Book - The Nervous System of Vertebrates (1907) 11
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Chapter XI. The Somatic Motor Division
This division of the nervous system directly controls the actions of the typical body muscles; namely, those derived from the dorsal mesoderm or somites. It would be expected that each segment of the body which has such muscles would have a pair of somatic motor nerves. Inasmuch as several somites in the occipital region degenerate in most vertebrates without producing muscle, these nerves are wanting in those segments. Somites i, 2 and 3 produce the eye muscles and these are innervated by the cranial nerves numbered III, IV and VI. Between these and the first somatic motor nerve of the spinal or trunk region is a gap owing to the absence of a variable number of postotic myotomes. In cyclostomes, where the postotic somites all develop into myotomes, one or more nerves are absent (Petromyzon), apparently because one or two somites only partially develop. In one cyclostome (Bdellostoma) it is now known that a complete series of somatic motor nerves is present in this region, one nerve for each postotic myotome. In the adult of this animal, however, the eye muscle nerves are wholly lacking. Finally, it is to be mentioned that one rudimentary somite is known in selachians anterior to those which produce the eye muscles. It is probable that the segment to which this somite belonged possessed muscles in primitive or ancestral vertebrates. The somatic motor division is to be thought of as incomplete owing to the loss, in various segments, of the muscles which it should innervate. Otherwise this division is the simplest and least modified of the four functional divisions of the nervous system.
In the trunk region (Fig. 102) the somatic motor nerve fibers arise from the cells of the ventral horn of the spinal cord and make their exit from the ventral surface of the cord as the ventral roots of the spinal nerves. In cyclostomes these ventral nerves arise opposite the middle of the myotomes, pass through the membranous skeleton and divide into dorsal and ventral rami. The rami of each nerve run upon the inner face of the myotome to which they belong and twigs from them penetrate the myotome to end in relation with the muscle fibers. These ventral nerves make no connection with the dorsal nerves, which lie hi the spaces between each two myotomes. In all higher classes each ventral nerve unites with an adjacent dorsal nerve to form a composite structure called a spinal nerve. The union takes place at or near the distal end of the ganglion of the dorsal nerve and the composite nerve immediately divides into dorsal and ventral rami. At about the same point, or from the ventral ramus, the ramus communicans is given off to the sympathetic ganglion. (See p. 200.) In urodeles (Bardeen) where the trunk muscles have the same simple segmental arrangement as in fishes, the spinal nerve lies in a myoseptum between two muscle segments. The sensory fibers are distributed to the skin both before and behind this septum, and the motor fibers enter both the adjacent muscle segments and innervate the muscle fibers at their ends. Each nerve therefore helps to innervate two muscle segments and any muscle fiber may be innervated from two spinal nerves. These facts seem to have an important bearing on the question of metamerism in the vertebrate body and also upon the problems of histogenesis of motor nerves (cf. p. 63). In higher vertebrates when the myotomes lose their simple segmental arrangement and become divided into special muscles, the relations of the nerves to the muscle segments become more obscure.
FIG. 102. A diagrammatic representation of the somatic motor components of a trunk segment.
The greatest modifications affecting the ventral nerves occur hi connection with the innervation of the limbs. The limbs are innervated by rami going out from plexuses formed by the union of the ventral rami of several spinal nerves. Two such plexuses are formed, the brachial for the fore limb, the lumbar plexus for the hind limb. The presence of such plexuses is explained by the mode of origin and evolution of the vertebrate limbs. The limbs first arose as folds of skin and muscle extending along the side of the body. These long limb folds are formed in the embryos of lower vertebrates by outgrowths or buds from a number of myotomes together with mesenchyme (Fig. 103). The myotome buds form the muscles of the limb; the mesenchyme gives rise to the skeleton and the connective tissue into which the blood vessels and nerves grow as the development of the limb proceeds. As many nerves are involved in the innervation of the limb muscles as there are myotomes involved in the limb fold. In higher vertebrates (and in later stages in the growth of the individual), as the limb grows in length it comes to have a shorter base where it is attached to the trunk. At the same time the nerves in order to reach their muscles which have shifted out into the limb, must converge to enter the narrow base of the limb. This together with various modifications of position due to the formation of the special muscles of the limbs have caused the nerves to unite into intricate plexuses.
plexus in a selachian, Spinax. The muscle buds are in dark shading, the nerves in black.
The most cephalic of the ventral spinal nerves form the so-called cervical plexus. This is closely related to the brachial plexus, some nerves usually contributing to both. The rami arising from the cervical plexus go to innervate muscles extending between the pectoral girdle and the region of the tongue, commonly called the tongue musculature. This musculature is formed by buds from a number of myotomes in the cervical region, and, in vertebrates which possess gills, these muscles are separated from the dorsal muscles and from the place of origin of the nerves by the expansion and shifting backward of the gill apparatus. In consequence, the nerves destined to the tongue muscles must run around behind and forward beneath the gills. As the more anterior nerves run back to get around the gills they unite with the more caudal ones and so give rise to the cervical plexus (Fig. 104). In higher vertebrates this plexus becomes somewhat modified. The anterior part of it, consisting of nerve roots arising in the occipital region and innervating the muscles of the tongue, becomes relatively independent of the plexus and is known as the hypoglossal nerve. The remainder of the plexus gives rise to rami supplying various muscles of the neck and is even connected with the spinal accessory nerve which belongs to the visceral motor system.
FIG. 104. The constitution of the cervical plexus in a selachian, Hexanchus. After Furbringer. vg., vagus; w,x,y,z, spino-occipital nerve roots; i and 2, spinal nerves; the letters d and v indicate respectively the dorsal and ventral roots; Rdl, rami for the dorso-lateral trunk muscles; Rsbsp, rami for the subspinal muscles; Rib, rami for the interbasal muscles.
The number and segmental position of the roots represented by the hypoglossus varies in different vertebrates, owing to the difference in the number of gills and to the extent caudally of the branchial apparatus. The tongue musculature arises from myotomes situated immediately behind the gills, and in fishes and higher vertebrates possessing five or four gills it is probable that one or two more anterior myotomes may enter into the tongue muscles than in forms like the cyclostomes where there are seven or more gills. In all higher vertebrates the hypoglossal roots include the first ventral roots following the eye muscle nerves. In cyclostomes, however, and to a less extent in selachians, a number of myotomes are preserved anterior to those which contribute to the tongue muscles and to supply these muscles ventral nerves are present anterior to the hypoglossal roots. In Petromyzon all the myotomes behind the ear form permanent muscles, while the first myotome to contribute to the tongue muscles is the seventh behind the ear (myotome 10). However, from one to three nerves are absent in different species, of Petromyzon in spite of the presence of permanent muscles formed from their myotomes, and there remain from three to five nerves anterior to those which enter into the formation of the hypoglossus. The most anterior ventral root present in Petromyzon dorsatus belongs to the same segment as the vagus (Fig. 51). The segment of the glossopharyngeus also has a somatic motor nerve in Bdellostoma, so that, as noted above, the series of somatic motor roots in this animal is complete from the segment of the glossopharyngeus backward.
The eye muscles are developed from the first, second and third somites. From the first somite are derived the rectus superior and inferior, the rectus internus and the obliquus inferior. From the second somite comes the obliquus superior, and from the third comes the rectus externus. The nerve which innervates the rectus externus muscle is the VI or abducens. It is the ventral motor nerve of the third somite and is comparable in every way with a ventral spinal nerve. In Petromyzon there is no VI nerve. The muscle which is usually regarded as the homologue of the rectus externus is innervated by a branch from the trigeminus ganglion which probably arises from the trigeminus motor nucleus. The source of the muscle whether from dorsal or lateral mesoderm is uncertain. The nerve innervating the obliquus superior muscle is the IV or trochlearis, which is the ventral nerve of somite 2. This also is comparable with a ventral spinal nerve except that its root starting from a ventral motor nucleus runs upward in the brain wall, decussates with its fellow in the roof of the brain and emerges from the dorsal or lateral surface between the cerebellum and tectum mesencephali. No satisfactory explanation has yet been found for the curious course of this nerve. The nerve which innervates the four muscles derived from the first somite is the III or oculomotorius, which arises from a ventral motor nucleus in the base of the mesencephalon. It is a noticeable peculiarity in the origin of this nerve that a large part of its fibers arise from the nucleus of one side and cross to enter the root of the opposite side (Fig. 105). The same arrangement is found in the roots of other ventral nerves but to a much less degree.
FIG. 105. A transverse section through the nucleus of origin of the III nerve in a cyclostome, Lampetra. n.III, nucleus of III nerve; b.M., bundle of Meynert (tractus habenulo-peduncularis) ; d.b.M., decussation of the same; n.b.M.^ endnucleus of the same; g.M., one of the giant cells of Mauthner.
All the somatic motor nerves arise from a portion of the gray matter which lies latero-ventral to the central canal or ventricle. In the brain region it is usually marked by a pair of special grooves or furrows in the floor of the ventricle, one at either side of the mid- ventral furrow (Figs. 3, 46, etc.). The motor cells are large and have large dendrites whose branches spread widely through the latero- ventral part of the brain or cord. The neurites from the cells which lie in the immediate vicinity of a nerve root may pass out in that root, but most neurites must run forward or backward in order to reach their nerve roots. Indeed, it is probable that the neurites do not all enter the nerve root which lies nearest to their cells of origin but that many neurites run through one or more complete segments before going out of the brain or cord as fibers of a ventral nerve. The neurites which run from segment to segment lie mesial to the motor nuclei, where they run in a definite bundle at either side of the mid-ventral groove of the ventricle. The large bundle of coarse fibers in this position is known as the fasciculus longitudinalis mediates (dorsalis or posterior), and forms one of the most conspicuous landmarks in the brain of any vertebrate. This contains, however, other fibers in addition to those here mentioned. That portion of it which is made up of motor fibers on their way to become fibers of ventral nerves may be called the somatic motor fasciculus. The motor cells together with this fasciculus make up the somatic motor column or zone of the spinal cord and brain. In those segments of the brain which have no ventral nerves the somatic motor cells are wanting and the column is represented only by the somatic motor fasciculus. This fasciculus continues forward beyond the oculomotor nerve and its fibers take their origin from the cells of a special nucleus cephalad from the nucleus of the III nerve in the ventral part of the central gray of the thalamus. This nucleus is frequently considered to have some relation to the fibers of the posterior commissure (cf. p. 265 and Fig. 134, p. 272).
Owing to the importance of bodily movement in nearly all the activities of the animal, the connections of the somatic motor centers with the rest of the nervous system are numerous and varied in character. Necessarily the motor neurones form a link in every reflex chain which leads to a bodily movement, whatever the source and character of the exciting stimulus. Some of the chief classes of impulses and the tracts which bring them to the motor centers will be mentioned here. The simplest case is that of tactile impulses which are brought into the spinal cord by the cutaneous fibers of the dorsal roots. Collateral branches of these fibers carry the impulses directly to the motor cells. A somewhat more complex course for similar impulses is illustrated in the relations of the acusticum and cerebellum to the motor centers. Impulses from the skin, the lateral line organs and the ear are given directly or indirectly to the large cells of the acusticum and cerebellum. The neurites from a part of these cells form bundles which have been described as going down close to the ventricle, to the somatic motor column (p. 135). Such bundles have been seen in selachians going to the nucleus of each of the eye-muscle nerves. A part of the fibers enter the fasciculus longitudinalis medialis and may go to the segments of the somatic motor column in the spinal cord which control the movements of the body and limbs. A third and more complex course for such impulses is that by way of the roof of the mesencephalon (cf. p. 117). The fibers descending from the tectum form the tractus tectobulbaris. The end branches of these fibers make direct or indirect connections with the motor centers in the medulla oblongata and spinal cord. Finally, when in a mammal or man such impulses are carried up to the cerebral cortex and give rise to a sensation, there may follow a voluntary motor impulse which descends over the fibers of the pyramidal tract and reaches the motor centers of the spinal cord. The simpler courses for tactile impulses from the cutaneous to the somatic motor nerves are indicated in Figure 59, p. 118. The student should construct similar diagrams to illustrate the course of impulses from the Template:Ear and the Template:Eye.
It is not yet certainly known whether visceral sensory impulses (from the general visceral surfaces, from taste organs or the olfactory organs) go directly to the somatic motor nuclei. Further study of the central gustatory apparatus may be expected to throw light on this question. Until such investigations are made it can only be said that in general -the connections between the visceral sensory and somatic motor nerves are much more indirect than those between the cutaneous and somatic motor nerves. Some of the indirect paths of taste impulses leading to somatic motor centers are shown in Figure 92, p. 174.
Demonstration or Laboratory Work
- Review the dissections of spinal and cranial nerves already made, with especial reference to the position of the ventral roots and the relation of the rami to the muscle segments. Dissect the brachial and cervical plexuses of a selachian.
- Examine carefully the somatic motor cranial nerves, noting their segmental arrangement with reference to the dorsal roots and the number and position of the "hypoglossal roots" present. Selachian brains are especially useful for this. Small roots which would ordinarily be overlooked can be brought sharply to view by painting the dissection with a one percent, solution of osmic acid which is well washed away with water as soon as the nerves are blackened.
- In sections by the Weigert or Golgi method study the somatic motor nuclei and the formation of the ventral roots. Selachian or frog.
- Upon the basis of the descriptions given in previous chapters construct diagrams representing the course of impulses the centers and tracts involved in somatic motor reflexes aroused by cutaneous, auditory, optic and olfactory stimuli.
Bardeen, C. R. : The Bimeric Distribution of the Spinal Nerves in Elasmobranchii and Urodela. Amer. Jour, of Anat., Vol. 3. 1904.
Fiirbringer, M.: Ueber die spino-occipitalen Nerven der Selachieru. s. w. Gegenbaur's Festschrift, Bd. 3. 1897.
Johnston, J. B.: The Cranial Nerve Components of Petromyzon. Morph. Jahrb., Bd. 34. 1905.
Koltzoff, N. K.: Entwickelungsgeschichte des Kopfes von Petromyzon planeri. Bull. Soc. Imper. d. Natural, de Moscou, Annee 1901, No. 3-4. 1902.
Neal, H. V.: The Development of the Hypoglossus Musculature in Petromyzon and Squalus. Anat. Anz., Bd. 13. 1897.
Neal, H. V.: The Development of the Ventral Nerves in Selachii. i. The Spinal Nerves. Mark Anniversary Volume. 1903.
van Wijhe, J. W.: Ueber die Mesodermsegmente und die Entwickelung der Nerven des Selachierkopfes. Amsterdam 1882.
Worthington, Julia: The Descriptive Anatomy of the Brain and Cranial Nerves of Bdellostoma Dombeyi. Quart. Jour. Mic. Sci., Vol. 49. 1905.
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