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

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Chapter XI Medulla Oblongata

There is no systematic description of the medulla oblongata of adult Amblystoma comparable with my paper on Necturus ('30) . Coghill's papers give many details of the early stages of development, but no comprehensive description has been written. The 38mm. larva was described in 1914, and subsequent study convinces me that the conditions found there and in Necturus are not fundamentally different in adult Amblystoma, though the latter is considerably more specialized. This is evident upon comparison of the figures of adult Amblystoma recently published ('446) with those of the two papers just cited, to which the reader is referred for general discussion. At metamorphosis the changes are far less in Amblystoma than in Salamandra and some other urodeles. Descriptions of the medulla oblongata and cerebellum of Cryptobranchus, Siren, Proteus, Salamandra, and some other urodeles are found in the papers cited on page 11. Some details of the schematic outline given in chapters iv and v will now be filled in.

SENSORY ZONE

The sensory zone forms the massive dorsolateral wall of the fourth ventricle from the calamus scriptorius forward, to and including the thick auricle under the cerebellum (p. 44). This zone is reduced in size but not suppressed in the floor of the lateral recess of the ventricle spinal ward of the auricle. In the remainder of the medulla oblongata the gray substance forms a low ridge bordering the taenia of the fourth ventricle and projecting into the ventricle, the acousticolateral area, which is smaller in the adult than in the larva (figs. 9, 89, 90). Ventrally of this area is a narrower ridge, which expands posteriorly — the visceral lobe (figs. 9, 88). This contains the nucleus of the fasciculus solitarius.

The white substance of the sensory zone is composed chiefly of the fascicles of sensory root fibers and the associated neuropil. There are, in addition, secondary arcuate fibers and two longitudinal bundles of correlating fibers — tracts a and h of Kingsbury (figs. 7, 9, 87-90).

The sensory root fibers are related peripherally with end-organs which seem to be physiologically as specific as those of man, though the specificities are of different types; but at the first synapse this specificity is no longer preserved in terms of localized gray centers or pathways of conduction. A neuron of the second order may have functional connection with afferent fibers of tactile, vestibular, lateral-line, or gustatory roots, singly or in any combination.

Since most of the sensory root fibers span the entire length of the medulla oblongata and have numberless collateral branches throughout this length, evidently there is little provision for localization of function in terms of fore-and-aft relations in space. Mass responses of the entire musculature are readily evoked by excitation of any or all components of this sensory complex, but how are selective local responses effected? When the arrangement of the secondary connections of these sensory root fibers is examined, it is evident that this question cannot be answered in terms of any mechanism of switchboard type. The axons of these secondary neurons do not take random courses. They tend to be assembled in bundles of fibers, which have a common direction and destination. Such bundles as ascend to higher levels (the lemniscus systems) evidently have some kind of specificity, for they retain their identity up to termination in definite and different places. This specificity may be determined by the source of their excitation — the modality of sense and the location in the body of the receptive end-organs — or it may be determined by the nature of the response to be evoked — muscular contraction, secretion, and the location of the effector organs activated. Probably both these factors are operative in establishing the pattern of arrangement of the higher centers of adjustment and their connecting tracts of fibers.

The problem of the apparatus employed in making discriminative responses to specific types of sensory excitation is not simplified by these observations. In higher animals the specificity of the various modalities of sense is preserved in central pathways of conduction leading up to higher centers of adjustment in the thalamus and the cerebral cortex. But somewhere in the course of this transmission these diverse, localized, functional systems are brought into relation with one another and are integrated in such a way as to result in appropriate responses to the total situation. In these amphibians the process of integration begins at the first synapse. The organization of the neurons of the second order is such as to facilitate activation of

MEDULLA OBLONGATA 155

large masses of musculature by sensory excitation of any kind, that is, reactions of total-pattern type as defined by Coghill. This is the only kind of response which can be made by Amblystoma in the earliest stages of development of motility ; and in the adult animal it is clear, from both the structural organization of the nervous system and the overt behavior, that the action system as a whole is predominantly of total-pattern type. One of the coarser features of the structural mechanism of this integrated behavior is seen in the waj^ the neurons of the second order act as collectors of excitations of diverse modalities and widely distributed peripheral origin.

Within this primordial apparatus of integrated mass action, as development advances, local areas are differentiated with progressively more restricted functional connections. This is manifested anatomically by contraction of the spread of the dendrites of the secondary elements so as to have synaptic contact with a smaller number of fascicles of root fibers. In midlarval stages some of these elements spread their dendrites through the whole extent of the white substance of the sensory zone, thus having contact with terminals of all fascicles of root fibers (fig. 9, no. 1). Some of these dendrites also reach downward into the zone of correlation, the reticular formation (fig. 9, no. 4). Other elements are in functional relation with a few or only one of the root fascicles (fig. 9, nos. 2, 3). In the adult the dendritic spread is not so wide, and a larger proportion of the neurons are in synaptic relation with one or a few contiguous fascicles of root fibers; yet even here there are few neurons of the second order which may be activated by root fibers serving a single modality of sense.

There is, accordingly, as development advances a progressive restriction and local specialization of the central apparatus of analysis of the manifold of sensory experience, and this is an important factor in the acquisition of the local reflexes which are individuated within the larger total patterns of behavior as this process has been described by Coghill. Furthermore, this process of local specialization of structure within the sensory field, as exhibited in ontogeny, has a parallel in phylogenetic history, as illustrated, for instance, by comparison of the organization of this field of Amblystoma with that of the frog, as described by Larsell ('34, p. 504), and with that of the specialized fishes ('44&).

In the tracts of fibers which ascend to higher levels of the brain stem from this sensory field — the lemniscus systems as described below — there is a similar lack of segregation of separate pathways for the several modalities of sense. The general bulbar lemniscus {Im. of the figures) carries fibers which may be activated from any or all of the sensory nerve roots. The segregation of some specific sensory pathways is incipient in the lemniscus systems, and this is seen with especial clarity in the secondary ascending visceral-gustatory path (tr.v.a.).

INTERMEDIATE ZONE

The reticular formation of the spinal cord is continued forward into the medulla oblongata (fig. 9; '14a, p. 378; '30, p. 59). In Weigert sections (figs. 89, 90) it may be recognized as a lighter field between the bulbar lemniscus (Im.) and the spinal lemniscus (Im.sp.). Here the spino-bulbar, spino-cerebellar, and spinal lemniscus tracts are assembled in the alba.

The neuropil of this field receives dendrites from the underlying gray and from the adjoining sensory and motor zones (fig. 9; '14a, figs. 22-43; '446, figs. 7, 8). Some of these cells are small, with local connections; others are very large, with wide dendritic spread and long axons (most of them myelinated), which descend as internal arcuate fibers to the ventral funiculi. Many of them decussate in the ventral commissure; and both crossed and uncrossed fibers, before or after crossing, bifurcate into ascending and descending branches. The descending bulbo-spinal fibers and the ascending bulbo-tegmental fibers are part of the neuromotor apparatus of bulbar and spinal reflexes. The lemniscus systems, on the other hand, arise chiefly in the sensory zone, though some of their fibers may be axons of neurons of the intermediate zone, a distinction of no great significance, in any event, because many neurons have extensive dendritic spread in both zones. The lemniscus systems also differ from the neuromotor apparatus just described, in their terminal connections, passing not to the motor zone but to higher levels of the sensory zone.

Among the large cells of the intermediate zone the two giant cells of Mauthner are of special interest. They lie at the level of the vestibular roots and have large dendrites directed outward among terminals of these root fibers ('14a, fig. 12). In the 12-mm. larva they are of enormous size ('14a, fig. 53), with thick dendrites, which ramify throughout the entire area of the white substance. In the adult the dendritic spread is less extensive, yet it is sufficiently wide to embrace the greater part of the alba of the intermediate and motor zones and especially the terminal area of vestibular fibers. The thick

MEDULLA OBLONGATA 157

myelinated axons immediately decussate and descend in the f . longitudinalis medialis tlirough the entire length of the spinal cord. These two cells are collectors of nervous impulses from many sources and are part of the apparatus of control of mass movements of the musculature of the trunk. Their specific functions have not been fully explained, though they have been much studied. They are magnified and highly specialized examples of the elements of the nucleus motorius tegmenti of the medulla oblongata.

We lack sufficient knowledge of the structural and physiological properties of this tegmentum to frame satisfactory hypotheses of the actual mechanism of the integrative and co-ordinating functions which seem to be operating here. The intermediate tegmentum is structurally so intimately interwoven with the tegmentum of the motor zone that both probably act in these operations as a functional unit.

At the anterior end of the medulla oblongata the trigeminal tegmentum merges with the isthmic tegmentum, and dorsally of this junction the nucleus cerebelli occupies the position of the intermediate zone. In higher animals the gray of this nucleus is incorporated within the body of the cerebellum as the deep nuclei. The definitive cerebellum, accordingly, is a derivative of both sensory and intermediate zones.

MOTOR ZONE

Little can be added here to the general description in chapter v. The larger cells of the motor field may spread their dendrites through the whole of the motor and intermediate zones and also upward into the sensory zone and across the ventral commissure to the opposite side ('14a, figs. 29, 30, 31, 37, 41, 42; '446, figs. 7, 8). This type of structure is extended forward into the isthmus, where it is more specialized and the small and large cells are segregated, though not completely so. Most axons of the large cells are myelinated in the adult. In midlarval and late larval stages, thick unmyelinated axons of the large cells take curious courses. Some descend uncrossed in the ventral or ventrolateral funiculi. Others decussate in the ventral commissure. The latter may cross transversely as internal or external arcuate fibers, or they may take long ascending or descending courses before crossing obliquely and then turning spinal ward. Before or after crossing, these fibers may divide into long ascending and descending branches, and they may give long branched collaterals along the entire course. The terminals of an individual fiber may reach a large part of the motor field of the medulla oblongata of both sides. How far forward the ascending branches may extend has not been determined; some of them certainly pass beyond the isthmus. Some illustrations of these fibers have been drawn ('396, figs. 42, 46, 51, 57, 58, 61, 66, 68). In the medulla oblongata they are mingled with similar fibers of the thalamo-bulbar, pedunculo-bulbar, and tegmento-bulbar systems ('396, fig. 23). Relatively few of them enter the f. longitudinalis medialis. The extensive and diffuse spread of the terminals of these fibers makes it evident that, if they are employed in any definitely local reactions, the localization is effected by some device other than the arrangement in space of their terminals.

In mammalian neurology the transverse segment of the brain stem, which lies under the cerebellum, is named the "pons" after its most conspicuous external feature. This name is obviously inappropriate here, where there is no pons or any recognizable primordium of it.

FIBER TRACTS OF THE MEDULLA OBLONGATA

The white substance of the medulla oblongata contains many compactly arranged, well-myelinated fibers. Some are very coarse and some thinner. Mingled with these, especially in the sensory and intermediate zones, there are many unmyelinated fibers. In most preparations the several systems are not clearly fasciculated, so that analysis is difficult in the adult. In larvae elective Golgi preparations have revealed many details, though much remains obscure. Most of the ventromedial fibers are descending. Laterally within and surrounding the reticular formation are fibers of local correlation and the ascending lemniscus systems; and dorsolaterally there are the fascicles of sensory root fibers, together with some correlating systems. Most of the latter appear as arcuate fibers, which are very numerous at the outer border of the gray, with some at intermediate and superficial depths of the alba (figs. 87-90).

Sensory fibers of the second order are of five sorts: (1) reflex connections by arcuate fibers with the bulbar motor zone of the same and the opposite side, either directly or with synapse in the intermediate zone; (2) bulbo-spinal connections, crossed and uncrossed, descending in the ventral and ventrolateral funiculi; (3) bulbo-cerebellar connections (see chap, xii); (4) correlation tracts a and h (of Kingsbury), intrinsic to the sensory zone; (5) the lemniscus systems, passing between the sensory zone and higher levels of the same zone, most of

MEDULLA OBLONGATA 1.59

them decussating in the ventral commissure. The fibers of all these kinds are widely dispersed, and their analysis is still incomplete. Arcuate fibers are everywhere present in the medulla oblongata, and each of the five groups of secondary connections mentioned above is represented in these arcuates. There are also deep arcuates from the intermediate and motor zones to the ventral commissure. These are doubtless part of the apparatus by which the patterns of performance of synergic groups of muscles are organized, but the details of actual operation of this apparatus are unknown. It seems to be well established that the primary organization of these motor patterns is intrinsic to the motor and intermediate zones. These intrinsic mechanisms may be activated (1) in a nonspecific way from the sensory zone, with resulting mass movement; (2) more specifically from parts of the sensory field dominated by one or another system of peripheral sense organs, with local motor response; (3) from higher centers of control which co-ordinate all motor activity in the interest of integrated behavior of the body as a whole. The second and third of these types of behavior are recognizably present in Ambly stoma, though at a very primitive level of specialization.

SENSORY CORRELATION TRACTS a AND b

All peripheral somatic sensory fibers which enter the medulla oblongata terminate in a common pool of neuropil, which pervades the sensory zone within which the various qualities of sense seem to be more or less completely merged. The secondary fibers which leave these fields are, however, not strictly equipotential. There is evidently an incipient localization of function among these fibers, but the apparatus by which this specialization is achieved is not clear. It seems probable that part of this apparatus is to be sought in two longitudinal tracts of correlation, which were first described by Kingsbury ('95) in Necturus. To these he gave noncommittal names, "tracts a and 6," one lying above, the other below, the fascicles of lateral-line nerve roots (figs. 9, 87-90). I have written a general description of these tracts of Amblystoma ('446) and, in particular, of their relations posteriorly with the dorsal funiculus of the spinal cord and the nucleus funiculi and anteriorly with the cerebellum. Some additional features may now be added.

Tract a. — This tract follows the taenia of the fourth ventricle for most of its length, anteriorly passing below and through the dorsal island of neuropil, under the floor of the lateral recess of the fourth

160 THE BRAIN OF THEiTIGER SALAMANDER

ventricle, and then arborizing in the lateral vestibular area of the auricle (figs. 7, 32, 33; '446, fig. 14). Posteriorly, at the calamus scriptorius, it converges with its opposite fellow, and the two tracts enter, respectively, the medial fascicles of the dorsal funiculi of the spinal cord. Tract a is regarded as a mixed collection of fibers of correlation, related primarily with the lateral-line nerve roots, a supposition which is supported by Kreht's observation ('30, p. 316; '31, p. 422) that in adult Salamandra, in which the lateral-line roots are atrophied, tract a also disappears. It is connected by arcuate fibers with the intermediate and motor zones of both sides, and probably some of these fibers are commissural, connecting the acousticolateral areas of the two sides.

The nucleus of the dorsal funiculi extends for a considerable distance anteriorly and posteriorly of the calamus scriptorius. Fibers of the slender median fascicle of the dorsal funiculus of the cord terminate in this nucleus, and some of them continue forward for an undetermined distance in tract a. The nucleus funiculi is connected through tract a with the lateral-line area farther forward by fibers, some of which are probably descending and some ascending. The latter may go as far as the cerebellar primordium in the auricle, though there is no demonstration of this. In Triturus, as described by Larsell ('31, p. 48), unmyelinated fibers descend into tract a from the auricle, thus providing for cerebellar discharge into the lateral-line area.

Trad h. — This tract is a mixed bundle of myelinated and unmyelinated fibers, believed to be related primarily and perhaps specifically with the vestibular roots (figs. 7, 31). The literature contains several descriptions of tract b of Amblystoma ('14a, p. 373; '396, p. 604; '446, p. 317; Larsell, '32) and other urodeles. In the aggregate, these give an incomplete account of it. From the data at hand it is concluded that it contains a variety of crossed and uncrossed, ascending and descending, fibers connecting the vestibular areas of both sides with each other, with the underlying tegmentum, with the cerebellum, and with the spinal cord. Kreht ('31, p. 423) describes in Proteus a probable connection with the f . longitudinalis medialis.

Anteriorly, tract h converges with tract a into the vestibular neuropil at the lateral border of the auricle (fig. 91). This is cerebellar territory, the primordium of the flocculus of mammals. Silver impregnations show that many of these fibers end in free arborizations in the auricle ('396, figs. 43, 67, 77, 98). This connection is clear in

MEDULLA OBLONGATA IGl

Triturus also (Larsell, '31, p. 50) and is comparable with the secondary vestibulo-cerebellar tract of mammals. Fibers descending from the auricle into tracts a and b are probably ancestral to the mammalian f. uncinatus of Russell.

At the bulbo-spinal junction, tract b merges with the spinal root of the vestibular nerve, and its fibers descend for an undetermined distance in the lateral fascicles of the dorsal funiculus of the cord. There is, accordingly, a dorsal vestibulo-spinal connection by both peripheral and secondary fibers, a connection which puts the vestibular apparatus into especially intimate relation with the neuropil of the nucleus funiculi. It is probable that fibers ascend from the nucleus funiculi in tract b, and some of these may extend as far as the auricle. As I have pointed out ('44&), this cerebellar connection, if confirmed, would be the obvious precursor of the system of arcuate cerebellar fibers described in primates by Ferraro and Barrera ('35) as passing from the external cuneate nucleus to the cerebellum by way of the corpus restiforme. Tract b connects only with the vestibular field of the cerebellum, and this cerebellar connection of the nucleus funiculi (if present) would have a physiological significance quite different from that of the larger connection by way of the tractus spinocerebellaris (fig. 3), for the latter connects only with the non vestibular body of the cerebellum.

In addition to the dorsal uncrossed vestibulo-spinal connection just described, there is an extensive crossed and uncrossed connection between tract b and the motor field by arcuate fibers. Many of these fibers bifurcate, with branches which ascend and descend in the ventral funiculus. These may correspond with the secondary vestibular fibers of the mammalian f. longitudinalis medialis, but here they are dispersed, and the details of their courses have not been described. In the absence of elective impregnations, our material does not reveal their courses. Their presence may be expected, in view of the fact that they comprise an important constituent of this fasciculus in mammals. An incomplete impregnation in an advanced larva gives some evidence of them. In figure 38 the contorted fiber directed medially from the region of entrance of the VIII root is probably a secondary vestibular fiber, which enters the f . longitudinalis medialis, and the two impregnated fibers seen in this fasciculus may belong to this system.

The inferior olive has not been identified in urodeles, but among the dispersed arcuate fibers are some with connections which suggest the presence of a primordium of this structure ('396, p. 604). Some of the smaller tegmental neurons in the vicinity of the VIII roots send axons laterally into tract b, where they divide with ascending and descending branches. Similar fibers are seen to enter this tract at all levels between the V and X roots. Some of these come from the opposite side. The ascending branches of these fibers pass under the auricular gray and recurve dorsalward to arborize within the gray of the rostral face of the auricle. This may represent a vestigial remnant of the large olive of some fishes, here reduced to insignificant proportions because of the small size of the cerebellum (Ariens Kappers, Huber, Crosby, '36, pp. 668-89).

THE LEMNISCUS SYSTEMS

The less myelinated field of alba of the medulla oblongata between the sensory and motor zones, which I term the "reticular formation," contains the ascending fibers of the lemniscus systems and a neuropil, which receives dendrites of neurons of the underlying gray and also of many neurons of the adjoining sensory and motor zones. This seems to be the field in which patterns of local bulbar reflexes are organized, but its texture and connections have not been satisfactorily analyzed. From the reticular formation and the overlying sensory gray, thick axons, some of which are myelinated, descend as internal and external arcuate fibers to the ventral funiculus. Many of these pass to the motor zone as part of the neuromotor apparatus of bulbar and spinal reflexes. These enter a mixed spino-bulbar and bulbo-spinal tract associated with the spinal lemniscus, as shown in figures 38 and 39. Others, usually thinner fibers, immediately decussate in the ventral commissure and then ascend in the lemniscus tracts to higher levels of the sensory zone.

In the lemnisci of mammals, fibers of the various sensory systems are segregated in separate tracts. In Ambly stoma the arrangement is radically different. Segregation of functional systems is incipient but so little advanced that mammalian names are inapplicable. Under this heading there are included here all ascending fibers from the spinal cord and medulla oblongata, which terminate in the sensory and intermediate zones at higher levels. These are arranged in four groups which take separate courses: (1) the spinal lemniscus complex (Im.sp.), (2) the general bulbar lemniscus (hi.), (3) tr. bulbo-tectalis lateralis (tr.t.b.L), and (4) the ascending secondary visceral-gustatory tract {tr.v.a.). The spino-cerebellar tract is closely associated with the

MEDULLA OBLONGATA 163

first group. There is some evidence of a separate trigeminal lemniscus, as described below.

1. THE SPINAL LEMNISCUS (fIGS. 3, 9, 10, 11, Im.Sp.)

The spinal lemniscus is an extensive system of fibers ascending from the spinal cord and here lying immediately ventrally of the spinal V root. In the cord these fibers arise as axons of cells of the dorsal gray column, which decussate in the ventral commissure. In the region of the calamus this tract receives extensive additions from the nucleus funiculi, the spinal V nucleus, and the commissural nucleus of the opposite side (figs. 3, 87). Some of these crossed fibers from the calamus region have thinner collaterals, which descend for a short distance in the ventral funiculus of the spinal cord. The ascending fibers, at first, lie in the ventral funiculus (figs. 41, 42) but soon turn dorsally to join those from lower levels of the cord ('446, figs. 9-11). The most lateral and ventral fibers of this mixed bundle terminate in the reticular formation of the medulla oblongata — tr. spino-bulbaris (figs. 3, 88, 89, 90). The others at the level of the V nerve roots separate into spino-cerebellar and spino-tectal tracts. Many of the spino-cerebellar fibers are collaterals of the spino-tectal fibers (fig. 10; '14a, p. 376). Some fibers of the spino-tectal tract continue past the midbrain to end in the dorsal thalamus (fig. 34). The entire course of this complex is shown in a diagram ('396, fig. 26) drawn from three adjoining sagittal sections of a larva, and this is confirmed by elective Golgi impregnations of the adult. Terminals of this tract in the dorsal thalamus are seen in figure 44.

2, THE GENERAL BULBAR LEMNISCUS (hn.)

This large fascicle, the general bulbar lemniscus, receives fibers from all parts of the sensory zone of the medulla oblongata (figs. 9, 11). These decussate in the ventral commissure and ascend medially of the reticular formation (figs. 89, 90). In the isthmus they turn dorsally and traverse the midbrain ventrally of the tectal formation, some fibers continuing forward into the dorsal thalamus (figs. 27-34, 91-94). These fibers may apparently carry nervous impulses activated by all kinds of sensory fibers which enter the medulla oblongata. Their terminals are spread quite uniformly throughout the entire extent of the tectum, and here they mingle with those of the spinal lemniscus (fig. 11; '396, fig. 96). There is little evidence of physiological specificity in either of these tracts, save that one comes from the spinal cord and the other from the medulla oblongata and that the latter may be activated by a wider variety of sensory excitations, including the vestibular, lateral-Hne, and visceral-gustatory systems. This lemniscus is one of the largest tracts of the urodele brain and is very large in fishes also. In anurans and higher forms it can be recognized with difficulty because its fibers are dispersed or segregated in other specialized tracts like the trigeminal lemniscus. Amblystoma has nothing comparable with the medial lemniscus of mammals.

There is some evidence of an incipient trigeminal lemniscus, though most of the secondary general cutaneous fibers ascend in the general bulbar lemniscus ('39&, p. 606). From the region of the superior sensory V nucleus under the auricle a large number of arcuate fibers descend to the ventral commissure. Some of these enter the general bulbar lemniscus. Others are added from the entire length of the spinal V nucleus, and in the calamus region similar fibers join the spinal lemniscus. These connections were observed by Woodburne ('36, p. 455). From the region of the superior V nucleus, fibers ascend uncrossed as far as the isthmic tegmentum, where they end. This may be the precursor of an uncrossed trigeminal lemniscus. In the preparation from which figure 39 was drawn, no fibers of the lemniscus systems are impregnated. The visible external and internal arcuate fibers make local bulbar connections, descend to the spinal cord, and ascend only as far as the isthmus. Some of these fibers divide into ascending and descending branches before or after decussation. Anteriorly on the right, a compact fascicle of external arcuates descends from the superior trigeminal neuropil, turns forward, and decussates close to the ventral surface within the interpeduncular neuropil. Most of them descend in the bulbo-spinal tract, but some turn forward as tr. bulbo-isthmialis. This may be a precursor of a separate crossed trigeminal lemniscus. Their course is parallel with tr. bulbo-tectalis lateralis (primordial lateral lemniscus), but deeper.

3. TRACTUS BULBO-TECTALIS LATERALIS (fIGS. U, 91, 9'2, tr.b.t.l.)

The tr. bulbo-tectalis lateralis is a mixed system of fibers, closely associated with the general bulbar lemniscus and of similar origin from the sensory zone, chiefly from its middle part in the vicinity of the vestibular nerve roots. In the isthmus it lies externally of the general bulbar lemniscus, and it terminates exclusively in the primordial inferior colliculus (nucleus posterior tecti) and the underlying isthmic neuropil. These fibers are mingled with those of a system

MEDULLA OBLONGATA 1G5

which descends from the inferior colhculus — tr. tecto-bulbaris posterior (figs. 1)2, 13, 29-34, 89, 90, tr.t.h.p.). Their segregation from those of the general bulbar lemniscus in the urodeles seems to be determined primarily by their terminal distribution; they end in the inferior colliculus, while the larger bulbar lemniscus ends chiefly in the superior colliculus.

This tract is probably the precursor of the lateral lemniscus, which first makes its appearance in definitive form in anurans. A "lateral lemniscus" has been described in the brains of fishes by many authors, but this is a misnomer. That large tract of fishes is comparable with the general bulbar lemniscus of urodeles and has none of the distinguishing features of the mammalian lateral lemniscus, though it doubtless includes the primordium of that tract. Its current name, "f. longitudinalis lateralis," is more appropriate.

Both these bulbar lemniscus tracts are well developed in Necturus and the reader is referred to their description in that animal for discussion of the phylogenetic relationships ('30, pp. 51 ff.; Ariens Kappers, Huber, Crosby, '36). Necturus lacks a cochlear rudiment, and hence the lateral bulbar lemniscus here probably has a very imperfect auditory function, if any. In Ambly stoma and most other urodeles there is such a rudiment, and here audition of a primitive type may be represented, along with other functions, in this tract. In adult Anura there is a well-developed primordium of the cochlea, from which a cochlear division of the VIII nerve arises. Correlated with this, there are cochlear nuclei, from which a true lateral lemniscus passes to the inferior colliculus. Parallel with this differentiation, both general and lateral lemniscus tracts, as seen in urodeles, are radically reorganized. Larsell ('34, p. 521), after thorough examination of this question, accepts my interpretation of the lateral tract of urodeles as an incipient lateral lemniscus, citing my remark ('30, p. 58) : "It may be that this incipience of an auditory lemniscus in Necturus is a central anticipation of a later peripheral specialization of the cochlear rudiment— ^that is, an apparatus for sorting out centrally the meager auditory component of the mixed functional complex of the undifferentiated VIII nerve. But it is more likely another evidence that Necturus is a degenerated or an arrested derivative of some more highly differentiated ancestor." A good brief summary of the evolution of the auditory apparatus has been given by Papez ('36). For further comments on this tract see pages 188, 214.

4. THE SECONDARY VISCERAL-GUSTATORY TRACT {tr.V.a.)

The fibers of the secondary visceral-gustatory tract are segregated in terms of physiological specificity more definitely than are those of the other lemniscus systems. They are axons of neurons of the nucleus of the f . solitarius and commissural nucleus, which ascend uncrossed ventrally of the spinal V root. Most of the visceral sensory fibers of the VII root are gustatory, and most of those of the vagus roots are general visceral. It is probable that the secondary fibers arising more anteriorly are activated mainly from taste buds, but these cannot be distinguished from those of general visceral sensibility. The central courses of these fibers have been fully described and illustrated in the larva ('14a), in the adult ('446), and in Necturus ('30). A useful summary of the comparative anatomy of this system has been published by Barnard ('36). The adult arrangement is shown here in figures 7, 8, 9, 23, 30, 37, 38, 87-90.

One is impressed by the stability of the general plan of these arrangements throughout the vertebrate series, despite the most extreme modifications of the details in adaptation to different modes of life. This applies particularly to the peripheral and bulbar connections. The ascending connections are less well known, and they probably show more radical changes in the series from lower to higher vertebrates. Fishes and amphibians exhibit a common plan, with infinite variety of detail; the arrangement in Amblystoma appears to present this plan reduced to its simplest form, and this generalized plan may be taken as a point of departure from which the cerebral apparatus of visceral-gustatory adjustments of higher animals has been derived.

All the peripheral fibers enter the f. solitarius, which spans the entire length of the medulla oblongata. They are thin, and many of them are myelinated. Most of those from the geniculate ganglion, which form the visceral sensory VII root, have a T-form division, with ascending and descending branches as they eiiter the f . solitarius. Most (perhaps all) of the prefacial f. solitarius is composed of these ascending facialis fibers, which are less heavily myelinated than are their descending branches and, presumably, are gustatory in function. These fibers ascend to the auricle, where they end in the same neuropil as the ascending V root (figs. 7, 30, 38; '14a, p. 365). By this arrangement, correlation may be effected between visceralgustatory and general somatic sensibility of the mouth cavity. The visceral sensory fibers of the IX and X roots are less myelinated than

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are those of the VII root. Most of them descend without division in the f. soHtarius, but some divide with branches, which ascend for short distances (fig. 37; '446, figs. 14, 17).

In Amblystoma, as in all other vertebrates, the two f . sohtarii converge at the calamus scriptorius into the commissural nucleus, and part of their fibers decussate here in the dorsal commissura infima of Haller. Here at the bulbo-spinal junction there is an important field of correlation between the visceral-gustatory systems and the general somatic sensory systems of the entire body. The details of this structure in Amblystoma, the comparative anatomy of this region, and its strategic importance for fundamental physiological problems have recently been discussed ('446) and are summarized here in chapter ix.

The nucleus of the f. solitarius contains some neurons with dendritic connections exclusively with this fasciculus, but most of these elements have other connections also. Some of their axons cross in the ventral commissure and are lost to view in the vicinity of the general bulbar lemniscus; others ascend uncrossed in the secondary visceral-gustatory tract to the isthmus, midbrain, and hypothalamus. Some of these secondary fibers bifurcate, with branches taking both these courses (fig. 9) . The first of these pathways is probably more primitive, for Barnard ('36, p. 513) writes : "There is no differentiated, uncrossed secondary gustatory tract in the lamprey. All connections with higher centers are through the bulbar lemniscus system." It is believed that in Amblystoma the decussating fibers are concerned chiefly with bulbar reflexes; whether any of them ascend to higher levels in the bulbar lemniscus is not clear. In any event the uncrossed tract is evidently the chief pathway to higher centers. Rostrally of the V roots this tract divides, some of its fibers continuing forward to area ventrolateralis pedunculi and some turning dorsad to the superior visceral nucleus in the isthmus. We have many elective preparations of these fibers which have been cut in various planes, and the entii-e course is clear.

In the ganoid fishes (Johnston, '01; Barnard, '36, p. 517) the secondary visceral tract takes essentially the same course as in Amblystoma; but the superior visceral nucleus lies more ventrally and posteriorly, under the cerebellum. In those teleosts in which the gustatory system is enormously enlarged (carp and catfish), the secondary visceral-gustatory nucleus shows corresponding enlargement and is displaced forward and dorsalward in the isthmus to a position similar to that seen in Amblystoma. This makes it probable that in Amblystoma the fibers of the secondary tract that pass through the isthmus and go directly forward to the peduncle and hypothalamus are general visceral in function and that those that take the more dorsal course to the superior nucleus are chiefly gustatory in function, though this distinction may not be sharply drawn. In support of this conclusion, Barnard ('36, p. 595) mentions the fact that this superior nucleus disappears in birds, in which the gustatory apparatus is greatly reduced. If we may accept the suggestion of Fox ('41, p. 418) and others that the ventral tegmental nucleus of mammals is comparable with the superior visceral-gustatory nucleus of lower forms, it is evident that the mammalian nucleus has a more posteroventral position, similar to that of the generalized ganoid fishes.

The isthmic secondary visceral nucleus was first identified in larval Amblystoma ('14a, pp. 364, 373) and subsequently in Necturus ('17, p. 248; '30, p. 62). Its cells form a low ventricular eminence at the posterior dorsal tip of the isthmic tegmentum (fig. 2B). They are of medium size, scattered and clumped in an open neuropil (fig. 34), with outlying cells dispersed in the alba. As seen in transverse sections, they lie immediately ventrally of the posterior end of the isthmic sulcus ('25, fig. 19; '42. fig. 43). Their long dendrites are directed laterally and ventrally into the posterior isthmic neuropil, where they engage terminals of the secondary visceral tract, ascending root fibers of the trigeminus, tr. bulbo-tectalis lateralis, tr. bulboisthmialis (fig. 38), collaterals of tr. tecto-bulbaris rectus (fig. 37; '42, fig. 67), and terminals of the dorsal tegmental fascicles, notably those of group (9), as described in chapter xx, from the cerebral hemispheres. There is also a diffuse connection with the cerebellar formation, and this is much more intimate in some fishes ('05) .

An extension of the secondary visceral-gustatory tract to the tectum has been described by Brickner ('30) in some teleostean fishes. In these fishes there is a large commissure connecting the two visceral-gustatory nuclei, which includes many decussating fibers of the secondary visceral-gustatory tract. A separate fascicle of the latter fibers turns forward, as tr. gustato-tectalis, to reach the anterior part of the tectum opticum. This connection has not been observed in Amphibia, but it may exist. It makes provision for correlation of visceral-gustatory with visual experience, and it is accompanied by

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a return path from the tectum to the visceral-gustatory nucleus byway of tr. tecto-bulbaris rectus — a typical reflex circle (p. 76).

At the outer border of the gray of this nucleus, in a few preparations there are large spindle-shaped neurons, with dendrites extending for long distances tangentially to the gray layer. One dendrite may ramify among terminals of the secondary visceral tract and dorsal tegmental fascicles of groups (7), (8), and (9) and the other in the deep neuropil and the f. tegmentalis profundus. One such element is illustrated in figure 101, showing an axon directed dorsally toward the decussatio veli, possibly a commissural connection between the two visceral nuclei similar to that described in the frog and in fishes.

These afferent connections suggest that this isthmic nucleus is a correlation center, where gustatory, general visceral, and a considerable variety of other types of experience are brought into relation, with the gustatory component dominant and all in the interest of reflexes concerned with feeding. The afferent fibers of the secondary gustatory tract terminate not only in the neuropil of the visceralgustatory nucleus but also in that of the adjacent isthmic and dorsal tegmentum ('25, fig. 19; '42, fig. 43), where reflexes of the jaw and hyoid musculature are believed to be organized (p. 190).

This hypothesis is supported also by the courses taken by the efferent fibers. These go out in several directions. First, there is a dispersed group of fibers which spread in the underlying tegmentum (figs. 8, 13). This is a direct connection with the neuromotor apparatus of the mouth and pharynx. The remainder enter the tertiary visceral tract (fig. 8, tr.v.t.), which has rather wide distribution. This tract of Amblystoma is relatively small, and its fibers are dispersed and so intimately mingled with others that analysis is difficult. The diagrams show what has been clearly seen. In figure 8 the dotted component of tr.v.t. is highly probable, though not confirmed by elective impregnations.

In the description of the brachium conjunctivum (p. 176) this fascicle is shown to descend close to the gray in the posterior lip of the isthmic sulcus to reach the decussation. In this part of their course the cerebellar fibers comprise a large component of the f . tegmentalis profundus. As they emerge from the cerebellar formation, they pass through the superior visceral nucleus, and here they are joined by a smaller number of similar thin unmyelinated axons of cells of this nucleus. These two systems of fibers are mingled, and analysis is impossible except in electively impregnated material. We have few such specimens of either Amblystoma or Necturus, but those we have show that in the ventral commissure immediately spinalward of the decussation of the f. retroflexus some of the visceralgustatory fibers turn forward in company with those of the ventral secondary visceral tract, and both groups of fibers arborize in the ventrolateral neuropil of the peduncle. Some of these fibers probably pass through this neuropil without synapse to reach the hypothalamus, as in fishes, in company with pedunculo-mamillary fibers and mingled with those of the mamillo-peduncular, mamillo-tegmental, and mamillo-interpeduncular tracts. At their decussation the tertiary visceral fibers spread in the alba of the isthmic tegmentum before and after crossing, and some of them probably reach the interpeduncular neuropil, though this needs confirmation. All these systems and others (including, perhaps, mamillo-cerebellar fibers) are mingled, and none of the tracts are closely fasciculated.

In some fishes these tracts are separately fasciculated, and the amphibian arrangement, so far as revealed in our material, conforms with the teleostean pattern. The tertiary visceral-gustatory tract is very large in some of these fishes ('05, pp. 420, 436, figs. 20, 23, 37, 38), passing from the secondary nucleus directly to the hypothalamus. This direct connection has been described by Larsell ('23, p. 109) in the frog; and in both Amblystoma and Triturus he saw some indications of it. In the frog these direct fibers do not take the deep course, as described above for Amblystoma, but are superficial, accompanying mamillo-cerebellar fibers. I find evidence of this superficial connection in Amblystoma also, but in the absence of elective impregnations no accurate description can be given. These fibers in some of our preparations are seen as a well-fasciculated tract. In the Cajal sections, from which figures 25-36 were drawn, it is not so clearly shown as in some other specimens, but its location can be identified. These fibers assemble at the lateral surface rostrally of tr.v.a. (fig. 34), in company with those of tr.t.b.p., as drawn in that figure. Ten sections farther ventrally, they are drawn, but not labeled, in figure 33 superficially of tr.t.h.r. In some Golgi preparations, fibers in this position connect with the hypothalamus, and this is the course taken by the tertiary visceral tract in the frog (Larsell, '23, figs. 2, 4). This is the position also of the tr. mamillo-cerebellaris,

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which I provisionally identified in Necturus ('14, p. 8) and subsequently ('17, p. 250) questioned and the presence of which has been confirmed by Larsell.

In the light of such fragmentary evidence as we now have, it seems to me probable that in Amblystoma the ascending tertiary visceralgustatory fibers take a deep course, accompanying the brachium conjunctivum, and that the superficial fascicle described above is a mamillo-cerebellar tract, which also connects with the isthmic tegmentum and the isthmic visceral-gustatory nucleus.

The bulbar apparatus of visceral-gustatory adjustment is simply organized in Amblystoma, as, indeed, it is in man, with minimal provision for local specialization. The division of the ascending path into dorsal and ventral moieties in the isthmus is similar to the divergence of the olfactory paths to epithalamus and hypothalamus. In both cases somatic correlations are effected dorsally and olfactovisceral correlations ventrally. The dorsal gustatory nucleus may directly activate the skeletal musculature concerned with feeding, through its connections with the underlying tegmentum, or it may discharge into the hypothalamus, where all visceral adjustments are organized.

The swallowing of a morsel of food in response to excitation of taste buds is one of the simplest acts of which the body is capable. This may be done reflexly through the local activation of the bulbar connections of the f. solitarius; but, simultaneously with this local action, nervous impulses may be transmitted to higher centers which are so interconnected as to bring this simple act into relation with any other activities that may be in process in response to internal or external stimuli. The final result of any particular sensory excitation is dependent upon the central excitatory state of the central nervous system as a whole and of every local part of it. And the quality of this excitatory state is determined not only by the stimuli presently acting upon it but also by the^past experience of the race and of the individual. This theme is amplified in a recent discussion of the apparatus of optic and visceral correlation ('44a).

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Reference

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

Cite this page: Hill, M.A. (2024, April 20) Embryology Book - The brain of the tiger salamander 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_brain_of_the_tiger_salamander_11

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