Book - The brain of the tiger salamander 15
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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.
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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 XV Midbrain
IN THE preceding chapters the structure of the rhombencephalon and the functions intrinsic to it were summarized. We now pass to the cerebrum, with a radically different type of organization, dominated by the optic and olfactory systems. In lower vertebrates the major optic terminals in the midbrain evidently have determined the course of its differentiation ; but, since optic fibers terminate also in the diencephalon, a separate chapter is devoted to the visual system as a whole.
Development
Because the mesencephalon is the site of a remarkable series of changes in the shape of the brain in early developmental stages, it is appropriate to review here some features of this growth. Attention has been called (p. 117) to the significance of von Kupffer's anterior and posterior intraencephalic sulci as stable landmarks in the development of all vertebrate brains. These mark the loci of two strong cerebral flexures, which are extremely variable in different species and among individuals of the same species. These flexures so modify the topographic arrangement of parts as to make morphological analysis difficult. The posterior sulcus of von Kupffer (the development of which is described in chap, xiii) marks the posterior limit of the great mesencephalic flexure, which bends the neural tube ventralward and backward so that, at the close of the early swimming stage of Amblystoma tigrinum (Harrison's stages 36-38), the hypothalamus is closely appressed against the ventral surface of the peduncle and isthmus ('38, figs. 1-4, 15-18). The tuberculum posterius marks the ventral fulcrum around which this bend is made.
Beginning somewhat later than this ventral bending, there is a flexure in the reverse direction more anteriorly at the site of von Kupffer's anterior sulcus, which bends the cerebral hemispheres forward and upward. In the early feeding stage (Harrison's stages 44, 45) this movement is far advanced ('386, figs. 1, 2); and from this stage onward to the adult stage these flexures appear to be somewhat straightened and masked by intussusception of newer tissues.
Baker and Graves (S*^) described the external form of the brain in six stages of A. jeffersonianum (3-17 mm). Their figures 1 and 2 show that in their 3-mm. embryo, which is 7 days and 16 hours old and in a premotile stage, the mesencephalic flexure has begun. In Harrison's stage 30 of A. punctatum, about 24 hours younger than Coghill's nonmotile stage, this flexure is far advanced ('38, fig. 1), and the reverse flexure leading toward the evagination of the cerebral hemisphere is incipient.
Correlated with these changes in external form, cells are proliferating internally, and the fibrous connections are established. CoghilFs charts show that differentiation is precocious in the peduncle, the chiasma ridge, and the olfactory area of the hemisphere. Other local areas of differentiation appear successively, and, before the early swimming stage is reached, confluence of these areas has begun, and fibers are descending from the peduncle and ventral thalamus in the primary motor tract, which is the precursor of the fasciculus longitudinalis medialis. This development is well advanced before connection is made between the sensory field of the tectum and the motor field of the peduncle (compare '37, figs. 1 and 2; '38, figs. 5 and 20). Fibers from the retina enter the brain in early swimming stages, but they do not reach the tectum until the close of this period. These optic fibers enter the tectum at its anterior end in the eminence of the posterior commissure, and the first connection between optic tectum and peduncle is by way of this commissure, in which fibers first appear in the coil stage (Harrison's stage 35), long before optic fibers reach the tectum.
The commissura tecti in the remainder of the midbrain roof appears much later, beginning as an extension spinalward from the posterior commissure. As late as the midlarval period (A. tigrinum of 38 mm.) this commissure has matured only in the anterior half of the tectum ('39a, figs. 1, 11, 12; '39/;, fig. 1). Full details of the development of the optic nerve and its connections have been published ('41), as well as some features of the development of the tectum ('42, p. 243).
In development the nucleus posterior tecti is retarded as compared with the tectum opticum, more so in A. tigrinum than in A. punctatum ('386, p. 413; '39a, p. 265). The ventricle is here dilated as the recessus posterior mesencephali, and in larval stages the deep sulcus isthmi ends dorsally in this recess ('38, fig. 18; '14a, fig. 2). The roof and side walls of the posterior recess remain relatively thin until late larval stages, and they are expanded so as to form a noticeable external eminence, which disappears in the adult except in the midplane.
Sensory Zone Tectum Opticum - Superior Colliculus
Nothing need be added here to the statements in chapters iv and xvi and the description of the mesencephalic V nucleus in chapter x. For details my paper of 1942 may be consulted.
Nucleus Posterior Tecti - Inferior Colliculus
The small nucleus posterior tecti is an undifferentiated primordium of the inferior colliculus, with perhaps some additions in higher animals from the posteroventral part of the optic tectum.
The most characteristic cells of the posterior nucleus are small elements with thick bushy dendrites directed forward into the tectum opticum. Slender axons arise from these dendrites and are directed forward to the thalamus as an important component of the brachium of the inferior colliculus ('42, p. 265 and figs. 51 and 79). Other efferent fibers go out in large numbers in tractus tecto-thalamicus et hypothalamicus cruciatus posterior (p. 297; '42, p. 221), of which the strongest component decussates in the postoptic commissure and distributes to the tegmentum in tegmental fascicles (8) and (6). Other shorter efferents reach all surrounding parts. The chief connections of this region are shown diagrammatically in figures 10-18 and 21.
In Amblystoma there is no sharp boundary between the optic tectum and the nucleus posterior. The distinguishing features of the latter are the absence of optic terminals and the presence of two special tracts — the primordial lateral lemniscus {tr.b.t.l.) and the efferent tr. tecto-bulbaris posterior (tr.t.b.p.). The spinal lemniscus and the general bulbar lemniscus terminate chiefly in the optic tectum, though the spinal lemniscus has a larger proportion of terminals in the nucleus posterior. The tecto-cerebellar tract arises chiefly in the nucleus posterior. These connections suggest proprioceptive functions (chap. xii). This suggestion is strengthened by the presence of many cells of the mesencephalic V nucleus here and by the undeveloped condition of the auditory apparatus. The dominance of the cochlear connection is a later acquisition in land animals, with resulting transformation of the nucleus posterior into the colliculus inferior and differentiation of the medial geniculate body. This transformation is far advanced in the frog, for Aronson and Noble ('45) report that the warning croak uttered during mating is abohshed by extensive injury of the inferior colHcuH, though it is not lost after complete ablation of the hemispheres, diencephalon, superior colliculus, cerebellum, and anterior parts of the tegmentum.
In Anura the optic tectum is greatly enlarged so as to cover the dorsal convexity of the midbrain. The nucleus posterior is also enlarged under the influence of the well-developed lateral lemniscus, and it is folded into the aqueduct as the so-called "corpus posticum" or "torus semicircularis." In some reptiles and in all mammals the definitive inferior colliculus reappears on the dorsal surface.
Intermediate Zone
The distinctive characteristics of the intermediate zone of Amblystoma are more clearly shown in the midbrain than elsewhere. This band of subtectal tissue is well differentiated from the overlying tectum and the underlying peduncle and isthmic tegmentum and is given a distinctive name.
Tegmentum Dorsale
The dorsal tegmentum is separated from the isthmus by the sulcus isthmi (s.is.) and from the peduncle by the limiting sulcus (s.) of the nucleus of the tuberculum posterius (fig. 2). On the external and ventricular surfaces there is no visible boundary between this zone and the overlying tectum, but the internal structure is sharply contrasted. The gray of the tectum is irregularly laminated, with plaques of cells separated by thin sheets of neuropil; that of the subtectal area is more homogeneous (figs. 34, 36, 93, 94). The texture of the white substance is even more distinctive.
The neurons are rather small and are imbedded in dense neuropil, which is continuous with that of adjoining areas. Typical illustrations are shown in figures 24, 48, and 49 (see also '42, figs. 29, 30, 44, 45, 48). Their dendrites ramify among dorsal and ventral tegmental fascicles and into the tectum, peduncle, and isthmic tegmentum. The axons arise from the dendritic tree, and most of them are short, arborizing locally and in adjoining areas. Many divide with ascending and descending branches. Axons of the larger elements (fig. 49) descend to the isthmic tegmentum in the dorsal tegmental fascicles (fig. 21, tr.tegds.), these fibers being the largest component of fascicles of group (7). All these neurons seem to be concerned with local correlation.
This is typical correlational tissue interpolated between the sensory zone above and the motor zone below, with both of which it is connected by numberless short fibers, by long dendrites, and by the intrinsic neuropil (fig. '24). In addition to these local connections, it receives longer fibers from the cerebral hemispheres, dorsal and ventral thalamus, hypothalanms, and tectum, many of these fibers decussating in the anterior and postoptic commissures. There are long efferent fibers which descend in the dorsal tegmental fascicles to the isthmic and trigeminal tegmentum.
This field evidently contains important components of the apparatus of sensori-motor adjustment, but how this mechanism operates is unknown. This relatively homogeneous area probably loses its identity in higher animals, in which its parts are specialized and dispersed in the brain stem or perhaps supplanted by the more efficient adjustors of the thalamus and hemispheres.
Motor Zone
PEDUNCLE
At the sharp ventral cerebral flexure of the brain stem, a ventricular eminence marks the position of the nucleus of the tuberculum posterius, which is called the "peduncle" in a restricted sense (p. 21). This peduncular area, which is boundetl by a shallow and variable sulcus (fig. 2, s.), is an arbitrarily defined field at the ventral surface of the midbrain, bounded dorsally by the dorsal tegmentum and posteriorly by the isthmic tegmentum. It arises from the anterior end of the embryonic basal plate of the neural tube, and nerve fibers appear within it very early in embryogenesis. In the coil stage, fibers are added to this tract from the ventral thalanuis and dorsal tegmentum ('37, fig. 1), and, before the early swimming stage is reached, there are additions also from the tectum and other neighboring parts ('37, fig. 2). This basal sector of the midbrain is the nucleus of origin of the first efferent fibers of this region for lower motor centers, and, as early as the S-reaction stage, fibers go out from it to the periphery in the oculomotor nerve. This is before any afferent fibers are connected with this region. The motor mechanism develops autonomously.
In Necturus the f . longitudinalis medialis is definitely organized in the peduncle and is a well-defined anatomical structure from here caudad ('36, p. 349); but in Amblystoma the fibers which compose this bundle are mingled with others in tegmental fascicles numbered (1), (4), (5), and (6) in the peduncle and isthmus, and below the isthmus a residue of fibers from all these fascicles is assembled to continue spinalward in the f. longitudinalis medialis ('36, figs. 9-16).
The gray layer is relatively thin, and in the mid-plane most of its cells are displaced by the massive decussations of the commissure of the tuberculum posterius (figs. 2C, 29, 30, 31, 94). More laterally, large and small cells are mingled in the gray, with little evidence of local segregation except for the nucleus of the oculomotor nerve at the posteroventral border.
The largest cells include dorsally the primordium of the nucleus of Darkschewitsch, which is related primarily with the posterior commissure (figs. 6, 18, '2'2, 24; shown but not named in fig. 32), more ventrally Cajal's interstitial nucleus of the f. longitudinalis medialis (figs. 18, 22, 104), and posteriorly the III nucleus (figs. 18, 22, 24, 93, 104). Larval forms of these cells have been illustrated ('396, figs. 42, 53, 72, 73). They resemble those of Necturus ('17, figs. 23, 24, 30, 33).
These cells evidently are collectors of a wide variety of nervous impulses brought into the peduncle by the numberless fibers which converge to end here and by fibers of passage with collaterals in this area. The axons of these cells descend in the ventral tegmental fascicles and seem to be primarily concerned with activation of mass movements, particularly of locomotion. Most of these thick fibers descend in ventral tegmental fascicles of group (5) and are uncrossed, but some of them decussate in the ventral commissure, mingled with the tecto-bulbar fibers of the ventral median fascicles of group (1) ('396, figs. 23, 24).
Small cells in the anteroventral part of the peduncle are intimately connected with the underlying hypothalamus and are doubtless concerned primarily with olfacto-visceral adjustments (figs. 18, 21, 53). There are numberless terminals here of the nervus terminalis, tr. hypothalamo-peduncularis from the ventral hypothalamus ('42, p. 226 and figs. 3, 22, 23, 68, 69), tr. mamillo-peduncularis from the dorsal hypothalamus ('396, fig. 22; '42, fig. 39), and tr. olfactopeduncularis from the anterior olfactory nucleus and primordial head of the caudate nucleus ('396, fig. 1). This area also receives terminals of the secondary and tertiary visceral-gustatory tracts, and there is a strong pedunculo-hypothalamic connection (fig. 8; '42, p. 227). Ventral tegmental fascicles (3) and (4) from the hypothalamus ('42, fig. 3) are related with this area ('42, figs. 39, 40, 44), and these fascicles, together with the related small cells, probably contain primordia of those components of the mammalian dorsal longitudinal fasciculus of Schutz which are related with the peduncle and hypothalamus (Thompson, '42, p. 249).
Cells of medium size at the posterior end of the peduncular gray in the vicinity of the nucleus of the III nerve (fig. 59) have the chief dendrites directed rostrad, some of them longer and more sharply bent forward than those shown in figure 59. These dendrites engage fibers from the basal optic tract, visceral-gustatory system, hypothalamus, tr. olfacto-peduncularis, and mesencephalic terminals of the f. retroflexus, among other systems. Their axons also are directed forward; how far they extend within the basal forebrain bundles is not evident, probably for a long distance because they are thick, smooth fibers similar to those of the long descending and ascending tracts. It is not improbable that they reach the primordial corpus striatum, where similar fibers end in wide terminal arborizations ('396, figs. 39, 41, 50, 72, 73; '42, fig. 69). These fibers may be comparable with those described in mammals as ascending from the entopeduncular nucleus and substantia nigra to the corpus striatum.
The neuropil of the alba resembles that of the ventral thalamus, but that of the gray substance is more dense and of different composition. In the white substance there are mingled terminals of axons derived from a great variety of sources — tectum, basal optic tract, posterior commissure and its nucleus, dorsal and isthmic tegmentum, the entire diencephalon, and the basal forebrain bundles (figs. 12, 14, 15, 17, 18, 20, 21, 22). There is little evidence of regional localization of these terminals except in two places.
Posteriorly, in the vicinity of the nucleus of the III nerve, the deep neuropil of the gray is very dense, with strands of fibers descending from the tectum and the dorsal and isthmic tegmentum (fig. 24). These clearly envelop the cell bodies of the III nucleus, and many of them decussate here in the ventral commissure. A second and more sharply localized area of neuropil is the area ventrolateralis pedunculi, extending superficially from the III root forward (fig. 23), as described in chapter iii.
The commissure of the tuberculum posterius includes all fibers of the ventral series of commissures which cross in the midbrain; for details of its connections see chapter xxi.
Summary. — The peduncle of Amblystoma as here defined receives fibers from practically all parts of the brain above the isthmus, and its primary function is control of mass movements of the trunk and limbs and conjugate movements of the eyes. Control of local reflexes is effected elsewhere. It is intimately connected with the hypothalamus, and various visceral motor adjustments are made here, though of these little is known.
