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C K Henkel

Publications and source records attributed to C K Henkel.

At least 37 records · Page 2Linked to original sources
PubMed1990

Synaptic organization in the adult ferret medial superior olive.

The ultrastructure of the medial superior olive (MSO) was studied in the adult ferret. The synaptic terminals were categorized on the basis of morphology and their distribution determined. There are three types of synaptic terminals: R terminals, containing round vesicles; Ov terminals, containing ovoid vesicles; and P terminals, containing vesicles of varying morphologies. R terminals are the dominant terminal type on both the somata and dendrites. Ov and P terminals are equally prevalent on the dendrites; however, P terminals are significantly more common than Ov terminals on the somata. Furthermore, P terminals are significantly more common on the somata than on the dendrites. These results suggest that there is some segregation of types of afferent synapses on MSO cells.

Animals↗
PubMed1988

Connections of the dorsal nucleus of the lateral lemniscus: an inhibitory parallel pathway in the ascending auditory system?

This study examines the dorsal nucleus of the lateral lemniscus (DNLL) and its afferent and efferent connections. In Nissl-stained material, DNLL has three parts: dorsal, ventral, and lateral. Although each part contains neurons with similar Nissl patterns, the subdivisions may be distinguished by the size, shape, and orientation of the cells. The lateral DNLL contains a mixture of DNLL neurons and cells from the sagulum. Afferent connections to DNLL were investigated with anterograde axonal transport techniques. Bilateral inputs to DNLL arise from the anteroventral cochlear nucleus and lateral superior olive, while unilateral inputs are provided by the ipsilateral medial superior olive and the contralateral DNLL. The inputs appear to have a tonotopic organization. Afferent fibers to DNLL form horizontal bands that are continuous both mediolaterally and rostrocaudally. All parts of DNLL do not share the same inputs, and a medial-to-lateral gradient in the labeling of some pathways is evident. To study the efferent connections of DNLL, both retrograde and anterograde axonal transport techniques were used. The DNLL projects to the inferior colliculus and the contralateral DNLL. The topography of these projections suggests that areas of similar tonotopic organization are connected. In the inferior colliculus, the projection is heaviest to the central nucleus and extends to the adjacent dorsal and caudal cortex, the rostral pole nucleus, and the ventrolateral nucleus. Axons from DNLL terminate along the fibrodendritic laminae of the central nucleus as bands that are prominent on the contralateral side, whereas those on the ipsilateral colliculus are more diffuse. The afferent and efferent connections of DNLL constitute a multisynaptic pathway, parallel to the other ascending pathways to the inferior colliculus. The other ascending pathways include the direct pathways from the cochlear nucleus to the inferior colliculus and the indirect pathways via the superior olivary complex. Ascending pathways are discussed as to their relationship to the subdivisions of the inferior colliculus, the laterality of their projections, and their banding patterns in the central nucleus. In contrast to the excitatory pathways to the inferior colliculus, the neurons in DNLL may use GABA as a neurotransmitter. Axons from the DNLL terminate in the inferior colliculus as bands that could have a unique inhibitory function. Thus, the multisynaptic, DNLL pathway may provide feed-forward inhibitory inputs to the inferior colliculus, bilaterally, and to the contralateral DNLL.

Animals↗
PubMed1988

Nucleus sagulum: projections of a lateral tegmental area to the inferior colliculus in the cat.

The nucleus sagulum, an area of the midbrain tegmentum, has been considered a component of a lateral tegmental system within the ascending auditory pathway to the thalamus. In this study, connections of the nucleus sagulum within the midbrain were investigated in adult cats. Tracing methods using anterograde and retrograde axonal transport of markers were employed. The nucleus sagulum was identified as a region of principally small neurons (261 +/- 79 micron2) at the margin of the midbrain and neighboring the nuclei of the lateral lemniscus. Injections of tritiated leucine in the nucleus sagulum labeled axons that ended in dense patches within the superficial layers of the caudal portion of the dorsal cortex of the inferior colliculus on the ipsilateral side. Retrograde experiments confirmed this connection. Other axonal projections labeled in the anterograde studies included fibers ending in the dorsomedial nucleus, the superficial layers of the dorsal cortex, and the rostral nucleus of the inferior colliculus with some bilateral distribution. Outside of the inferior colliculus, sagulum injections labeled other axons ending in the ventral intercollicular tegmentum on both sides and in a dorsal and rostral region of the contralateral nucleus sagulum that appeared contiguous with the dorsal nucleus of the lateral lemniscus. The latter region included a population of larger neurons (340-540 micron2) and had different connections with the inferior colliculus. The distribution of axonal labeling after injections in the nucleus sagulum was contrasted with the distribution of projections from several neighboring areas of the lateral tegmentum, including the dorsal nucleus of the lateral lemniscus. None of these areas exhibited connections with the superficial layers of the caudal cortex of the inferior colliculus, which was the major target in the inferior colliculus of the nucleus sagulum. Thus, the results indicated that the nucleus sagulum is distinguished from adjacent regions of the lateral tegmentum by its connectivity. Its association with midbrain auditory pathways is supported by these connections as well as ascending ones to the auditory thalamus.

Animals↗
PubMed1987

Banding of lateral superior olivary nucleus afferents in the inferior colliculus: a possible substrate for sensory integration.

In this study the organization of the projection from the lateral superior olivary nucleus (LSO) to the inferior colliculus was investigated in the cat by using anterograde tract-tracing techniques. The findings indicated that LSO projected bilaterally to the central nucleus of the inferior colliculus as well as to the ventrolateral and rostral pole nuclei. In the central nucleus a larger medial component of the projection ended in pars medialis and centralis. A smaller lateral component ended in the region of the pars lateralis. Both components of the projection appeared to be topographically organized, but in the lateral component the low-frequency part of LSO appeared to have greater representation. The uncrossed and crossed LSO projections to the inferior colliculus exhibited several important differences in their distribution. First, periodic bands of dense labeling were more prominent in the distribution of the uncrossed projection. The bands measured 150-200 micron in thickness and in some cases interruptions or gaps were present along the length of the bands. The distribution of the crossed projection was more diffuse, but some banding was also apparent. Second, the positions of the bands of dense labeling on the two sides were not homotopic as determined by labeling projections from the ipsilateral and contralateral LSO in the same tissue. The dense bands labeled with WGA-HRP from an injection in LSO on one side and bands labeled with 3H-leucine from an injection in LSO on the other side either were interdigitating or were only partially overlapping. Finally, the area over which the uncrossed projection distributed endings varied in size with respect to that of the crossed projection. The variation in size of the area of the projections was a function of the frequency representation. A model based on the three-dimensional reconstruction of bands as projection sheets is proposed as a substrate for selective integration of afferents in the inferior colliculus.

Afferent Pathways↗
PubMed1987

Descending projections from the superior olivary complex to the cochlear nucleus of the cat.

Subdivisions of the cochlear nuclear complex give rise to a number of discrete projections to certain cell groups of the superior olivary complex and also received substantial descending projections from the periolivary nuclei. In the present study, we sought to determine by means of retrograde transport of horseradish peroxidase (HRP), and anterograde transport of radiolabeled protein, if the periolivary nuclei give rise to discrete projections to the various subdivisions of the cochlear nuclear complex. Following medium to large injections of HRP into the cochlear nucleus, irrespective of location, labeled cells were found in all periolivary nuclei bilaterally. In every case more than 40% of the labeled cells were found in the lateral nucleus of the trapezoid body on the same side and the ventral nucleus of the trapezoid body of both sides. Other periolivary nuclei contributing more than 5% of the total number of cells in individual cases were the contralateral lateral nucleus of the trapezoid body and the ipsilateral anterolateral and dorsal periolivary nuclei. Injections of tritiated leucine into periolivary nuclei gave rise to axonal labeling to the trapezoid body and the dorsal acoustic stria, usually bilaterally, and to terminal labeling that was widely distributed within the cochlear nuclear complex. In several cases with small injections, particularly in the lateral nucleus of the trapezoid body, the projections from the periolivary nuclei to the anteroventral and dorsal cochlear nuclei connected areas described as having similar best-frequency representation. The autoradiographic data corroborated the main results from the HRP experiments and provided additional information permitting these conclusions: the projections from the periolivary nuclei to the cochlear nuclear complex are organized tonotopically, at least in part; each periolivary nucleus (and perhaps individual cells), projects widely throughout the cochlear nuclear complex; the pattern of termination of projections from different periolivary nuclei to a given region of the cochlear nuclear complex are similar, as seen in autoradiograms, and the lateral and dorsal periolivary nuclei project mainly ipsilaterally, while the medial periolivary nuclei project bilaterally with a contralateral bias. The magnitude of these projections and their widespread distribution within the cochlear nuclear complex would suggest an important role for the descending projections in the normal functioning of the cochlear nucleus.

Acetylcholinesterase↗
PubMed1985

The projections of principal cells of the medial nucleus of the trapezoid body in the cat.

Previous studies suggest that the principal cells of the medial nucleus of the trapezoid body (MNTB) give rise to the projection from MNTB to the lateral superior olivary nucleus (LSO) of the same side, where they mediate rapid inhibitory effects of contralateral sound stimulation. In the present study, we explored certain morphological features of this connection as well as several other projections of the MNTB by using anterograde and retrograde axonal tracing methods. Following injections of tritiated leucine into MNTB, labeled axons reached LSO by passing ventral to, dorsal to, and through the medial superior olivary nucleus, and gave rise to labeling around the somata and proximal dendrites of LSO fusiform cells. As measured in autoradiograms of 2 micron plastic sections, these axons had a modal diameter of 5-6 micron. Terminal labeling, tentatively attributed to principal cell axons, was also seen in the ventral nucleus of the lateral lemniscus (VNLL) and the dorsomedial and ventromedial periolivary nuclei. HRP injections into the LSO and the VNLL showed that the principal cell projected to both of these nuclei and revealed a topographic arrangement of the projection to the LSO which is consistent with tonotopic maps determined electrophysiologically. Control HRP injections demonstrated that other minor projections of the MNTB arose from minor cell populations in this nucleus. The findings provide a morphological correlate of certain physiological findings and suggest a wider role for the MNTB in the ascending auditory system than previously has been supposed.

Animals↗
PubMed1985

Evidence of collateral axonal projections to the superior olivary complex.

In this study we investigated the collateral axonal projections to the superior olivary complex using the combined anterograde and retrograde transport of wheat-germ agglutinin conjugated with horseradish peroxidase. Small injections of this tracer were placed in the lateral or medial superior olivary nuclei in cats, and the location of anterograde label in the alternate nuclei of the superior olivary complex was determined. Injections of [3H]leucine were also placed in these nuclei for control purposes. After wheat-germ agglutinin-horseradish peroxidase injections in the lateral superior olivary nucleus anterograde label was observed bilaterally in the medial superior olivary nuclei. Likewise, after injections in the medial superior olivary nucleus anterograde label was observed in the contralateral medial and lateral superior olivary nuclei. The topography of the anterograde label was always precise and varied predictably as a function of the injection site. Most retrogradely labeled cells were located in the ipsilateral anteroventral cochlear nucleus and the medial nucleus of the trapezoid body. Various interpretations of the data are considered. Our primary conclusion is that cells in the anteroventral cochlear nucleus are a major source of collaterals to both the ipsilateral and contralateral nuclei of the superior olivary complex.

Animals↗
PubMed1984

Topographical organization of the inferior collicular projection and other connections of the ventral nucleus of the lateral lemniscus in the cat.

The topographic distribution of projections from the ventral nucleus of the lateral lemniscus (VNLL) in the cat was investigated with the autoradiographic tracing method. The origin of minor projections was verified by retrograde tracing methods. Small injections of tritiated leucine were placed in restricted zones of VNLL. A major afferent fiber system to the inferior colliculus was labeled in all cases. From the injection site labeled fibers coursed through and around the nuclei of the lateral lemniscus to enter the ipsilateral inferior colliculus. Regardless of the position or small size of the injection, labeled fibers distributed widely in the inferior colliculus. Fibers ended in the central nucleus and deeper layers of the dorsal cortex in most cases. There was also labeling in the ventrolateral nucleus, but very few fibers ended as lateral as the lateral nucleus. A small number of labeled fibers passed from the inferior colliculus into the nucleus of the brachium of the inferior colliculus and adjacent tegmental areas. Some labeled fibers entered the commissure of the inferior colliculus where they were traced into the dorsal cortex and rostral pole of the inferior colliculus on the side contralateral to the injection site. Though the projections labeled in individual cases were similar in their divergent pattern within the central nucleus of the inferior colliculus, specific variations in the pattern were found. The dorsal zone of VNLL projected more heavily to the deeper layers of the dorsal cortex and an adjacent field in the central nucleus than the other zones. Dorsal injections in the middle zone of VNLL, on the other hand, labeled the medial part of the central nucleus more heavily, whereas ventral injections in the middle zone resulted in heavier lateral labeling. The ventral zone of VNLL projected heavily to a central field in the central nucleus. In addition to this major afferent system of VNLL to the inferior colliculus, a smaller descending projection was found. The descending projection ended mainly in the dorsomedial periolivary region and ventral nucleus of the trapezoid body. However, in some cases a few fibers were traced to the cochlear nuclei. Finally, we observed projections to the medial geniculate body from the dorsal and ventral zones of VNLL that ended diffusely in the medial division of the medial geniculate body. Possibly some fibers from the dorsal zone contribute to a broader projection of the lateral tegmentum to the dorsal division of the medial geniculate body.

Animals↗
PubMed1983

Organization of the efferent projections of the medial superior olivary nucleus in the cat as revealed by HRP and autoradiographic tracing methods.

Features of the organization of the efferent axonal projections from the medial superior olivary nucleus (MSO) in the cat were studied. In order to determine the origin and distribution of projections from MSO, the retrograde horseradish peroxidase (HRP) and autoradiographic tracing methods were used. The results showed that (1) in both HRP and autoradiographic studies the projection to the inferior colliculus was largely ipsilateral, although a contralateral component was present; (2) the projection field of MSO was confined to the ventral division of the central nucleus of the inferior colliculus, and within this field the labeling was heavier in the rostral and dorsolateral parts of the ventral division; (3) the projection to the inferior colliculus was topographic with ventral parts of MSO projecting ventrally and dorsal parts of MSO projecting dorsolaterally; (4) the projection field in the central nucleus formed successive laminae oriented from ventrolateral to dorsomedial; (5) the axonal course was via the medial or internal segment of the lateral lemniscus; and (6) some fibers in this course ended additionally within the dorsal nucleus of the lateral lemniscus. This latter projection was also topographically organized. These observations supported previously described features of lamination and tonotopic order for afferents of the inferior colliculus, as well as recent suggestions that functional segregation of afferent connections exists within the laminated portion of the central nucleus of the inferior colliculus.

Animals↗
PubMed1983

Evidence of sub-collicular auditory projections to the medial geniculate nucleus in the cat: an autoradiographic and horseradish peroxidase study.

Connection of a posteromedial region of the ventral nucleus of the lateral lemniscus were examined in the cat using the autoradiographic tracing method. This sub-collicular region previously had been shown, using retrograde transport of horseradish peroxidase, to send axons to the superior colliculus. The autoradiographic findings revealed that many axons from the posteromedial region of the ventral nucleus of the lateral lemniscus that entered the superior colliculus continued into the midbrain reticular formation. Moreover, other axons traced rostral to the interior colliculus into the thalamus ended in the medial geniculate nucleus, bilaterally. Experiments in which horseradish peroxidase was placed in the medial geniculate nucleus retrogradely labeled the large neurons in the posteromedial region supporting the autoradiographic observations. Other sub-collicular regions also contained labeled cells in these cases, including the main body of the ventral nucleus of the lateral lemniscus and scattered cell groups around the superior olivary complex.

Animals↗
PubMed1982

Localization of the motor neurons to the tensor tympani muscle.

The localization of the motor neurons to the tensor tympani (TT) muscle was studied using the method of retrograde transport of horseradish peroxidase (HRP). After intramuscular injections of HRP, specifically labeled neurons were found in the ventral, parvocellular portion of the trigeminal motor nucleus. These cells had a medial relation to the rootlets of the trigeminal nerve and, rostrally, to the lateral lemniscus. The results are compared to those of other investigators and a generalization is suggested for the localization of these neurons which allows for species variation. Other incidental findings are also discussed.

Animals↗
PubMed1981

Afferent sources of a lateral midbrain tegmental zone associated with the pinnae in the cat as mapped by retrograde transport of horseradish peroxidase.

A paralemniscal zone in the lateral midbrain tegmentum of the cat has been identified in a possible pathway from the superior colliculus to the facial nucleus that may control pinna movements (Henkel and Edward, '78). Other brainstem afferent projections to this paralemniscal zone have been mapped in the present study using the retrograde horseradish peroxidase tracing method and are discussed in three groups. First, potential sources of auditory afferents were limited mainly to the external nucleus of the inferior colliculus, the nucleus sagulum, and the dorsomedial periolivary cell group. Labeled cells in other superior olivary regions and the dorsal cochlear nucleus were apparently related to uptake of horseradish peroxidase from the axons of the lateral lemniscus. Second, afferents from several premotor regions involved in aspects of gaze control were identified. These were mainly from the nucleus prepositus hypoglossi and adjacent pontine reticular formation, but also included projections from the medial vestibular an abducens nuclei and possibly subthalamic regions such as the zona incerta and fields of Forel. Third, a relatively large group of midbrain afferents was closely related to the origin of the collicular projection to the paralemniscal zone. This group consisted of labeled cells in the periaqueductal gray matter, nucleus cuneiformis, and pretectum. The relatively sparse labeling in auditory regions that projected to the paralemniscal zone seems to indicate that the sensorimotor integration necessary to guide pinna movements does not take place primarily in the lateral midbrain tegmentum. The interaction of gaze-related sources with the pinna-related pathway is also discussed.

Afferent Pathways↗
PubMed1979

Sources of subcortical projections to the superior colliculus in the cat.

A comprehensive search for subcortical projections to the cat superior colliculus was conducted using the retrograde horseradish peroxidase (HRP) method. Over 40 different subcortical structures project to the superior colliculus. The more notable among these are grouped under the following categories. Visual structures: ventral lateral geniculate nucleus, parabigeminal nucleus, pretectal area (nucleus of the optic tract, posterior pretectal nucleus, nuclei of the posterior commissure). Auditory structures: inferior colliculus (external and pericentral nuclei), dorsomedial periolivary nucleus, nuclei of the trapezoid body, ventral nucleus of the lateral lemniscus. Somatosensory structures: sensory trigeminal complex (all divisions, but mainly the gamma division of nucleus oralis), dorsal column nuclei (mostly cuneate nucleus), and the lateral cervical nucleus. Catecholamine nuclei: locus coeruleus, raphe dorsalis, and the parabrachial nuclei. Cerebellum: medial, interposed, and lateral nuclei, and the perihypoglossal nuclei. Reticular areas: zona incerta, substantia nigra, midbrain tegmentum, nucleus paragigantocellularis lateralis, and the hypothalamus. Evidence is presented that only the parabigeminal nucleus, the nucleus of the optic tract, and the posterior pretectal nucleus project to the superficial collicular layers (striatum griseum superficiale and stratum opticum), while all other afferents terminate in the deeper layers of the colliculus. Also presented is information concerning the rostrocaudal distribution of some of these afferent connections. These findings stress the multiplicity and diversity of inputs to the deeper collicular layers, and more specifically, identify multiple sources of the physiologically well-known representations of the somatic and auditory modalities in the colliculus.

Animals↗
PubMed1978

The superior colliculus control of pinna movements in the cat: possible anatomical connections.

Possible anatomical pathways mediating superior colliculus control of pinna movements were determined in the cat using the orthograde autoradiographic tracing method and the retrograde horseradish peroxidase technique. This was done in the following manner. First, the division of the facial nucleus that innervates the pinna muscles was determined by injecting the pinna muscles with HRP and surveying the facial nucleus for retrogradely filled cells. Second, the brainstem regions that project the facial nucleus were identified using the horseradish peroxidase method. Third, the superior colliculus projections to these areas were studied using the autoradiographic tracing method. The results suggest that superior colliculus control of pinna movements is mediated entirely by indirect connections with the facial nucleus and that these connections occur mainly in a paralemniscal zone in the lateral midbrain. Of all the brainstem regions shown by the horseradish peroxidase experiments to project to the facial nucleus only this midbrain paralemniscal zone received a projection from the superior colliculus that was dense and overlapped precisely the region containing facial projecting neurons. Further autoradiographic tracing revealed that the facial nucleus was the primary brainstem target of this paralemniscal zone and that all paralemniscal fibers projecting to the facial nucleus ended in the subdivision that innervates the pinna muscles. Other paralemniscal efferents terminate in the opposite paralemniscal zone. The data suggest that other connections between the superior colliculus and the facial nucleus may occur in the cuneiform nucleus of the midbrain, the region around the oculomotor complex, and the reticular formation dorsal to the superior olive.

Animals↗
PubMed1978

Superior colliculus connections with the extraocular motor nuclei in the cat.

Direct and indirect projections from the cat superior colliculus to the extraocular motor nuclei were studied using the orthograde autoradiographic tracing method, the retrograde horseradish peroxidase technique, and Golgi methods. The results show that the superior colliculus projects to the central gray matter directly overlying the oculomotor complex. This projection arises almost entirely from the rostral third of the colliculus, and it terminates most heavily over the rostral half of the oculomotor complex. Dendrites of oculomotor cells extend into this tectal termination zone, making direct tecto-oculomotor contacts possible. Central gray cells within this termination zone project bilaterally to the abducens nuclei. It is proposed that the superior colliculus projection to the supraoculomotor central gray matter and the projection from the central gray matter to the abducens nuclei play a role in convergent eye movements. The superior colliculus projects lightly to a cell group directly ventrolateral to the trochlear nucleus. The superior colliculus sends a small direct projection to the contralateral abducens nucleus and a substantial projection to wide regions of the reticular formation that have been shown previously to project, in turn, to the abducens nucleus. Colliculus cells projecting to the abducens nucleus and adjacent reticular formation are located only in the caudal three-fourths of the colliculus, where they become increasingly concentrated at successively more caudal levels. It is proposed that the graded density of the cells of origin of this projection is the basic structural mechanism by which the colliculus generates horizontal foveating saccades of different amplitudes. Laminar analysis of the origin of all the superior colliculus projections to the extraocular motor regions described here revealed that they arise mostly from the stratum griseum intermedium.

Abducens Nerve↗
PubMed1977

The vestibular complex of the American opossum, Didelphis virginiana. I. Conformation, cytoarchitecture and primary vestibular input.

Degeneration experiments reveal that the vestibular nerve of the adult opossum distributes to an extensive and diverse area of the brainstem and, as in placental mammals, the traditionally named nuclei do not receive a uniform distribution of primary afferent fibers. Nevertheless, such nuclei as well as other nearby cell groups have been considered as vestibular since they share other mutual connections. Except for subgroups "l", "g" and the nucleus supravestibularis all of the vestibular subdivisions generally recognized in more specialized placentals (e.g., the cat) have been identified in the marsupial opossum. Each of the vestibular nuclei and "extra-nuclear" cell stations are described as to their boundaries, cytoarchitecture and dendritic domain. Whenever possible we have employed a terminology consistent with that used for the cat. In several instances we have used results from experimental degeneration material to identify nuclear boundaries and divisions which are not apparent in routine Nissl, Golgi or silver preparations.

Animals↗
PubMed1977

The vestibular complex of the American opossum didelphis virginiana. II. Afferent and efferent connections.

We have demonstrated the connectivity of the opossum's vestibular nuclei using degeneration, autoradiographic and horseradish peroxidase techniques and have found it to be generally comparable to that reported for the cat. Apart from the primary input described in Part I of our study, the cerebellum provides the major source of afferent connections to the vestibular complex. Axons from the cerebellar cortex distribute mainly to vestibular areas which receive no primary afferent projections, e.g., the dorsal part of the lateral vestibular nucleus, the dorsolateral margin of the inferior vestibular nucleus as well as cell groups comparable to "f" and "x." In contrast, fastigial fibers show considerable overlap with primary vestibular input, particularly in the ventral part of the lateral nucleus, the central part of the inferior nucleus and the medial nucleus. Axons of fastigial origin also distribute to the superior vestibular nucleus, to subnuclei "f" and "x" and to the parasolitary region. Although spinal fibers are diffuse within the main vestibular nuclei, they ramify quite densely within subnucleus "x." Most of the spinovestibular projection appears to arise in the cervical spinal cord of the opossum. Ipsilateral connections from the interstitial nucleus of Cajal and surrounding areas end predominantly, but not exclusively, in the medial vestibular nucleus. A crossed midbrain projection has been verified from the red nucleus to cell group "x" and the lateral part of the inferior nucleus, as well as to an area possibly comparable to cell group "z," as described for the cat. In Part I of our study we have shown that the major targets of primary vestibular fibers are the central part of the superior nucleus, a portion of the parabrachial complex possibly comparable to subnucleus "y"," the ventral part of the lateral nucleus and the medial nucleus. All of these primary zones give rise to fibers supplying extraocular nuclei and surrounding areas (present study). The ascending midbrain fibers from the superior nucleus end mainly ipsilaterally, whereas those from the putative subnucleus "y" and the medial vestibular nucleus distribute contralaterally for the most part. Although the dorsal part of the lateral vestibular nucleus has no primary vestibular input, it does receive a major projection from the cerebellar cortex. This same region of the lateral nucleus projects to the spinal cord, but not to extraocular nuclei. The ventral part of the lateral nucleus, and perhaps the medial nucleus, also relay to the spinal cord. Additional projections from all vestibular nuclei to the reticular formation provide indirect routes through which the vestibular nuclei can potentially effect multiple systems, including those innervating the spinal cord. Finally, commissural vestibular connections of the opossum are shown to arise within all four major nuclei.

Afferent Pathways↗