Selachiern (Acanthias vulgaris). Verhandl. K. Acad. Wetensch. Amsterdam. 1900. 2. Die Entwickelungsgeschichte des Sympathicus bei den Urodelen. Ibid. 1902.
Huber, G. C.: Lectures on the Sympathetic Nervous System. Jour. Comp. Neur., Vol. 7. 1897.
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Langley, J. N. : The Arrangement of the Sympathetic Nervous System based chiefly on Observations on Pilo-motor Nerves. Jour, of Physiol., Vol. 15. 1893.
Langley, J. N.: A Short Account of the Sympathetic System. Pamphlet Physiol. Congress Berne. 1895.
Langley, J. N.: The Sympathetic and other related systems of nerves. Text -book of Physiology. Edited by Schafer. Vol. 2. 1900.
Langley, J. N.: The Autonomic Nervous System. Brain. Vol. 26. 1903.
Langley, J. N.: Das Sympathische und verwandte nervose Systeme der Wirbelthiere (autonomes nervoses System). Asher u. Spiro's Ergebnisse. II. Jahrg., II. Abtheil. 1903.
Onuf and Collins: Experimental Researches on the Central Localization of the Sympathetic with a Critical Review of its Anatomy and Physiology. Arch. Neur. and Psychopath., Vol. 3. 1900.
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