Book - The brain of the tiger salamander 12

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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander

Chapter XII Cerebellum

Ii Urodeles the cerebellum is incompletely separable from the brain stem, and the entire rhombencephalon evidently acts as a closely integrated unit. Larsell's papers ('20, '31, '32, '45) on the cerebellum of Amblystoma and Triturus give good accounts of its development and structure.


Three of the primordia from which the mammalian cerebellar complex has been assembled are here clearly separate in relations easily recognized. The fourth mammalian component — the pontile system — is not represented in Amblystoma. The three components present are: (1) the vestibulo-lateralis system in the auricles, primordia of the floccular part of the fiocculonodular lobes ; (2) the median body of the cerebellum, which is ancestral to the larger part of the vermis and adjoining regions; and (3) the nucleus cerebelli, internal to the other two and in intimate relations with both of them. The topographic arrangement of these parts as seen in transverse sections is shown in figure 91. In higher brains these three components are variously merged, and the nucleus cerebelli is subdivided and incorporated within the cerebellar mass as the deep nuclei. The connections of the vestibular and lateral-line systems with the auricle have been described above. The afferent connections of the body of the cerebellum are shown in figure 10. These include primary and secondary sensory trigeminal fibers, spino-cerebellar fibers, tectocerebellar fibers, and probably a hypothalamo-cerebellar tract. Evidence from both comparative anatomy and embryology indicates that cerebellar differentiation began within the sensory zone, and in the adults of all vertebrates it retains some features characteristic of this zone, including termination within it of vestibular root fibers and (in many species) of trigeminal root fibers also. It is morphologically supra-segmental and physiologically supra-sensory, an adjustor of higher order, not primarily concerned with determining the pattern of performance but rather with facilitation and regulation of its execution. The motor zone plays no part in it genetically or functionally except as a sort of accessory after the fact. In later phylogenetic stages, with the appearance of the pons and cortico-pontile connections, this situation is changed; but even in mammals these connections, though topographically in the motor zone, are foreign to its intrinsic organization, for the pons and all its connections are dependencies of a supra-segmental apparatus and the cells of the pontile nuclei are embryologically derived from the sensory zone. In the most primitive vertebrates in which cerebellar differentiation is minimal (almost absent in myxinoids, Jansen, '30; Larsell, '47a) the sensory zone of the medulla oblongata abuts against that of the midbrain, which in these animals is the dominant adjustor of all somatic (nonvisceral) activities. Necturus presents a very early stage in the fabrication of the cerebellar complex ('14), scarcely more advanced than in cyclostomes. In Ambly stoma the process is further advanced. It is evident that cerebellar differentiation began under the influence of several kinds of sensory excitations, viz., the vestibular, lateralline, general cutaneous, and deep sensibility of the muscles, joints, etc. ('24 ; Larsell, '47, '47a) . Connection with the visual system of the tectum was effected in early phylogenetic stages. With the elaboration of the cerebral cortex in mammals, another major component was added — the cortico-pontile system. This history has been thoroughly explored and documented by Larsell ('20-'47a). Brodal ('40) and Brodal and Jansen ('46) have greatly extended our knowledge of the connections between the inferior olive and the cerebellum in mammals, but we have little exact information about them in lower forms.


The isthmus is a critical junctional field. Immediately spinalward of it the sensory and intermediate zones of the medulla oblongata are enlarged to form the massive auricle, within which are terminals of the ascending vestibular and associated lateral-line roots (fig. 7). This is probably the first primary component of cerebellar architectonic. More medially the ascending sensory root of the trigeminus reaches its terminal (superior) nucleus. Some of these V root fibers, accompanied by secondary V fibers from the superior nucleus, continue into the median body of the cerebellum (figs. 10, Sl-33, 91), where they are joined by fibers of the spino-cerebellar tract. Some fibers of the mixed bundle decussate in the commissura cerebelli (com.cb.). This is the second primary component of the cerebellum from which the median mass (corpus cerebelli) of larger cerebella has been derived and into which converge sensory influences of all sorts except those from the internal ear and the lateral-line organs. This median mass receives fibers from the mesencephalic tectum and (in lower-vertebrates) also from the hypothalamus. The tecto-cerebellar tract may be activated from the mesencephalic V nucleus and from terminals of the optic and lemniscus systems.


The very rudimentary cerebellum of Myxine has been the subject of much controversy. Holmgren ('46, p. 54) described in the embryo two possible primordia, one in the auricle of the rhombencephalon and one at the posterior end of the mesencephalic tectum. Larsell's comprehensive study ('47, '47a) of this region of cyclostomes has clarified the problem. Myxine has the most primitive cerebellum known, and, so far as its differentiation has gone, it conforms with the typical vertebrate pattern.


The spino-tectal terminals in the inferior colliculus are primitively contiguous with the vestibular and lateralis terminals in the auricle, and from the wedlock of these two systems the cerebellum was born. The primordial inferior colliculus, accordingly, was primitively concerned with proprioception. Upon this foundation the cochlear system of higher animals was built in much the same way that the bulbar cochlear nuclei emerged from the lateral-line nuclei of the medulla oblongata (p. 138). It is significant that in Amblystoma the primordial inferior colliculus contains dense collections of cells of the mesencephalic V nucleus, that fibers of the mesencephalic root of the V nerve penetrate cerebellar tissue, and that cells of the mesencephalic V nucleus are found sparsely scattered in the cerebellar gray (p. 140). It seems probable that the mesencephalic V nucleus was first differentiated at the posterior end of the tectum and that from this focus its cells spread forward into the optic tectum and, in smaller number, backward into the cerebellum.


In the gray of both the median body of the cerebellum and the lateral auricle some of the neurons are specialized as Purkinje cells, in contrast with smaller cells, which are precursors of the cerebellar granules. The largest and best developed Purkinje cells are loosely arranged at the outer border of the central gray, but they are not delaminated from the granular layers as in mammals. If cortex is defined as laminated superficial gray, Amblystoma has no cerebellar cortex, though its primordium is clearly evident. The Purkinje cells, though of simple form, are quite distinctive. From them and from the nucleus cerebelli, efferent fibers stream downward, forward, and backward as cerebello-tegmental fibers, and one large fascicle of these extends farther forward parallel with the sulcus isthmi as brachium conjunctivum.


In, my earlier papers a ventricular swelling under the cerebellum was termed "eminentia subcerebellaris tegmenti." This has subsequently been analyzed into several distinctive areas, of which the posterodorsal member was named "eminentia cerebellaris ventralis" ('35a, fig. 1). It is now clear that this is the primordium of the deep nuclei of the cerebellum, and it is here named "nucleus cerebelli" (figs. 2, 10, 32, 33, 91, nvc.cb.). This is an ill-defined region of the intermediate zone, not clearly separable from its surroundings. Some fibers of the brachium conjunctivum arise from these cells, though only a small part of them.


The spino-cerebellar tract ascends in company with the spinal lemniscus, some cerebellar fibers being collaterals of lemniscus fibers, as described above. Cerebellar fibers join this tract from the nucleus of the dorsal funiculus and the sensory zone of the medulla oblongata. One confused issue regarding this tract of Amblystoma can now be clarified. A dorsal slip of the spino-cerebellar tract passing through or above the entering fibers of the sensory V root was described by me ('14, p. 7) and by Larsell ('20, p. 277). These observations could not be confirmed by either of us (Herrick, '14a, p. 375; Larsell, '32, p. 414), the supposed tract being then interpreted as the ascending trigeminal root. It now appears that both the earlier and the later accounts are correct. In one of our adult brains, cut in a favorable oblique plane (no. 2245), there is partial Golgi impregnation of the sensory V root fibers and of the spino-cerebellar tract, with no other fibers stained in this vicinity. A separate fascicle of the ventral spinocerebellar tract turns dorsally at the level of the V nerve, its fibers interdigitate with those of the entering sensory V root, and rostrally of this level the spino-cerebellar fibers and the ascending sensory V fibers are mingled as far as their entrance into the cerebellar commissure. Here a fortunate elective impregnation clarifies the confusion as neatly as could be done by an experimental degeneration (compare Salamandra as described by Kreht, '30, p. 294).


In the literature our tractus spino-cerebellaris is sometimes designated "tractus cerebello-spinalis," implying conduction spinalward. Some descending fibers may be present in this tract of Amblystoma, though our preparations have not revealed them. That the larger part of the tract is ascending is clearly evident.


The tr. tecto-cerebellaris was originally described ('*25, p. 483 and figs. 39-41) from two series of sagittal Golgi sections of the adult, and this description has been confirmed by Larsell ('3'2, p. 425). Additional details can now be added. In several series of Golgi sections of advanced larvae and adults these thin unmyelinated fibers arise from neurons of the nucleus posterior tecti with the dendrites directed forward into the posterior border of the optic tectum. Their axons form a rather compact fascicle close to the dorsal surface of the nucleus posterior, which passes backward though the anterior medullary velum and laterally of it, accompanying thick myelinated fibers of the mesencephalic root of the V nerve which take a similar course. At the anterior border of the cerebellum they turn ventrad and spread out along the anterior side of the body of the cerebellum, some reaching the underlying nucleus cerebelli.


A tr. mamillo-cerebellaris was described by Larsell ('20, p. 279; '32, p. 424), and some evidence of it has been seen in our preparations; but in the absence of elective impregnations no satisfactory description can be given. In view of the large size of this connection in fishes, its presence in urodeles may be expected.


BRACHIUM CONJUNCTIVUM

Large numbers of unmyelinated and lightly myelinated fibers descend from the body of the cerebellum, the auricle, and the nucleus cerebelli. These cerebello-tegmental fibers are in addition to the connections of the auricle with the vestibulo-lateralis sensory field in correlation tracts a and b; they are directed in dispersed arrangement ventrally, posteriorly, and anteriorly. A large and rather compact fascicle of these fibers, a few of them myelinated, is directed antero ventrally and is clearly a primordial brachium conjunctivum (figs. 10, 21, 71). These axons arise from cells of the body of the cerebellum (fig. 47) and the nucleus cerebelli (14a, fig. 52); they assemble anteriorly of the body of the cerebellum and pass forward through the visceral-gustatory nucleus in the isthmus (figs. 33, 34). Here they are joined by fibers of the tertiary visceral tract, which arises in this nucleus (p. 169).


This mixed tract with accessions of fibers from other sources has been termed "fasciculus tegmentalis profundus" ('36, p. 304, figs. 14, 23, f.teg.p.) ; its analysis is possible only with the aid of elective impregnations, and these, fortunately, are available (see the further description in chap. xx). This fascicukis extends forward and ventralward from the cerebellum at the outer border of the gray in the lips and floor of the isthmic sulcus, finally to reach the ventral commissure, where some of its fibers decussate a short distance spinalward of the fovea isthmi, mingled with those of the decussation of the f. retroflexus and spinalward of it. Its course as seen in horizontal sections is shown in figures 29-32 (f.teg.p.), and in sagittal sections in figure 104 and previously published figures of the same specimen ('36, figs. 19-21, trxb.teg.).


Elective impregnations show that the brachium conjunctivum is the largest component of this mixed fasciculus (figs. 71, 72). Many of its fibers spread outward into the alba of the isthmic tegmentum on the side of origin, and these include all the myelinated fibers of the brachium. A residue of the unmyelinated fibers decussates in the ventral commissure. Elective Golgi impregnations show that, at the decussation, fibers of the brachium conjunctivum and tertiary visceral tract are mingled and that both enter a dense superficial neuropil laterally of the crossing. Here the tertiary visceral fibers turn forward in company with those of the secondary visceral tract to reach the area ventrolateralis pedunculi, and the cerebellar fibers turn posterodorsally and spread out in the alba of the isthmic tegmentum. This primordial brachium conjunctivum may activate almost the entire extent of the isthmic tegmentum diffusely on both sides. No evidence has been found of a concentration of cells related with it which could be regarded as a nucleus ruber.


THE CEREBELLAR COMMISSURES

In the body of the cerebellum there are two commissures, which differ in position, connections, and functions. They are specifically related with the two chief subdivisions of the cerebellar complex.


The com. cerebelli (com.cb.) is a compact fascicle of well-myelinated fibers crossing more ventrally than the less myelinated com. vestibulo-lateralis cerebelli (com.cb.i.), as shown in figures 10 and 91. It crosses between the levels of figures 34 and 35. The com. cerebelli is composed of fibers from two sources, which terminate chiefly in the median body of the cerebellum. These are (1) fibers of the tr. spinocerebellaris (figs. 32, 33, 91) and (2) fibers of the sensory trigeminus system. The latter are, in part, fibers of the ascending sensory V root terminating in the cerebellum and, in part, axons of cells of the superior sensory V nucleus terminating in the cerebellum and the neuropil of the V nucleus of the opposite side (figs. 31, 32, 91). The latter is a true trigeminal commissure.


The com. vestibulo-lateralis cerebelli is a more dispersed collection of unmyelinated and lightly myelinated fibers assembled from the auricles (figs. 31 36, 91). Included among them are root fibers of the vestibular nerve {comxb.VIII) and secondary fibers of the lateralline system of nerves {com.cb.l.L). These are mingled at their crossing in the cerebellar alba. Whether any primary root fibers of the lateralline nerves decussate in this commissure is not clear in our material ; apparently they do not.


PROPRIOCEPTIVE FUNCTIONS OF THE CEREBELLUM

In the initial differentiation of the cerebellum and in its normal functions in all animals, proprioception has played a dominant part. Nevertheless, it must be recognized that all sensory systems may participate in cerebellar control of muscular movements (Larsell, '45), a control that is applied to the motor adjustors as going concerns at every phase of their operations. The significance of the cerebellum as "the head ganglion of the proprioceptive system" (Sherrington) is discussed in chapter x in connection with a critique of the proprioceptive system as a whole.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander

Reference

Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.


Cite this page: Hill, M.A. (2019, September 24) Embryology Book - The brain of the tiger salamander 12. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_brain_of_the_tiger_salamander_12

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