Book - The Nervous System of Vertebrates (1907) 12
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Chapter XII. The Visceral Efferent Division
The visceral efferent division controls the smooth muscles in the viscera and elsewhere in the body, the muscles of the heart and blood vessels, certain striated muscles derived from the lateral mesoderm, and the glands of the body. In higher vertebrates at least, the smooth muscle and glands do not receive impulses directly from the central nervous system, but the sympathetic system offers intermediate neurones by which cerebro-spinal impulses are carried (see following chapter). In the lower vertebrates where the sympathetic system is poorly developed, it is probable that some of the functions performed by it in higher forms are performed by the visceral efferent nerves and centers. As it is not possible at present to distinguish the excito-glandular from the excito-motor cells in the central nervous system, the nuclei and nerve components may be described under the general name of visceral efferent structures.
FIG. 106. A diagrammatic representation of the visceral efferent components in a trunk segment.
The visceral efferent nuclei in the spinal cord occupy a position dorsal to the ventral horn, between it and the visceral afferent column (Fig. 106). The visceral efferent nuclei in the higher vertebrates and man form here a lateral projection of the gray matter known as the lateral horn. The neurites from the cells of this column pass out of the cord in the dorsal nerve roots in lower vertebrates. In higher vertebrates a part of these fibers, and in some cases all of them, pass out by way of the ventral roots. The fibers then go through the white rarni communicantes into the ganglia of the sympathetic chain. Their further relations will be given in the next chapter.
In lower vertebrates the visceral efferent column of the cord is continued forward as a column in the brain constituting the nuclei of origin of the efferent roots of the X, IX, VII and V nerves. In fishes, where the voluminous musculature of the gill apparatus is to be innervated, this is a large and important column of gray matter at either side of the ventricle as far forward as the cerebellar segment. In higher vertebrates, where the gills have been lost, the column becomes less important and is divided into dorsal and ventral portions. The former is the dorsal vago-glossopharyngeal nucleus, and gives rise to efferent sympathetic fibers. The ventral portion consists of discontinuous masses known as the nucleus ambiguus, and gives rise to fibers which innervate striped muscles (Fig. 107). The neurites from the cells of this column form the visceral efferent component of the X, IX, VII and V nerves in all vertebrates. In the gill breathing forms (Figs. 51, 63, 79) these components run in the posttrematic ramus of each of the branchial nerves. This ramus runs down along the anterior side of each branchial arch and hence behind the gill slit. The fibers in question are distributed to the muscles which control the gill arches in respiratory movements. The component in the trigeminus supplies more specialized muscle in all vertebrates. In fishes this component runs in the mandibular ramus of the trigeminus which holds the same relation to the mandibular arch and mouth . that the posttrematic rami of the branchial nerves hold to the gill arches and slits. The motor fibers of the trigeminus supply the muscles which move the mandibular arch, i.e. the chief muscles of mastication. In all gnathostome vertebrates the muscles of mastication are supplied in this way; with the exception of the posterior belly of the digastric, which is supplied by the corresponding component in the VII nerve. In higher vertebrates the motor component of the facial nerve also controls highly specialized muscles which move the skin of the face, of the scalp and ears, the muscles of expression. This mode of innervation indicates that these muscles are derivatives of the branchial muscles of the hyoid segment which have spread forward to their present position, whither the motor branches of the facial nerve have followed them. This- is one example of the way in which terms ill-adapted to the comparative description of the nervous system of vertebrates have come into use. The nerve is named the facial from the distribution of its larger branches in man, but the comparative anatomy shows that this distribution has been secondarily acquired and only in higher vertebrates. The facial nerve is really the nerve of the hyoid segment.
FIG. 107. Diagram to show the central relations of the IX, X and XI nerves in mammals. From Onuf and Collins. N.X.dors., dorsal vagus or vago-glossopharyngeal nucleus, nucleus for the visceral or vegetative (sympathetic) efferent fibers; N.amb., nucleus ambiguus, nucleus for the striped muscle innervated by the IX, X and XI nerves; sol., fasciculus solitarius; N.homol.CL, nucleus homologous with Clarke's column; N.XTI, nucleus of hypoglossus; spin.V.R., spinal V tract.
As in the case of the somatic motor nuclei the course of the neurites of the visceral motor cells is not always simple. Not only do the fibers run longitudinally in the brain for longer or shorter distances before going out in their nerve roots, but there have occurred shiftings of segmental relations which are at first sight difficult to understand. In the case of the trigeminus, a part of the nucleus of origin of its motor component lies in all vertebrates caudal to the root, and the fibers from this part of the nucleus run forward lateral to, or in the lateral part of, the fasciculus longitudinalis medialis to join the remainder of the root. In the case of the facialis the entire nucleus always lies caudal to the plane of exit of the root and the fibers run fonvard as a distinct bundle in the fasciculus longitudinalis medialis and turn laterad to form the root. In man this is known as the geniculated root of the facialis. The roots and nuclei of the IX and X nerves are arranged in much the same way as those of the V nerve. This condition is more pronounced in the vagus and the fibers do not all unite into one root but the vagus has a number of motor as well as of sensory rootlets. This is explained by the fact that the vagus has gathered into it all the branchial nerves of the segments following that of the glossopharyngeus. The roots of the more caudal branchial nerves have gradually shifted forward until they have united with that of the vagus, but the union is not complete in any class of vertebrates. In some fishes the number of rootlets approaches twenty and they are scattered for a considerable distance along the side of the medulla oblongata (Figs. 2, 3, 7, 12). The fibers of the most caudal of these motor rootlets in fishes go to supply certain muscles connected with the pectoral arch which are homologized with a part of the trapezius musculature in mammals. In higher vertebrates the nuclei of these more caudal rootlets apparently extend farther back in the spinal cord and the roots take their exit from the dorso-lateral surface of the cord between the dorsal and ventral roots of the spinal nerves, run forward along the side of the medulla oblongata and join the trunk of the vagus (Figs. 20, 32). These roots have the name of the spinal accessory or XI cranial nerve. In mammals the nucleus and roots of this nerve extend farther caudad than in other classes, are more variable in position and show a greater tendency to a segmental arrangement and a closer relation with the dorsal roots of the spinal nerves. The more caudal roots are smaller, more nearly segmental and are placed nearer the dorsal roots (Lubosch). It seems that above the amphibia the increasing importance of the trapezius muscles have been correlated with an increase in the extent of the accessorius nucleus. The fact that skeletal muscles important in the movements of the fore limb are innervated by nerves arising from the visceral motor column requires a word of explanation. All other muscles involved in general bodily movements are derived from the dorsal mesoderm and are innervated by nerves from the ventral motor column. A somewhat similar anomaly is seen in the muscles of mastication. Although these are derived from the lateral mesoderm and are innervated by visceral motor fibers they are voluntary muscles which move skeletal parts whose functions are much more than merely visceral functions. Although primarily all muscles derived from the lateral mesoderm may have been related solely to the walls of the alimentary canal and have been involuntary in their action, it is evident that neither of these characters are retained by all such muscles. What is found to be constant is that muscles derived from the lateral mesoderm are innervated by nerves arising from the visceral motor column. The chief question regarding the trapezius musculature, then, is how it comes to be attached to the skeleton of the arm. The only probable explanation is that the shoulder girdle or pectoral arch did not have its origin as a part of the skeleton of a limb, but existed as a branchial arch before the limb was formed. It is believed that primitive vertebrates possessed a considerably larger number of gills than are now found in most vertebrates and it is supposed that the skeleton of one or more branchial arches has been retained as a girdle for the attachment of the fore limb. The muscles which moved this branchial arch, or perhaps those of several arches, have in part been preserved as muscles of the limb girdle. These muscles and their nerves were of course visceral muscles and nerves like those of other branchial arches, and they have secondarily acquired somatic functions. This interpretation is supported by the following facts: (i) The position of the brachial plexus in mammals shows that the shoulder girdle has shifted backward from its primitive position. (2) The most primitive vertebrates now possess a large number of gills, as many as thirty-five (Price). (3) There are in amphibian embryos signs of gill slits extending back into the trunk region caudal to the position of the fore limb (Platt).
As pointed out in the last chapter, the somatic and visceral efferent nuclei differ in the source of the impulses which come to them and in the tracts which bring them. Tracts from higher brain centers bring impulses to both sets of motor nuclei, but much remains to be done in order to explain the mechanisms by which somatic and visceral activities are correlated.
Collaterals from afferent visceral fibers directly to the visceral efferent nuclei are probably present in mammals (see Figs. 52 and 78). The short viscero-motor connections described in a previous chapter (p. 162) form a two linked chain between the visceral sensory and the visceral motor apparatus. The tertiary connections of the inferior secondary gustatory center are not known. From the superior secondary gustatory center a large tract goes to the inferior lobes of the diencephalon, from which tracts go to the cerebellum and medulla oblongata. The greater part of the tract to the medulla oblongata ends in the region of the visceral motor nuclei and it is undoubtedly chiefly these nuclei which receive the impulses. Olfactory impulses also may come over the same tract to the visceral motor nuclei. It is probable that even in fishes tracts from other correlating centers, such as the cerebellum or the mesencephalic nuclei, bring impulses to the visceral motor nuclei for the control of some of the more complex movements, especially for the coordination of somatic and visceral muscles in the act of seizing food.
Demonstration or Laboratory Work
- Review the dissections already made, with especial reference to the viscero-motor rami of the cranial nerves.
- Study the spinal accessory nerve in a mammal, either by dissection or in embryos such as the pig. Compare the arrangement of the X and XI roots in a 12 mm. pig embryo with that in a selachian or the frog.
- In Golgi or Weigert sections offish or frog brain study the origin of the viscero-motor roots of the cranial nerves.
Coghill, G. E.: The Cranial Nerves of Amblystoma. Jour. Comp. Neur., Vol. 12. 1902.
Fiirb ringer, M.: Spino-occipitalen Nerven der Selachier u. s. w. Gegenbaur's Festschrift, Bd. 3. 1897.
Herrick, C. Judson: The Cranial and First Spinal Nerves of Menidia. Jour. Comp. Neur., Vol. 9. 1899.
Johnston, J. B.: The Morphology of the Vertebrate Head, etc. Jour. Comp. Neur. and Psych., Vol. 15. 1905.
Lubosch: Vergleichend-anatomische Untersuchungen iiber den Ursprung und die Phylogenese des N. Accessorius Willisii. Arch. f. mik. Anat., Bd. 54. 1899.
Osborn, H. F.: A Contribution to the Internal Structure of the Amphibian Brain. Jour. Morph., Vol. 2. 1888.
Platt, Julia B.: Ontogenetische Differenzirung des Ektoderms in Necturus. Arch. f. mik. Anat., Bd. 43. 1896.
Price, G. C.: Some Points in the Development of a Myxinoid (Bdellostoma Stouti L.) Verhdl. Anat. Ges. 10. Vers. Berlin. 1896.
Stannius, H.: Das peripherische Nervensystem der Fische, u. s. w. Rostock. 1849.
Streeter, G. L.: The Development of the Cranial and Spinal Nerves in the Occipital Region of the Human Embryo. Amer. Jour. Anat., Vol. 4. 1904.
Strong, O. S.: The Cranial Nerves of Amphibia. Jour. Morph., Vol. 10. 1895.
See also the list at the close of the following chapter.
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