- Email: [email protected]
Fiber degeneration following lesions in the posteroventral cochlear nucleus of the cat
EXPERIMENTAL
NEUROLOCY
23,14Q-155
(1969)
Fiber Degeneration following Lesions in the Posterovkntral Cochlear Nucleus of the Cat Defiartment
W. BRUCEWARR 1 of Anatomy, Boston Unizrersity Boston, Received
School
of Medicine,
MassacRusetts 02118 October
25,1968
The results of studying fiber projections of the anteroventral cochlear nucleus (AVCN) by the Nauta and protargol methods were described in a previous paper. In the present study using similar methods, lesions in the posteroventral cochlear nucleus (PVCN) produced degeneration in the intermediate acoustic stria of Held and in the trapezoid body. Neither of these tracts projects to principal nuclei of the superior olivary complex, but rather they have overlapping projections to ipsilateral periolivary cell groups which partially surround the lateral superior olivary nucleus. Contralaterally, axons in the stria of Held project to: (i) The dorsomedial periolivary cell group; (ii) the anterolateral periolivary cell group; (iii) the ventral nucleus of the lateral lemniscus; and (iv) the inferior colliculus. Fibers in the trapezoid body project to the contralateral ventromedial periolivary cell group. The possibility that these projections represent, in part, inputs to cells which give rise to the olivocochlear bundle is discussed. In contrast to AVCN which projects to the lateral and medial superior olivary nuclei proper, PVCN projects to adjacent periolivary cell groups, but both AVCN and PVCN project in an overlapping fashion to the ventral nucleus of the lateral lemniscus; bilaterally. This result lends further support to the idea that the cochlear nucleus is a complex of subnuclei each of which has specific sets of projections. Introduction
The mammalian cochlear nucleus serves both as the central terminus of the cochlear nerve and as the origin of fiber projections to the nuclei of the superior olivary complex, lateral lemniscus, and inferior colliculus.
Each fiber of the cochlear nerve ramifies within each of several histologically distinct regions of the cochlear nucleus (8, 14, ZO), but the neurons in particular regions may have more limited projections to central auditory nuclei. For example, the anteroventral cochlear nucleus projects r This work was supported in part by Research Grant NB-01344, from the National Institutes of Health, and in part by Public Health Service Grant FR-05485 during the author’s tenure at the Eaton-Peabody Laboratory at the Massachusetts Eye and Ear Infirmary. It is a pleasure to thank N. Y. S. Kiang and D. K. Morest for their helpful advice and criticism during all phases of this work. The assistance of Mrs. S. S. Guinan during the surgical operations is gratefully acknowledged. 140
COCHLEAB
NUCLEUS
141
to the medial and lateral superior olivary nuclei, but not to most of the periolivary cell groups of the superior olivary complex (23). In the present paper, projections from the posteroventral cochlear nucleus are analyzed with degeneration methods. In contrast to the anteroventral cochlear nucleus, the posteroventral nucleus projects to periolivary cell groups. Methods
The methods used in this study were the same as those described previously (23). The material consisted of 65 adult cats with lesions located in the various subdivisions of the cochlear nucleus, including the posteroventral, interstitial, anteroventral, and dorsal cochlear nuclei. In two animals (cats 61 and 62), the lesion was limited to the posterior two-thirds of the posteroventral cochlear nucleus. Lesions were also placed in fiber tracts associatedwith the cochlear nucleus for the purpose of studying retrograde chromatolysis. Control lesions included destruction of cerebellar cortex, dentate nucleus, and the cochlear nerve. All surgery was performed under asceptic conditions and pentobarbital anesthesia. The cochlear nucleus was visualized from its posterior aspect following retraction of the overlying cerebellum for a distance of approximately 3 mm and for a period of about 15 sec. During this time, radiofrequency lesions were made by inserting a 26-gauge Nichrome electrode, insulated except for 0.5 mm at the tip, into the cochlear nucleus. Lesions limited to the posteroventral cochlear nucleus were made by penetrating its free surface located along the posteromedial margin of the cochlear nucleus where it approachesthe inferior cerebellar peduncle. After 3-28 days, the animals were perfused through the heart (7) with a fixative containing 10% formalin and 3% potassium dichromate (suggested by G. L. Rasmussen). The modal survival time for the 65 cats was 7 days. Frozen sections were cut transversely at 25 p and a group of sections spaced 100 ,u apart were stained en masse (4) by the Nauta method (10). Sections were routinely counterstained with buffered thionin (24) in order to permit the study of retrograde chromatolysis. In all animals discussed below, a group of intervening sections spaced 200 p apart were stained with Protargol, as described by Rasmussen (16), with the exception that no mordant was used. The criteria used in identifying degenerated fibers and their pericellular end-arborizations were as described in a previous study (23). The nomenclature used for nuclei of the brain stem is based on that of Taber (22). Results
Lesions limited to the posterior two-thirds of the posteroventral cochlear nucleus (PVCN) did not produce preterminal degeneration in the principal cell massesof the superior olivary complex, namely, the lateral and medial
142
WARR
superior olivary nuclei. Rather, preterminal degeneration was found in several small cell groups which lie adjacent to these principal olivary nuclei. In a recent study with Golgi methods, Morest (9) used the general term “peri-olivary” to refer to these cell groups, and applied standard relational adjectives, such as dorsomedial and ventromedial, to refer to specific groups of cells located in the periolivary region. For example, the dorsomedial periolivary cell group is located dorsomedial to the medial superior olivary nucleus. This same convention will be adopted in the present paper. In the two cats with the lesion limited to the posterior two-thirds of PVCN, the course of degenerated fibers and the sites of preterminal degeneration were identical. Degenerated fibers leave the lesion to reach the superior olivary complex by way of two distinct fiber tracts: The intermediate acoustic stria of Held ; and the trapezoid body. Ipsilateral Degeneration in tlze Stria of Held. Degenerated fibers destined to form the intermediate acoustic stria of Held depart from the lesion in a dorsal direction, and upon reaching the lateral border of PVCN form a compact bundle consisting of large and moderate sized fibers (2-3 p in Protargol-stained material). This bundle travels along the lateral contour of the inferior cerebellar peduncle (Fig. lA, B, SH), and upon reaching the dorsal surface of the peduncle, breaks up into small fascidles which turn .sharply ventral and penetrate, in order, the spinal vestibular nucleus, the spinal tract of the trigeminal nerve and the oral division of its nucleus. Turning medially, these fascicles penetrate the motor nucleus of the facial nerve, or the rubrospinal tract, and arborize among cells lying directly posterior to, and extending anteriorly over the dorsal surface of the lateral superior olivary nucleus. Three groups of cells surrounding the lateral superior olivary nucleus receive an ipsilateral input from the stria of Held : The posterior periolivary cell group (Fig. lD, p) ; the dorsolateral periolivary cell group (Fig. lE, dl) ; and the anterolateral periolivary cell group (Fig. lF, al). As Taber (22) observed in Nissl preparations, neurons in the region corresponding to the posterior periolivary cell group (the posterior extremity of the lateral nucleus of the trapezoid body) are characteristically “medium-sized multipolar cells, each with a central nucleus and coarse deeply stained Nissl granules.” In the present material, the region also contains cell clusters composed of neurons similar in form to the principal neuron of the medial nucleus of the trapezoid body (9, 22). Preterminal degeneration was found only among the medium-sized multipolar cells (Fig. 2A) and appeared to be associated with only a fraction of the multipolar cells present in the posterior periolivary cell group even after total section of the stria of Held. Preterminal degeneration was present throughout the anteroposterior extent of the dorsolateral periolivary cell group (Fig. IE, &), but it was
COCHLEAR
NUCLEUS
I43
most profuse posteriorly, where there are numerous transverse degenerated fibers of the stria of Held (Fig. 2B). At more anterior levels, preterminal degeneration was supplied by fibers coursing longitudinally among the cells. A few of these fibers continue forward to project sparse preterminal degeneration to the anterolateral periolivary cell group (Fig. lF, al), and to a small cluster of cells in the dorsomedial portion of the ventral nucleus of the lateral lemniscus (Fig. lG, VLL). A very few degenerated fibers continue dorsally beyond this latter nucleus, but their site of termination could not be established. Many degenerated fibers of Held’s stria emerge medially from the posterior periolivary cell group. They regain a dorsal position by passing between the lateral and medial superior olivary nuclei and joining the more anterior fascicles of the stria as it courses toward the midline (Fig. lE,
SH). As the stria travels toward the midline, occasional fibers depart from the parent bundle and provide an extremely sparse preterminal arborization to a cell group adjacent to the dorsomedial extremity of the medial superior olivary nucleus (Fig. lE, dm). This cell group is located within the dorsomedial periolivary nucleus (9). Preterminal degeneration remains limited to this cell group even after complete section of the stria of Held. Contralateral Degeneration in fhe Stria of Held. Degenerated fibers of the stria of Held decussate dorsal to the trapezoid body and enter the contralateral dorsomedial periolivary cell group, where sparse preterminal degeneration was found (Fig. 1E and F, dm; Fig. 2C). Continuing its course to the lateral lemniscus, the stria of Held divides into two separate bundles of approximately equal size, one passing ventral to and the other dorsal to the medial superior olivary nucleus (Fig. 1E and F). The two bundles begin to reunite just rostra1 to the lateral superior olivary nucleus, where both supply moderate preterminal degeneration to the anterolateral periolivary cell group (Fig. lF, al; Fig. 2D). At this same anteroposterior level, the more ventral fibers provide profuse preterminal degeneration to the most posterior cells of the ventral nucleus of the lateral lemniscus (Fig. lF, VLL). At slightly more anterior levels, both ventral and dorsal components of Held’s stria ramify profusely among cells of the ventral nucleus of the lateral lemniscus (Fig. lG, VLL). Among the most posterior cells, preterminal degeneration is both profusely and uniformly distributed. At progressively more anterior levels it shifts medially (Fig. 3A), and becomes less profuse. In the most anterodorsal portions of the ventral nucleus of the lateral lemniscus, preterminal degeneration is sparse and tends to gather in small clusters (Fig. 1H). Little, if any, preterminal degeneration is present among cells of the ventral nucleus of the lateral lemniscus which correspond in position to
144
WARR
FIG. 1. Drawings of transverse Nauta-stained sections showing the course of degenerated fibers (dashes) and site of preterminal degeneration (stippling) consequent to the lesions (solid black areas) in cat 62. Arrow in F indicates anterior continuation of trapezoid body fibers. The area enclosed by broken lines in G is the posteromedial division of the ventral nucleus of the lateral lemniscus. A is the most posterior section. Postoperative survival time : 8 days. Abbreviations : al, anterolateral periolivary cell group; AV, anteroventral cochlear nucleus; CNIC, central nucleus of the inferior colliculus ; DAS, dorsal acoustic stria; DC, dorsal cochlear nucleus ; DLL, dorsal nucleus of the lateral lemniscus ; dl, dorsolateral periolivary cell group ; dm, dorsomedial periolivary cell group ; FN, facial nerve; G, genu of the facial nerve; ICP, inferior cerebellar peduncle; IH, interstitial nucleus of Held’s stria ; IN, interstitial nucleus of the cochlear nerve; LL, lateral lemniscus ; LSO, lateral superior olivary nucleus; LTB, lateral nucleus of the trapezoid body; LVN, lateral vestibular nucleus; MCP, middle cerebellar peduncle; ML, medial lemniscus; MSO, medial superior olivary nucleus ; MTB, medial nucleus of the trapezoid body; P,
COCFILEAR
NUCLEUS
145
those which receive an ipsilateral projection. No preterminal degeneration is found in the posteromedial division of the ventral nucleus of the lateral lemniscus (Fig. 1G, areas encircled by broken lines). The relatively few degenerated fibers which reach the most anterodorsal levels of the ventral nucleus of the lateral lemniscus are of small or medium size and travel in the most lateral portion of the lateral lemniscus. They provide little, if any, preterminal degeneration to the dorsal nucleus of the lateral lemniscus. A few enter the inferior colliculus where they arborize in the posterior part of the central nucleus. No degenerated fibers are present in the brachium or in the commissure of the inferior colliculus. Lesions of the dorsal cochlear nucleus, interstitial cochlear nucleus, and anteroventral cochlear nucleus did not produce degeneration in the intermediate stria of Held. Ipsilateral Degeneration in the Trapezoid Body. Degenerated fibers travel anteriorly from the lesion site and enter the posterior trapezoid body where, scattered throughout its dorsoventral extent, they course toward the superior olivary complex (Fig. 1D and E, TB). Small fascicles of small- and medium-sized degenerated fibers depart from the parent bundle, usually at nearly right angles, and form terminal arborizations among cells of the posterior periolivary cell group, dorsolateral periolivary cell group, and ventrolateral periolivary cell group (Fig. lD, p, and E, dl, and vl). Within the posterior periolivary cell group, only the multipolar cells (see above) are apparently involved. Most of the fibers destined to reach the dorsolateral periolivary cell group arch over the posterior surface of the lateral superior olivary nucleus but some actually pass through its medial half (Fig. lE, LSO) and emerge from its dorsal surface where they ramify. The fibers coursing through the lateral superior olivary nucleus appear to be fibers of passage. The ventrolateral periolivary cell group is a region with a relatively sparse neuronal population located just ventral to the medial hilus of the lateral superior olivary nucleus (Fig. lE, vl). It lies directly in the path of degenerated fibers which depart at right angles from the trapezoid body and which appear to continue dorsally to ramify within the dorsolateral periolivary cell group. Total section of the intermediate acoustic stria of Held produced little, if any, preterminal degeneration in the ventrolateral pyramidal tract; p, posterior periolivary cell group; PB, pontobulbar body; PN, pontine nucleus ; PV; posteroventralcochlear nucleus ; SCP, superior cerebellar peduucle ; SH, stria of Held ; ST, spinal tract of the trigeminal nerve ; TB, trapezoid body; VLL, ventral nucleus of the lateral lemniscus; VI, ventrolateral periolivary cell group; vm, ventromedial periolivary cell group ; VN, vestibular nerve ; IV. v., fourth ventricle; V.m.n., motor nucleus of the trigeminal nerve; VII. n., nucleus of the facial nerve.
146
WARR
FIG. 2. Preterminal degeneration in periolivary nuclei of cat 62. A. Ipsilateral posterior periolivary cell group showing medium-sized multipolar cells, X300. B. IpsiX 120. C. Contralateral dorsomedial lateral dorsolateral periolivary cell group, periolivary cell group, X 300. D. Contralateral anterolateral periolivary cell group, X300. Postoperative survival time: 8 days. Nauta method, thionine counterstain. Abbreviations : LSO, Lateral superior olivary nucleus.
.
FIG. in the level Nauta eration group stain,
COCHLEAB
NUCLEUS
147
3. A. Photomicrograph showing the distribution of preterminal degeneration contralateral ventral nucleus of the lateral lemniscus at an anteriorposterior comparable to that of Fig. IH. Cat 62. Postoperative survival time: 8 days. method, thionine counterstain, X80. B. Photomicrograph of preterminal degenfrom the trapezoid body in the contralateral ventromedial periolivary cell of cat 61. Postoperative survival time : 10 days. Nauta method, thionine counterx300.
148
WARR
periolivary cell group and no degenerated fibers of passage in the medial half of the lateral superior olivary nucleus. In addition to these prominent connections of the trapezoid body, a very few degenerated fibers were found entering the caudal extremity of the ventromedial periolivary cell group (Fig. lD, vnz). The precise rostrocaudal level at which this connection occurs is not represented in Fig. 1, but lies between drawings C and D in the most posterior portion of the superior olivary complex. -4 larger contralateral projection by way of the trapezoid body to the ventromedial periolivary cell group is described below. Contralateral Degeneration in the Trapesoid Body. Degenerated fibers, ranging in size from small to large, cross the midline throughout the depth of the posterior third of the trapezoid body (Fig. 1D and E) . There is no clear segregation of fibers according to their size either in the dorsoventral or the anteroposterior axis of the trapezoid body, The approximate anterior limit of these crossing fibers is at the level where the lateral superior olivary nucleus assumes its characteristic “S” shape in transverse section. Degenerated fibers either traverse or pass ventral to the medial nucleus of the trapezoid body. Most of them ramify within the ventromedial periolivary cell group (Fig. 1D and E, VW). Preterminal degeneration within this cell group is scattered among its characteristic large pale neurons, four or five of which commonly appear in a 25-p transverse section (Fig. 3B). Preterminal degeneration within the ventromedial periolivary cell group gradually diminishes in quantity and finally cannot be demonstrated at all in progressively more anterior levels of the cell group (Fig. lF, vm). Complete section of the stria of Held in two animals produced no preterminal degeneration in this cell group, although degenerated fibers comprising the ventral component of the stria do traverse the ventromedial periolivary cell group (Fig. lE, F, VWL) . A small proportion of the most ventral fibers of the trapezoid body turns rostrally and travels in a small bundle on the surface of the trapezoid body (Fig. lF, arrow). This bundle gradually becomes smaller and ultimately vanishes as its degenerated fibers turn dorsally into the trapezoid body and mingle with the ventral component of the stria of Held. The terminations of this superficial bundle could not be determined. As mentioned above, the more dorsal of the degenerated trapezoid fibers traverse the medial nucleus of the trapezoid body (Fig. 1D and E). Study of Nauta-stained sections produced no convincing evidence of preterminal degeneration in this nucleus. Moreover, it was observed that many of the fibers entering the medial nucleus of the trapezoid body continue on to terminate in the ventromedial periolivary cell group. However, the possibility that some of the fibers, especially the largest ones, end in the medial
COCHLEAR
NUCLEUS
149
nucleusof the trapezoid body in the form of calyces of Held (1, 3, 9, 14, 21) was checked in Protargol-stained frozen sections in cats 61 and 62. In normal material stained with Protargol, the calyces of Held can be reliably impregnated and found in association with cells characterized by an ovoid or spherical cell body, an eccentric nucleus and deeply staining cytoplasm (9). If some of the degenerated fibers in the present material terminate as calyces, then some of the cells meeting these criteria of identification should lack calyces. The possibility that transneuronal degenerative changes (13) would invalidate these criteria of identification would appear to be slight because the postoperative survival times (8 and 10 days) were too brief. In each cat, ‘ten transverse sections through the relevant portion of the medial nucleus of the trapezoid body were selected. Each cell meeting the criteria stated above was examined under an oil immersion objective for the presence of a calyx. On the contralateral side in cat 62, only three cells lacking calyces were counted among more than 300 cells examined. None was found ipsilaterally. In cat 61, none was found on either side. Degeneration Within the Cochlear Nucleus. In addition to the projections from PVCN already described, degenerated fibers were also traced to : (i) A small, hitherto undescribed nucleus, located along the lateral edge of, and partially embedded in, the stria of Held, and (ii) the dorsal cochlear nucleus. The former nucleus, referred to here as the interstitial nucleus of the stria of Held (Fig. lB, IH), contains a dense network of fine degenerated fibers (Fig. 4). These fine fibers are traceable dorsally from the lesion in PVCN to their termination. .Very few of these fibers continue beyond this nucleus with the stria of Held. Whether any of the characteristically larger fibers in the stria of Held contribute to this nucleus could not be determined. Neither destruction of the spiral ganglion of the cochlea nor lesions in any of the other subdivisions of the cochlear nuclear complex produce preterminal degeneration in the interstitial nucleus of the stria of Held. The fine degenerated fibers destined to reach the dorsal cochlear nucleus leave the lesion site in a dorsomedial direction and follow the medial border of the PVCN until they reach its dorsal extremity. Here they turn laterally and ramify in the dorsal cochlear nucleus (Fig. lC, DC). The exact layer of the dorsal cochlear nucleus in which these fibers terminate remains unknown, because lesions of PVCN unavoidably interrupt descending branches of cochlear nerve fibers, the ultimate terminations of which are the middle two layers of this four-layered nucleus (8). These fibers projecting from PVCN to the dorsal cochlear nucleus take an intranuclear course similar to that of an efferent bundle described by Lorente de No (8) and Rasmussen ( 17, 19). However, it is doubtful that the two are
150
WARR
Fig. 4. Photomicrograph of preterminal degeneration in the interstitial nucleus of the stria of Held in the ipsilateral cochlear nucleus. Arrows indicate cell bodies of this nucleus. Cat 61. Postoperative survival time: 10 days. Nauta method, thionine Dorsal cochlear nucleus; ICP, inferior counterstain, X 90. Abbreviations : DC, cerebellar peduncle ; PV, posteroventral cochlear nucleus.
the same. First, the lesions in cats 61 and 62 did not encroach upon the well-described route of the efferent bundle. Second, the degenerated fibers in the present material emanated directly from the lesion. Origin of the Intermediate Acoustic Stvia of Held. In two cats the stria of Held was completely severed at the point where it crosses the dorsal surface of the inferior cerebellar peduncle. Following survival times of 10 and 12 days, chromatolytic changes were noted in neurons of PVCN, that is, the affected neurons exhibited marked dissolution of their Nissl substance and eccentrically placed nuclei. In the animal surviving 10 days, these changes were especially clear. Sections from this animal were used to map the distribution of PVCN neurons exhibiting unequivocal chromatolytic changes. Most of the affected cells are located dorsolaterally within PVCN (Fig. 5). Within this regioh, the chromatolytic cells constitute a substantial majority of the neurons present. A comparison of the lesion location in cat 62 (Fig. lA-C) with the distribution of chromatolytic neurons in cat 46 (Fig. 5) suggests that: (i) The lesion in cat 62 destroyed cells in the dorsolateral PVCN, which apparently project into the stria of Held, (ii) the lesion also destroyed
COCHLEAR
NUCLEUS
151
FIG. 5. Camera lucida drawings of thionine-counterstained Nauta sections showing distribution of chromatolytic neurons in PVCN after destruction of the stria of Held in cat 46. A is most posterior. Sections are spaced 150 p apart. Abbreviations: DC, Dorsal cochlear nucleus ; PV, posteroventral cochlear nucleus.
PVCN which apparently do not project into the stria of Held, (iii) these ventromedial cells may project either by way of the trapezoid body or to loci within the cochlear nucleus itself. In a control experiment, the dorsal acoustic stria was severed without damaging the stria of Held. No chromatolytic neurons were found in PVCN, but marked chromatolytic changes were present in the dorsal cochlear nucleus. cells in the ventromedial
Discussion
Multiple Projections from PVCN. Lesions in PVCN produce fiber degeneration in four distinct pathways. Most prominent among these are the stria of Held and the trapezoid body which have overlapping projections to ipsilateral periolivary cell groups. In addition, two intranuclear projections were described, one to the dorsal cochlear nucleus and the other to a nucleus partially embedded in the stria of Held. Such a multiplicity of projections from PVCN is possibly related to those anatomical and electrophysiological observations which indicate that this region is heterogeneous. In a study of the cochlear nucleus with the Golgi method, Lorente de N6 (8) recognized three histologically distinct regions arranged along an anteroposterior axis within the PVCN. Each region was shown to contain one or more characteristic types of neurons. Evidence of functional heterogeneity within PVCN is found in electrophysiological studies (6, 11, 12) which indicate that at least three classes of unit activity are encountered there. Although the lesions in cats 61 and 62 probably involved only the two most posterior regions of the PVCN, these regions appear to contain a heterogeneous neuronal population (8). At present, however, it is not possibleto correlate the individual projections found in the present study with the detailed neuronal morphology or electrophysiology of PVCN. The extensive overlap in the ipsilateral projections of the stria and trapezoid body confirms the fundamental notion of Held (2) that the
152
WARR
stria is not a fasciculus with unique connections. The present findings are that many neurons in the lateral PVCN project by way of the stria of Held (Fig. 5)) but this does not necessarily represent a significant division of the nucleus with regard to its detailed neuronal morphology. One of Held’s semischematic drawings shows neurons of apparently the same type sending their axons to the superior olivary complex either by way of the stria or the trapezoid body. Furthermore, Lorente de Ni, (8) subdivided the PVCN along an anteroposterior axis, rather than the mediolateral axis evident in Fig. 5. That the stria of Held shares its nucleus of origin and field of termination with certain fibers of the trapezoid body must not obscure the fact that some differences do exist in the terminations of these two tracts. Most prominent among these are the fibers which originate in the PVCN and end in the contralateral ventromedial periolivary cell group. Covnparkon of PVCN and AVCN Projections. The projections of the posterior two-thirds of PVCN presented here and those of the anterior two-thirds of AVCN previously described (23) were studied with similar methods. It may, therefore, be justifiable to present the results of these studies in a single diagram for purposes of comparison (Fig. 6).” Within the superior olivary complex, the projections of PVCN and AVCN are almost mutually exclusive: PVCN projects exclusively to periolivary cell groups and AVCN projects to the lateral and medial superior olivary nuclei and to the anterior part of the lateral nucleus of the trapezoid body. Only in the ipsilateral anterolateral periolivary cell group do projections from PVCN and AVCN appear to coincide (Fig. 6, POal) . At the level of the lateral lemniscus, there is considerable overlap in PVCN and AVCN projections, as shown in detail in Fig. 6. Ipsilaterally, terminations are limited to a small dorsomedial cell group (Fig. 6, VLLdnz) which receives a strong input from AVCN. but only a sparse one from PVCN. Also represented at this level are the few degenerated fibers from both PVCN and AVCN which continue dorsally with the ipsilateral lateral lemniscus to an unknown destination. Earlier findings (23) that indicated the existence of an ipsilateral projection from AVCN to the dorsal nucleus of the lateral lemniscus and to the central nucleus of the inferior colliculus have not been confirmed in more recent replications of that experiment. Contralaterally, pathways from both PVCN and AVCN project massively upon the principal cell mass of the ventral nucleus of the lateral 2 An earlier version of this diagram appeared elsewhere (6). On the basis of data acquired since then, the present diagram contains the following changes: Minor PVCN inputs to the ipsilateral dorsomedial, ventromedial and ventrolateral periolivary cell groups; and no PVCN input to the contralateral medial nucleus of the trapezoid body.
COCHLEAR
NUCLEUS
FIG. 6. Schematic diagram of the projection pathways from the posteroventral and anteroventral cochlear nuctei. (See footnote 2). Solid lines indicate anatomically distinct fiber pathways. Branches of these pathways do not necessarily indicate collaterals of individual axons. Broken lines indicate that uncertainty exists concerning whether a given pathway is a source of a known input. Large solid arrowheads represent major inputs; empty arrowheads represent relatively minor inputs. Small arrowheads indicate direction of neural conduction. Abbreviations : AV, anteroventral cochlear nucleus ; CNIC, central nucleus of the inferior colliculus; DC, dorsal cochlear nucleus; DLL, dorsal nucleus of the lateral lemniscus; IH, interstitial nucleus of Held’s stria; LSO, lateral superior olivary nucleus; LTB, lateral nucleus of the trapezoid body; MSO, medial superior olivary nucleus ; MTB, medial nucleus of the trapezoid body ; POal, anterolateral periolivary cell group ; POdl, dorsolateral periolivary cell group; POdm, dorsomedial periolivary cell group; Pop, posterior periolivary cell group; POvl, ventrolateral periolivary cell group; POvm, ventromedial periolivary cell group ; PV, posteroventral cochlear nucleus ; VLL, ventral nucleus of the lateral lemniscus; VLLdm, dorsomedial division of the ventral nucleus of the lateral lemniscus ; VLLpm, posteromedial division of the ventral nucleus of the lateral lemniscus.
lemniscus. This degree of overlap in projections from PVCN and AVCN probably represents a convergence on the same cells. Such a convergence of heterogeneous inputs is entirely lacking, at least with respect to PVCN and AVCN, in the large-celled posteromedial portion of the ventral nucleus of the lateral lemniscus (Fig. 6, YLLpm) and in the dorsal nucleus of the lateral lemniscus (Fig. 6, DLL).
1.54
WARR
With regard to the local projections of PVCN and AVCN, both nuclei project to the dorsal cochlear nucleus (Fig. 6, DC). It has long been known that there are reciprocal connections between AVCN and the dorsal cochlear nucleus (8). The present findings, together with Rasmussen’s (19) demonstration that lesions of layers one through three of the dorsal cochlear nucleus produce preterminal degeneration in PVCN, indicate that PVCN and the dorsal cochlear nucleus are reciprocally related also. There is no evidence for the existence of reciprocal connections between PVCN and AVCN. The PVCN is the only known source of afferent fibers for the interstitial nucleus of the stria of Held (Fig. 6, IH). PVCN Relations with Descending Auditory Pathways. Periolivary and lemniscal cell groups give rise to segments of descending pathways which have terminations in the cochlea and cochlear nucleus. The dorsolateral periolivary cell group (17, 19) and the dorsomedial periolivary nucleus (15, 19) are sources of uncrossed and crossed fibers, respectively, of the olivocochlear bundle. The ventral nucleus of the lateral lemniscus (17) projects to the dorsal cochlear nucleus of the opposite side. The present findings support Rasmussen’s (17) conclusion that the olivocochlear bundle terminating in a given ear arises from periolivary cell groups which receive projections predominantly, but not exclusively, from the cochlear nucleus of that same ear. Recent evidence suggests that parts of the cochlear nucleus other than PVCN may also have functional relationships with cells in the periolivary region, and that the medial nucleus of the trapezoid body is an important intermediary in the chain of neurons which ultimately terminates in the cochlea (9, 18). In conclusion, the present findings indicate that the marked differences in histological appearance between PVCN and AVCN are matched by differences in their projections to the superior olivary complex. This result lends further support to the idea that the cochlear nucleus is a complex of subnuclei each of which has specific sets of output projections. References 1. HARRISON, afferent 2. HELD, H. Jg. UWJ 3. IRVING, R., trapezoid
J. M., and R. IRVING. 1964. Nucleus of the trapezoid body: dual innervation. Science 143 : 473-474. 1893. Die centrale Gehorleitung. ArcR. Amt. Elzt~cklzlngsgesckirhte : 201-248. and J. M. HARRISON. 1965. Origin of the large fiber component of the body of the rat. Amt. Record 151: 458.
4. JACOBSON, S. 1963. Handling sections in bulk, technique. Stain Technol. 38 : 262-263.
S’. KrANG, N. Y. S. 1968. A physiology.
Amt.
Otol.
survey
Rhirko,?.
with
special
reference
of recent developments in Lar)l,lgol.
77 : 656-675.
the
to the Nauta
study
of auditory
COCHLEAR
N~XLEUS
155
6. KIA~TG, N. Y. S., R. R. PFEIFPEI~, W. B. WARR, ad A. S. N. B.+CKUS. 1965. Stimulus coding in the cochlear nucleus. Am. Otol. Kkinol. Laryngol. 74: 463485. 7. KOENIG, H., R. A. GROAT, and W. F. WINDLE. 1945. A physiological approach to perfusion-fixation of tissues with formalin. St& Techrzol. 20: 13-22. 8. LORENTE DE N6, R. 1933. Anatomy of the eighth nerve. III. General plan of structure of the primary cochlear nucleus. Laqwgoscope 43: 327-350. 9. MOREST, D. K. 1968. The collateral system of the medial nucleus of the trapezoid body of the cat, its neuronal architecture and relation to the olivo-cochlear bundle. Brain Res. 9 : 28&311. 10. NAUTA, W. J. H. 1957. Silver impregnation of degenerating axons, pp. 11-26. In “New Research Techniques of Neuroanatomy.” W. F. Windle [ed.]. Thomas, Springfield, Illinois. il. PFEIFFER, R. R. 1966. Classifi-ation of response patterns of spike discharges for units in the cochlear nucleus: tone burst stimulation. Exptl. Bvain Res. 1: 220-235. 12. PFEIFFER, R. R., and N. Y. S. KIAXG. 1965. Spike discharge patterns of spontaneous and continuously stimulated activity in the cochlear nucleus of anesthetized cats. Biophys. 1. 5 : 301-316. 13. POWELL, T. P. S., and S. D. ERULKAR. 1962. Transneuronal cell degeneration in the auditory relay nuclei of the cat. J. Anat. 96: 249-268. 14. RAM~N Y CAJAL, S. 1904. “Textura de1 Sistema Nervioso de Hombre y de 10s Vertebredos.” Tomo II, Primera Parte, pp. 108-170. Imprenta y Liberia, Nicolas Moya, Madrid. 15. RASMUSSEN, G. L. 1946. The olivary peduncle and other fiber projections of the superior olivary complex. J. Comb. Neural. 84 : 141-219. 16. RASMUSSEN, G. L. 1957. Selective silver impregnation of synaptic endings, pp. 27-39. In “New Research Techniques of Neuronanatomy.” W. F. Windle [ed.]. Thomas, Springfield, Illinois. 17. RASMUSSEN, G. L. 1960. Efferent fibers of the cochlear nerve and cochlear nucleus, pp. 105-115. In “Neural Mechanisms of the Auditory and Vestibular Systems.” G. L. Rasmussen, and W. F. Windle reds.]. Thomas, Springfield, Illinois. 18. RASMUSSEN, G. L. 1964. Anatomical relationships of the ascending and descending auditory systems, pp. I-19. Zn “Neurological Aspects of Auditory and Vestibular Disorders.” W. S. Fields, and B. R. Alford [eds.]. Thomas, Springfield, Illinois. 19. RASMUSSEN, G. L. 1967. Efferent connections of the cochlear nucleus, pp. 61-75. In “Sensorineural Hearing Processes and Disorders.” A. B. Graham [ed.]. Little, Brown, Boston, Massachusetts. 20. SANDO, I. 1%5. The anatomical interrelationship of the cochlear nerve fibers. Acta Oto-laryngol. 59 : 417-436. 21. STOTLER, W. A. 1953. An experimental study of the cells and connections of the superior olivary complex of the cat. J. Corn@. Neural. 98: 401-432. 22. TABER, E. 1961. The cytoarchitecture of the brain stem of the cat. I. Brain stem nuclei of the cat. J. Corn). Neurol. 116 : 27-69. 23. WARR, W. B. 1966. Fiber degeneration following lesions in the anterior ventral cochlear nucleus of the cat. Exptl. Neurol. 14 : 453+74. 24. WINDLE, W. F., R. RHINES, and J. RANKIN. 1943. A Nissl method using buffered solution of thionin. Stain Tec/znol. 18 : 77-86.
NEUROLOCY
23,14Q-155
(1969)
Fiber Degeneration following Lesions in the Posterovkntral Cochlear Nucleus of the Cat Defiartment
W. BRUCEWARR 1 of Anatomy, Boston Unizrersity Boston, Received
School
of Medicine,
MassacRusetts 02118 October
25,1968
The results of studying fiber projections of the anteroventral cochlear nucleus (AVCN) by the Nauta and protargol methods were described in a previous paper. In the present study using similar methods, lesions in the posteroventral cochlear nucleus (PVCN) produced degeneration in the intermediate acoustic stria of Held and in the trapezoid body. Neither of these tracts projects to principal nuclei of the superior olivary complex, but rather they have overlapping projections to ipsilateral periolivary cell groups which partially surround the lateral superior olivary nucleus. Contralaterally, axons in the stria of Held project to: (i) The dorsomedial periolivary cell group; (ii) the anterolateral periolivary cell group; (iii) the ventral nucleus of the lateral lemniscus; and (iv) the inferior colliculus. Fibers in the trapezoid body project to the contralateral ventromedial periolivary cell group. The possibility that these projections represent, in part, inputs to cells which give rise to the olivocochlear bundle is discussed. In contrast to AVCN which projects to the lateral and medial superior olivary nuclei proper, PVCN projects to adjacent periolivary cell groups, but both AVCN and PVCN project in an overlapping fashion to the ventral nucleus of the lateral lemniscus; bilaterally. This result lends further support to the idea that the cochlear nucleus is a complex of subnuclei each of which has specific sets of projections. Introduction
The mammalian cochlear nucleus serves both as the central terminus of the cochlear nerve and as the origin of fiber projections to the nuclei of the superior olivary complex, lateral lemniscus, and inferior colliculus.
Each fiber of the cochlear nerve ramifies within each of several histologically distinct regions of the cochlear nucleus (8, 14, ZO), but the neurons in particular regions may have more limited projections to central auditory nuclei. For example, the anteroventral cochlear nucleus projects r This work was supported in part by Research Grant NB-01344, from the National Institutes of Health, and in part by Public Health Service Grant FR-05485 during the author’s tenure at the Eaton-Peabody Laboratory at the Massachusetts Eye and Ear Infirmary. It is a pleasure to thank N. Y. S. Kiang and D. K. Morest for their helpful advice and criticism during all phases of this work. The assistance of Mrs. S. S. Guinan during the surgical operations is gratefully acknowledged. 140
COCHLEAB
NUCLEUS
141
to the medial and lateral superior olivary nuclei, but not to most of the periolivary cell groups of the superior olivary complex (23). In the present paper, projections from the posteroventral cochlear nucleus are analyzed with degeneration methods. In contrast to the anteroventral cochlear nucleus, the posteroventral nucleus projects to periolivary cell groups. Methods
The methods used in this study were the same as those described previously (23). The material consisted of 65 adult cats with lesions located in the various subdivisions of the cochlear nucleus, including the posteroventral, interstitial, anteroventral, and dorsal cochlear nuclei. In two animals (cats 61 and 62), the lesion was limited to the posterior two-thirds of the posteroventral cochlear nucleus. Lesions were also placed in fiber tracts associatedwith the cochlear nucleus for the purpose of studying retrograde chromatolysis. Control lesions included destruction of cerebellar cortex, dentate nucleus, and the cochlear nerve. All surgery was performed under asceptic conditions and pentobarbital anesthesia. The cochlear nucleus was visualized from its posterior aspect following retraction of the overlying cerebellum for a distance of approximately 3 mm and for a period of about 15 sec. During this time, radiofrequency lesions were made by inserting a 26-gauge Nichrome electrode, insulated except for 0.5 mm at the tip, into the cochlear nucleus. Lesions limited to the posteroventral cochlear nucleus were made by penetrating its free surface located along the posteromedial margin of the cochlear nucleus where it approachesthe inferior cerebellar peduncle. After 3-28 days, the animals were perfused through the heart (7) with a fixative containing 10% formalin and 3% potassium dichromate (suggested by G. L. Rasmussen). The modal survival time for the 65 cats was 7 days. Frozen sections were cut transversely at 25 p and a group of sections spaced 100 ,u apart were stained en masse (4) by the Nauta method (10). Sections were routinely counterstained with buffered thionin (24) in order to permit the study of retrograde chromatolysis. In all animals discussed below, a group of intervening sections spaced 200 p apart were stained with Protargol, as described by Rasmussen (16), with the exception that no mordant was used. The criteria used in identifying degenerated fibers and their pericellular end-arborizations were as described in a previous study (23). The nomenclature used for nuclei of the brain stem is based on that of Taber (22). Results
Lesions limited to the posterior two-thirds of the posteroventral cochlear nucleus (PVCN) did not produce preterminal degeneration in the principal cell massesof the superior olivary complex, namely, the lateral and medial
142
WARR
superior olivary nuclei. Rather, preterminal degeneration was found in several small cell groups which lie adjacent to these principal olivary nuclei. In a recent study with Golgi methods, Morest (9) used the general term “peri-olivary” to refer to these cell groups, and applied standard relational adjectives, such as dorsomedial and ventromedial, to refer to specific groups of cells located in the periolivary region. For example, the dorsomedial periolivary cell group is located dorsomedial to the medial superior olivary nucleus. This same convention will be adopted in the present paper. In the two cats with the lesion limited to the posterior two-thirds of PVCN, the course of degenerated fibers and the sites of preterminal degeneration were identical. Degenerated fibers leave the lesion to reach the superior olivary complex by way of two distinct fiber tracts: The intermediate acoustic stria of Held ; and the trapezoid body. Ipsilateral Degeneration in tlze Stria of Held. Degenerated fibers destined to form the intermediate acoustic stria of Held depart from the lesion in a dorsal direction, and upon reaching the lateral border of PVCN form a compact bundle consisting of large and moderate sized fibers (2-3 p in Protargol-stained material). This bundle travels along the lateral contour of the inferior cerebellar peduncle (Fig. lA, B, SH), and upon reaching the dorsal surface of the peduncle, breaks up into small fascidles which turn .sharply ventral and penetrate, in order, the spinal vestibular nucleus, the spinal tract of the trigeminal nerve and the oral division of its nucleus. Turning medially, these fascicles penetrate the motor nucleus of the facial nerve, or the rubrospinal tract, and arborize among cells lying directly posterior to, and extending anteriorly over the dorsal surface of the lateral superior olivary nucleus. Three groups of cells surrounding the lateral superior olivary nucleus receive an ipsilateral input from the stria of Held : The posterior periolivary cell group (Fig. lD, p) ; the dorsolateral periolivary cell group (Fig. lE, dl) ; and the anterolateral periolivary cell group (Fig. lF, al). As Taber (22) observed in Nissl preparations, neurons in the region corresponding to the posterior periolivary cell group (the posterior extremity of the lateral nucleus of the trapezoid body) are characteristically “medium-sized multipolar cells, each with a central nucleus and coarse deeply stained Nissl granules.” In the present material, the region also contains cell clusters composed of neurons similar in form to the principal neuron of the medial nucleus of the trapezoid body (9, 22). Preterminal degeneration was found only among the medium-sized multipolar cells (Fig. 2A) and appeared to be associated with only a fraction of the multipolar cells present in the posterior periolivary cell group even after total section of the stria of Held. Preterminal degeneration was present throughout the anteroposterior extent of the dorsolateral periolivary cell group (Fig. IE, &), but it was
COCHLEAR
NUCLEUS
I43
most profuse posteriorly, where there are numerous transverse degenerated fibers of the stria of Held (Fig. 2B). At more anterior levels, preterminal degeneration was supplied by fibers coursing longitudinally among the cells. A few of these fibers continue forward to project sparse preterminal degeneration to the anterolateral periolivary cell group (Fig. lF, al), and to a small cluster of cells in the dorsomedial portion of the ventral nucleus of the lateral lemniscus (Fig. lG, VLL). A very few degenerated fibers continue dorsally beyond this latter nucleus, but their site of termination could not be established. Many degenerated fibers of Held’s stria emerge medially from the posterior periolivary cell group. They regain a dorsal position by passing between the lateral and medial superior olivary nuclei and joining the more anterior fascicles of the stria as it courses toward the midline (Fig. lE,
SH). As the stria travels toward the midline, occasional fibers depart from the parent bundle and provide an extremely sparse preterminal arborization to a cell group adjacent to the dorsomedial extremity of the medial superior olivary nucleus (Fig. lE, dm). This cell group is located within the dorsomedial periolivary nucleus (9). Preterminal degeneration remains limited to this cell group even after complete section of the stria of Held. Contralateral Degeneration in fhe Stria of Held. Degenerated fibers of the stria of Held decussate dorsal to the trapezoid body and enter the contralateral dorsomedial periolivary cell group, where sparse preterminal degeneration was found (Fig. 1E and F, dm; Fig. 2C). Continuing its course to the lateral lemniscus, the stria of Held divides into two separate bundles of approximately equal size, one passing ventral to and the other dorsal to the medial superior olivary nucleus (Fig. 1E and F). The two bundles begin to reunite just rostra1 to the lateral superior olivary nucleus, where both supply moderate preterminal degeneration to the anterolateral periolivary cell group (Fig. lF, al; Fig. 2D). At this same anteroposterior level, the more ventral fibers provide profuse preterminal degeneration to the most posterior cells of the ventral nucleus of the lateral lemniscus (Fig. lF, VLL). At slightly more anterior levels, both ventral and dorsal components of Held’s stria ramify profusely among cells of the ventral nucleus of the lateral lemniscus (Fig. lG, VLL). Among the most posterior cells, preterminal degeneration is both profusely and uniformly distributed. At progressively more anterior levels it shifts medially (Fig. 3A), and becomes less profuse. In the most anterodorsal portions of the ventral nucleus of the lateral lemniscus, preterminal degeneration is sparse and tends to gather in small clusters (Fig. 1H). Little, if any, preterminal degeneration is present among cells of the ventral nucleus of the lateral lemniscus which correspond in position to
144
WARR
FIG. 1. Drawings of transverse Nauta-stained sections showing the course of degenerated fibers (dashes) and site of preterminal degeneration (stippling) consequent to the lesions (solid black areas) in cat 62. Arrow in F indicates anterior continuation of trapezoid body fibers. The area enclosed by broken lines in G is the posteromedial division of the ventral nucleus of the lateral lemniscus. A is the most posterior section. Postoperative survival time : 8 days. Abbreviations : al, anterolateral periolivary cell group; AV, anteroventral cochlear nucleus; CNIC, central nucleus of the inferior colliculus ; DAS, dorsal acoustic stria; DC, dorsal cochlear nucleus ; DLL, dorsal nucleus of the lateral lemniscus ; dl, dorsolateral periolivary cell group ; dm, dorsomedial periolivary cell group ; FN, facial nerve; G, genu of the facial nerve; ICP, inferior cerebellar peduncle; IH, interstitial nucleus of Held’s stria ; IN, interstitial nucleus of the cochlear nerve; LL, lateral lemniscus ; LSO, lateral superior olivary nucleus; LTB, lateral nucleus of the trapezoid body; LVN, lateral vestibular nucleus; MCP, middle cerebellar peduncle; ML, medial lemniscus; MSO, medial superior olivary nucleus ; MTB, medial nucleus of the trapezoid body; P,
COCFILEAR
NUCLEUS
145
those which receive an ipsilateral projection. No preterminal degeneration is found in the posteromedial division of the ventral nucleus of the lateral lemniscus (Fig. 1G, areas encircled by broken lines). The relatively few degenerated fibers which reach the most anterodorsal levels of the ventral nucleus of the lateral lemniscus are of small or medium size and travel in the most lateral portion of the lateral lemniscus. They provide little, if any, preterminal degeneration to the dorsal nucleus of the lateral lemniscus. A few enter the inferior colliculus where they arborize in the posterior part of the central nucleus. No degenerated fibers are present in the brachium or in the commissure of the inferior colliculus. Lesions of the dorsal cochlear nucleus, interstitial cochlear nucleus, and anteroventral cochlear nucleus did not produce degeneration in the intermediate stria of Held. Ipsilateral Degeneration in the Trapezoid Body. Degenerated fibers travel anteriorly from the lesion site and enter the posterior trapezoid body where, scattered throughout its dorsoventral extent, they course toward the superior olivary complex (Fig. 1D and E, TB). Small fascicles of small- and medium-sized degenerated fibers depart from the parent bundle, usually at nearly right angles, and form terminal arborizations among cells of the posterior periolivary cell group, dorsolateral periolivary cell group, and ventrolateral periolivary cell group (Fig. lD, p, and E, dl, and vl). Within the posterior periolivary cell group, only the multipolar cells (see above) are apparently involved. Most of the fibers destined to reach the dorsolateral periolivary cell group arch over the posterior surface of the lateral superior olivary nucleus but some actually pass through its medial half (Fig. lE, LSO) and emerge from its dorsal surface where they ramify. The fibers coursing through the lateral superior olivary nucleus appear to be fibers of passage. The ventrolateral periolivary cell group is a region with a relatively sparse neuronal population located just ventral to the medial hilus of the lateral superior olivary nucleus (Fig. lE, vl). It lies directly in the path of degenerated fibers which depart at right angles from the trapezoid body and which appear to continue dorsally to ramify within the dorsolateral periolivary cell group. Total section of the intermediate acoustic stria of Held produced little, if any, preterminal degeneration in the ventrolateral pyramidal tract; p, posterior periolivary cell group; PB, pontobulbar body; PN, pontine nucleus ; PV; posteroventralcochlear nucleus ; SCP, superior cerebellar peduucle ; SH, stria of Held ; ST, spinal tract of the trigeminal nerve ; TB, trapezoid body; VLL, ventral nucleus of the lateral lemniscus; VI, ventrolateral periolivary cell group; vm, ventromedial periolivary cell group ; VN, vestibular nerve ; IV. v., fourth ventricle; V.m.n., motor nucleus of the trigeminal nerve; VII. n., nucleus of the facial nerve.
146
WARR
FIG. 2. Preterminal degeneration in periolivary nuclei of cat 62. A. Ipsilateral posterior periolivary cell group showing medium-sized multipolar cells, X300. B. IpsiX 120. C. Contralateral dorsomedial lateral dorsolateral periolivary cell group, periolivary cell group, X 300. D. Contralateral anterolateral periolivary cell group, X300. Postoperative survival time: 8 days. Nauta method, thionine counterstain. Abbreviations : LSO, Lateral superior olivary nucleus.
.
FIG. in the level Nauta eration group stain,
COCHLEAB
NUCLEUS
147
3. A. Photomicrograph showing the distribution of preterminal degeneration contralateral ventral nucleus of the lateral lemniscus at an anteriorposterior comparable to that of Fig. IH. Cat 62. Postoperative survival time: 8 days. method, thionine counterstain, X80. B. Photomicrograph of preterminal degenfrom the trapezoid body in the contralateral ventromedial periolivary cell of cat 61. Postoperative survival time : 10 days. Nauta method, thionine counterx300.
148
WARR
periolivary cell group and no degenerated fibers of passage in the medial half of the lateral superior olivary nucleus. In addition to these prominent connections of the trapezoid body, a very few degenerated fibers were found entering the caudal extremity of the ventromedial periolivary cell group (Fig. lD, vnz). The precise rostrocaudal level at which this connection occurs is not represented in Fig. 1, but lies between drawings C and D in the most posterior portion of the superior olivary complex. -4 larger contralateral projection by way of the trapezoid body to the ventromedial periolivary cell group is described below. Contralateral Degeneration in the Trapesoid Body. Degenerated fibers, ranging in size from small to large, cross the midline throughout the depth of the posterior third of the trapezoid body (Fig. 1D and E) . There is no clear segregation of fibers according to their size either in the dorsoventral or the anteroposterior axis of the trapezoid body, The approximate anterior limit of these crossing fibers is at the level where the lateral superior olivary nucleus assumes its characteristic “S” shape in transverse section. Degenerated fibers either traverse or pass ventral to the medial nucleus of the trapezoid body. Most of them ramify within the ventromedial periolivary cell group (Fig. 1D and E, VW). Preterminal degeneration within this cell group is scattered among its characteristic large pale neurons, four or five of which commonly appear in a 25-p transverse section (Fig. 3B). Preterminal degeneration within the ventromedial periolivary cell group gradually diminishes in quantity and finally cannot be demonstrated at all in progressively more anterior levels of the cell group (Fig. lF, vm). Complete section of the stria of Held in two animals produced no preterminal degeneration in this cell group, although degenerated fibers comprising the ventral component of the stria do traverse the ventromedial periolivary cell group (Fig. lE, F, VWL) . A small proportion of the most ventral fibers of the trapezoid body turns rostrally and travels in a small bundle on the surface of the trapezoid body (Fig. lF, arrow). This bundle gradually becomes smaller and ultimately vanishes as its degenerated fibers turn dorsally into the trapezoid body and mingle with the ventral component of the stria of Held. The terminations of this superficial bundle could not be determined. As mentioned above, the more dorsal of the degenerated trapezoid fibers traverse the medial nucleus of the trapezoid body (Fig. 1D and E). Study of Nauta-stained sections produced no convincing evidence of preterminal degeneration in this nucleus. Moreover, it was observed that many of the fibers entering the medial nucleus of the trapezoid body continue on to terminate in the ventromedial periolivary cell group. However, the possibility that some of the fibers, especially the largest ones, end in the medial
COCHLEAR
NUCLEUS
149
nucleusof the trapezoid body in the form of calyces of Held (1, 3, 9, 14, 21) was checked in Protargol-stained frozen sections in cats 61 and 62. In normal material stained with Protargol, the calyces of Held can be reliably impregnated and found in association with cells characterized by an ovoid or spherical cell body, an eccentric nucleus and deeply staining cytoplasm (9). If some of the degenerated fibers in the present material terminate as calyces, then some of the cells meeting these criteria of identification should lack calyces. The possibility that transneuronal degenerative changes (13) would invalidate these criteria of identification would appear to be slight because the postoperative survival times (8 and 10 days) were too brief. In each cat, ‘ten transverse sections through the relevant portion of the medial nucleus of the trapezoid body were selected. Each cell meeting the criteria stated above was examined under an oil immersion objective for the presence of a calyx. On the contralateral side in cat 62, only three cells lacking calyces were counted among more than 300 cells examined. None was found ipsilaterally. In cat 61, none was found on either side. Degeneration Within the Cochlear Nucleus. In addition to the projections from PVCN already described, degenerated fibers were also traced to : (i) A small, hitherto undescribed nucleus, located along the lateral edge of, and partially embedded in, the stria of Held, and (ii) the dorsal cochlear nucleus. The former nucleus, referred to here as the interstitial nucleus of the stria of Held (Fig. lB, IH), contains a dense network of fine degenerated fibers (Fig. 4). These fine fibers are traceable dorsally from the lesion in PVCN to their termination. .Very few of these fibers continue beyond this nucleus with the stria of Held. Whether any of the characteristically larger fibers in the stria of Held contribute to this nucleus could not be determined. Neither destruction of the spiral ganglion of the cochlea nor lesions in any of the other subdivisions of the cochlear nuclear complex produce preterminal degeneration in the interstitial nucleus of the stria of Held. The fine degenerated fibers destined to reach the dorsal cochlear nucleus leave the lesion site in a dorsomedial direction and follow the medial border of the PVCN until they reach its dorsal extremity. Here they turn laterally and ramify in the dorsal cochlear nucleus (Fig. lC, DC). The exact layer of the dorsal cochlear nucleus in which these fibers terminate remains unknown, because lesions of PVCN unavoidably interrupt descending branches of cochlear nerve fibers, the ultimate terminations of which are the middle two layers of this four-layered nucleus (8). These fibers projecting from PVCN to the dorsal cochlear nucleus take an intranuclear course similar to that of an efferent bundle described by Lorente de No (8) and Rasmussen ( 17, 19). However, it is doubtful that the two are
150
WARR
Fig. 4. Photomicrograph of preterminal degeneration in the interstitial nucleus of the stria of Held in the ipsilateral cochlear nucleus. Arrows indicate cell bodies of this nucleus. Cat 61. Postoperative survival time: 10 days. Nauta method, thionine Dorsal cochlear nucleus; ICP, inferior counterstain, X 90. Abbreviations : DC, cerebellar peduncle ; PV, posteroventral cochlear nucleus.
the same. First, the lesions in cats 61 and 62 did not encroach upon the well-described route of the efferent bundle. Second, the degenerated fibers in the present material emanated directly from the lesion. Origin of the Intermediate Acoustic Stvia of Held. In two cats the stria of Held was completely severed at the point where it crosses the dorsal surface of the inferior cerebellar peduncle. Following survival times of 10 and 12 days, chromatolytic changes were noted in neurons of PVCN, that is, the affected neurons exhibited marked dissolution of their Nissl substance and eccentrically placed nuclei. In the animal surviving 10 days, these changes were especially clear. Sections from this animal were used to map the distribution of PVCN neurons exhibiting unequivocal chromatolytic changes. Most of the affected cells are located dorsolaterally within PVCN (Fig. 5). Within this regioh, the chromatolytic cells constitute a substantial majority of the neurons present. A comparison of the lesion location in cat 62 (Fig. lA-C) with the distribution of chromatolytic neurons in cat 46 (Fig. 5) suggests that: (i) The lesion in cat 62 destroyed cells in the dorsolateral PVCN, which apparently project into the stria of Held, (ii) the lesion also destroyed
COCHLEAR
NUCLEUS
151
FIG. 5. Camera lucida drawings of thionine-counterstained Nauta sections showing distribution of chromatolytic neurons in PVCN after destruction of the stria of Held in cat 46. A is most posterior. Sections are spaced 150 p apart. Abbreviations: DC, Dorsal cochlear nucleus ; PV, posteroventral cochlear nucleus.
PVCN which apparently do not project into the stria of Held, (iii) these ventromedial cells may project either by way of the trapezoid body or to loci within the cochlear nucleus itself. In a control experiment, the dorsal acoustic stria was severed without damaging the stria of Held. No chromatolytic neurons were found in PVCN, but marked chromatolytic changes were present in the dorsal cochlear nucleus. cells in the ventromedial
Discussion
Multiple Projections from PVCN. Lesions in PVCN produce fiber degeneration in four distinct pathways. Most prominent among these are the stria of Held and the trapezoid body which have overlapping projections to ipsilateral periolivary cell groups. In addition, two intranuclear projections were described, one to the dorsal cochlear nucleus and the other to a nucleus partially embedded in the stria of Held. Such a multiplicity of projections from PVCN is possibly related to those anatomical and electrophysiological observations which indicate that this region is heterogeneous. In a study of the cochlear nucleus with the Golgi method, Lorente de N6 (8) recognized three histologically distinct regions arranged along an anteroposterior axis within the PVCN. Each region was shown to contain one or more characteristic types of neurons. Evidence of functional heterogeneity within PVCN is found in electrophysiological studies (6, 11, 12) which indicate that at least three classes of unit activity are encountered there. Although the lesions in cats 61 and 62 probably involved only the two most posterior regions of the PVCN, these regions appear to contain a heterogeneous neuronal population (8). At present, however, it is not possibleto correlate the individual projections found in the present study with the detailed neuronal morphology or electrophysiology of PVCN. The extensive overlap in the ipsilateral projections of the stria and trapezoid body confirms the fundamental notion of Held (2) that the
152
WARR
stria is not a fasciculus with unique connections. The present findings are that many neurons in the lateral PVCN project by way of the stria of Held (Fig. 5)) but this does not necessarily represent a significant division of the nucleus with regard to its detailed neuronal morphology. One of Held’s semischematic drawings shows neurons of apparently the same type sending their axons to the superior olivary complex either by way of the stria or the trapezoid body. Furthermore, Lorente de Ni, (8) subdivided the PVCN along an anteroposterior axis, rather than the mediolateral axis evident in Fig. 5. That the stria of Held shares its nucleus of origin and field of termination with certain fibers of the trapezoid body must not obscure the fact that some differences do exist in the terminations of these two tracts. Most prominent among these are the fibers which originate in the PVCN and end in the contralateral ventromedial periolivary cell group. Covnparkon of PVCN and AVCN Projections. The projections of the posterior two-thirds of PVCN presented here and those of the anterior two-thirds of AVCN previously described (23) were studied with similar methods. It may, therefore, be justifiable to present the results of these studies in a single diagram for purposes of comparison (Fig. 6).” Within the superior olivary complex, the projections of PVCN and AVCN are almost mutually exclusive: PVCN projects exclusively to periolivary cell groups and AVCN projects to the lateral and medial superior olivary nuclei and to the anterior part of the lateral nucleus of the trapezoid body. Only in the ipsilateral anterolateral periolivary cell group do projections from PVCN and AVCN appear to coincide (Fig. 6, POal) . At the level of the lateral lemniscus, there is considerable overlap in PVCN and AVCN projections, as shown in detail in Fig. 6. Ipsilaterally, terminations are limited to a small dorsomedial cell group (Fig. 6, VLLdnz) which receives a strong input from AVCN. but only a sparse one from PVCN. Also represented at this level are the few degenerated fibers from both PVCN and AVCN which continue dorsally with the ipsilateral lateral lemniscus to an unknown destination. Earlier findings (23) that indicated the existence of an ipsilateral projection from AVCN to the dorsal nucleus of the lateral lemniscus and to the central nucleus of the inferior colliculus have not been confirmed in more recent replications of that experiment. Contralaterally, pathways from both PVCN and AVCN project massively upon the principal cell mass of the ventral nucleus of the lateral 2 An earlier version of this diagram appeared elsewhere (6). On the basis of data acquired since then, the present diagram contains the following changes: Minor PVCN inputs to the ipsilateral dorsomedial, ventromedial and ventrolateral periolivary cell groups; and no PVCN input to the contralateral medial nucleus of the trapezoid body.
COCHLEAR
NUCLEUS
FIG. 6. Schematic diagram of the projection pathways from the posteroventral and anteroventral cochlear nuctei. (See footnote 2). Solid lines indicate anatomically distinct fiber pathways. Branches of these pathways do not necessarily indicate collaterals of individual axons. Broken lines indicate that uncertainty exists concerning whether a given pathway is a source of a known input. Large solid arrowheads represent major inputs; empty arrowheads represent relatively minor inputs. Small arrowheads indicate direction of neural conduction. Abbreviations : AV, anteroventral cochlear nucleus ; CNIC, central nucleus of the inferior colliculus; DC, dorsal cochlear nucleus; DLL, dorsal nucleus of the lateral lemniscus; IH, interstitial nucleus of Held’s stria; LSO, lateral superior olivary nucleus; LTB, lateral nucleus of the trapezoid body; MSO, medial superior olivary nucleus ; MTB, medial nucleus of the trapezoid body ; POal, anterolateral periolivary cell group ; POdl, dorsolateral periolivary cell group; POdm, dorsomedial periolivary cell group; Pop, posterior periolivary cell group; POvl, ventrolateral periolivary cell group; POvm, ventromedial periolivary cell group ; PV, posteroventral cochlear nucleus ; VLL, ventral nucleus of the lateral lemniscus; VLLdm, dorsomedial division of the ventral nucleus of the lateral lemniscus ; VLLpm, posteromedial division of the ventral nucleus of the lateral lemniscus.
lemniscus. This degree of overlap in projections from PVCN and AVCN probably represents a convergence on the same cells. Such a convergence of heterogeneous inputs is entirely lacking, at least with respect to PVCN and AVCN, in the large-celled posteromedial portion of the ventral nucleus of the lateral lemniscus (Fig. 6, YLLpm) and in the dorsal nucleus of the lateral lemniscus (Fig. 6, DLL).
1.54
WARR
With regard to the local projections of PVCN and AVCN, both nuclei project to the dorsal cochlear nucleus (Fig. 6, DC). It has long been known that there are reciprocal connections between AVCN and the dorsal cochlear nucleus (8). The present findings, together with Rasmussen’s (19) demonstration that lesions of layers one through three of the dorsal cochlear nucleus produce preterminal degeneration in PVCN, indicate that PVCN and the dorsal cochlear nucleus are reciprocally related also. There is no evidence for the existence of reciprocal connections between PVCN and AVCN. The PVCN is the only known source of afferent fibers for the interstitial nucleus of the stria of Held (Fig. 6, IH). PVCN Relations with Descending Auditory Pathways. Periolivary and lemniscal cell groups give rise to segments of descending pathways which have terminations in the cochlea and cochlear nucleus. The dorsolateral periolivary cell group (17, 19) and the dorsomedial periolivary nucleus (15, 19) are sources of uncrossed and crossed fibers, respectively, of the olivocochlear bundle. The ventral nucleus of the lateral lemniscus (17) projects to the dorsal cochlear nucleus of the opposite side. The present findings support Rasmussen’s (17) conclusion that the olivocochlear bundle terminating in a given ear arises from periolivary cell groups which receive projections predominantly, but not exclusively, from the cochlear nucleus of that same ear. Recent evidence suggests that parts of the cochlear nucleus other than PVCN may also have functional relationships with cells in the periolivary region, and that the medial nucleus of the trapezoid body is an important intermediary in the chain of neurons which ultimately terminates in the cochlea (9, 18). In conclusion, the present findings indicate that the marked differences in histological appearance between PVCN and AVCN are matched by differences in their projections to the superior olivary complex. This result lends further support to the idea that the cochlear nucleus is a complex of subnuclei each of which has specific sets of output projections. References 1. HARRISON, afferent 2. HELD, H. Jg. UWJ 3. IRVING, R., trapezoid
J. M., and R. IRVING. 1964. Nucleus of the trapezoid body: dual innervation. Science 143 : 473-474. 1893. Die centrale Gehorleitung. ArcR. Amt. Elzt~cklzlngsgesckirhte : 201-248. and J. M. HARRISON. 1965. Origin of the large fiber component of the body of the rat. Amt. Record 151: 458.
4. JACOBSON, S. 1963. Handling sections in bulk, technique. Stain Technol. 38 : 262-263.
S’. KrANG, N. Y. S. 1968. A physiology.
Amt.
Otol.
survey
Rhirko,?.
with
special
reference
of recent developments in Lar)l,lgol.
77 : 656-675.
the
to the Nauta
study
of auditory
COCHLEAR
N~XLEUS
155
6. KIA~TG, N. Y. S., R. R. PFEIFPEI~, W. B. WARR, ad A. S. N. B.+CKUS. 1965. Stimulus coding in the cochlear nucleus. Am. Otol. Kkinol. Laryngol. 74: 463485. 7. KOENIG, H., R. A. GROAT, and W. F. WINDLE. 1945. A physiological approach to perfusion-fixation of tissues with formalin. St& Techrzol. 20: 13-22. 8. LORENTE DE N6, R. 1933. Anatomy of the eighth nerve. III. General plan of structure of the primary cochlear nucleus. Laqwgoscope 43: 327-350. 9. MOREST, D. K. 1968. The collateral system of the medial nucleus of the trapezoid body of the cat, its neuronal architecture and relation to the olivo-cochlear bundle. Brain Res. 9 : 28&311. 10. NAUTA, W. J. H. 1957. Silver impregnation of degenerating axons, pp. 11-26. In “New Research Techniques of Neuroanatomy.” W. F. Windle [ed.]. Thomas, Springfield, Illinois. il. PFEIFFER, R. R. 1966. Classifi-ation of response patterns of spike discharges for units in the cochlear nucleus: tone burst stimulation. Exptl. Bvain Res. 1: 220-235. 12. PFEIFFER, R. R., and N. Y. S. KIAXG. 1965. Spike discharge patterns of spontaneous and continuously stimulated activity in the cochlear nucleus of anesthetized cats. Biophys. 1. 5 : 301-316. 13. POWELL, T. P. S., and S. D. ERULKAR. 1962. Transneuronal cell degeneration in the auditory relay nuclei of the cat. J. Anat. 96: 249-268. 14. RAM~N Y CAJAL, S. 1904. “Textura de1 Sistema Nervioso de Hombre y de 10s Vertebredos.” Tomo II, Primera Parte, pp. 108-170. Imprenta y Liberia, Nicolas Moya, Madrid. 15. RASMUSSEN, G. L. 1946. The olivary peduncle and other fiber projections of the superior olivary complex. J. Comb. Neural. 84 : 141-219. 16. RASMUSSEN, G. L. 1957. Selective silver impregnation of synaptic endings, pp. 27-39. In “New Research Techniques of Neuronanatomy.” W. F. Windle [ed.]. Thomas, Springfield, Illinois. 17. RASMUSSEN, G. L. 1960. Efferent fibers of the cochlear nerve and cochlear nucleus, pp. 105-115. In “Neural Mechanisms of the Auditory and Vestibular Systems.” G. L. Rasmussen, and W. F. Windle reds.]. Thomas, Springfield, Illinois. 18. RASMUSSEN, G. L. 1964. Anatomical relationships of the ascending and descending auditory systems, pp. I-19. Zn “Neurological Aspects of Auditory and Vestibular Disorders.” W. S. Fields, and B. R. Alford [eds.]. Thomas, Springfield, Illinois. 19. RASMUSSEN, G. L. 1967. Efferent connections of the cochlear nucleus, pp. 61-75. In “Sensorineural Hearing Processes and Disorders.” A. B. Graham [ed.]. Little, Brown, Boston, Massachusetts. 20. SANDO, I. 1%5. The anatomical interrelationship of the cochlear nerve fibers. Acta Oto-laryngol. 59 : 417-436. 21. STOTLER, W. A. 1953. An experimental study of the cells and connections of the superior olivary complex of the cat. J. Corn@. Neural. 98: 401-432. 22. TABER, E. 1961. The cytoarchitecture of the brain stem of the cat. I. Brain stem nuclei of the cat. J. Corn). Neurol. 116 : 27-69. 23. WARR, W. B. 1966. Fiber degeneration following lesions in the anterior ventral cochlear nucleus of the cat. Exptl. Neurol. 14 : 453+74. 24. WINDLE, W. F., R. RHINES, and J. RANKIN. 1943. A Nissl method using buffered solution of thionin. Stain Tec/znol. 18 : 77-86.