Distribution of the vestibular neurons projecting to the oculomotor and trochlear nuclei in rabbits

Horseradish peroxidase and Fast Blue were injected into the oculomotor and trochlear nuclei of rabbits so as to study the distribution of vestibular neurons that project to these nuclei. After the oculomotor nucleus was injected, labelled neurons were found in the superior, medial, and descending vestibular nuclei as well as in cell group Y. In the superior nucleus, most of the neurons (510 +/- 46) were ipsilateral to the injection, although contralaterally labelled neurons were also observed (104 +/- 19) more peripherally. In cell group Y, 186 +/- 24 contralaterally labelled neurons were observed, whereas hardly any (8 +/- 3) were found on the ipsilateral side. The largest group of labelled neurons (811 +/- 65) constituted a neuronal band located contralaterally in the medial nucleus and rostral part of the descending nucleus. This band rostromedially continued with the caudal part of the group of internuclear neurons of the abducens nucleus. Only 190 +/- 31 neurons were labelled in the medial and descending nucleus ipsilateral to the injected oculomotor nucleus. After injection of the trochlear nucleus, labelled neurons were found in the ipsilateral superior nucleus and contralateral medial and descending nuclei: a few labelled cells were also observed in the ipsilateral medial and descending nuclei as well as in the contralateral cell group Y.     

The motor nuclei and sensory neurons of the IIIrd, IVth, and VIth cranial nerves in the monitor lizard, Varanus exanthematicus

The motor nuclei of the oculomotor, trochlear, and abducens nerves of the reptile Varanus exanthematicus and the neurons that subserve the sensory innervation of the extraocular muscles were identified and localized by retrograde and anterograde transport of horseradish peroxidase (HRP). The highly differentiated oculomotor nuclear complex, located dorsomedially in the tegmentum of the midbrain, consists of the accessory oculomotor nucleus and the dorsomedial, dorsolateral, intermediate, and ventral subnuclei. The accessory oculomotor nucleus projects ipsilaterally to the ciliary ganglion. The dorsomedial, dorsolateral, and intermediate subnuclei distribute their axons to the ipsilateral orbit, whereas the ventral subnucleus, which innervates the superior rectus muscle, has a bilateral, though predominantly contralateral projection. The trochlear nucleus, which rostrally overlaps the oculomotor nuclear complex, is for the greater part a comma-shaped cell group situated lateral, dorsal, and medial to the medial longitudinal fasciculus. Following HRP application to the trochlear nerve, almost all retrogradely labeled cells were found in the contralateral nucleus. The nuclear complex of the abducens nerve consists of the principal and accessory abducens nuclei, both of which project ipsilaterally. The principal abducens nucleus is located just beneath the fourth ventricle laterally adjacent to the medial longitudinal fasciculus and innervates the posterior rectus muscle. The accessory abducens nucleus has a ventrolateral position in the brainstem in close approximation to the ophthalmic fibers of the descending trigeminal tract. It innervates the retractor bulbi and bursalis muscles. The fibers arising in the accessory abducens muscles form a loop in or just beneath the principal abducens nucleus before they join the abducens nerve root. The afferent fibers conveying sensory information from the extraocular muscles course in the oculomotor nerve and have their perikarya in the ipsilateral trigeminal ganglion, almost exclusively in its ophthalmic portion.     

Afferents to the abducens nucleus in the monkey and cat

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant to visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was processed by the tetramethyl benzidine (TMB) method of Mesulam. In both species the largest source of afferents to the abducens nucleus was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal.(ABSTRACT TRUNCATED AT 400 WORDS)     

The vestibular nuclei in the domestic hen (Gallus domesticus): VI. Afferents from the cerebellum

Following horseradish peroxidase (HRP) injections in the superior, in the Deiters', in the medial, and in the descending vestibular nuclei in the hen, labeled cells are found in lateral longitudinal zones in the ipsilateral cerebellar cortex, most numerously in the anterior lobe, the nodulus, the uvula, and the auricle. Furthermore, labeled cells are found bilaterally in the ventral parts of the medial and intermediate cerebellar nuclei. Lesions in the cerebellar cortex of the anterior lobe and in anterior parts of the posterior lobe result in terminal degeneration, mainly in the nucleus Deiters dorsalis, but also scantily, in peripheral regions of the superior nucleus, the nucleus Deiters ventralis, the ventrolateral part of the medial nucleus and, mainly medially, in the descending nucleus. Lesions in the posterior part of the uvula, in the nodulus, and in the auricle result in much denser degeneration, most heavily affecting the nucleus Deiters dorsalis, but also affecting peripheral regions of the superior nucleus, the nucleus Deiters ventralis, the entire descending and medial nuclei, and the tangential nucleus. Lesions in the medial cerebellar nucleus result in degeneration bilaterally in the vestibular complex, most heavily affecting the nucleus Deiters ventralis and cell group B, but also affecting peripheral regions of the superior nucleus, the medial nucleus—mainly in dorsomedial regions, lateral and caudal parts of the descending nucleus and, very scantily, in the nucleus Deiters dorsalis. The findings are discussed in the light of the data concerning the organization of the cerebellovestibular projections in mammals and the known connections of the vestibular nuclei in birds.     

Internuclear neurons in the ocular motor system of frogs.

Medial and lateral rectus motoneurons of frogs were localized after retrograde labeling with horseradish peroxidase (HRP) injected in the medial rectus muscle or applied on the cut end of the abducens nerve. Coordinates of these cell columns were used as target areas for the injection of small amounts of HRP (20-60 nl) and [3H]leucine (25-40 nl) and as search areas for retrogradely and anterogradely labeled internuclear neurons (INT) in in vivo and in vitro experiments. HRP injection in the medial rectus subdivision of the oculomotor nucleus (n = 6) resulted in retrograde labeling of cell bodies in the contralateral principal abducens nucleus. On the average about 16 cells per animal were found. Somatic diameters were about 13.5 +/- 2.8 microns (n = 32). The number and the size of these abducens internuclear neurons (AbINT) are smaller than those of lateral rectus motoneurons (n = 75; diameter: 19 +/- 3.2 microns). A crossed projection of AbINT to medial rectus motoneurons in the contralateral oculomotor nucleus is further supported by autoradiographic results. Following injection of [3H]leucine into the abducens nucleus, a high density of silver grains was visible within the contralateral oculomotor nucleus, mainly in the caudal part of the oculomotor nucleus, where medial rectus motoneurons are located. Injection of [3H]leucine in vivo (n = 4) and in vitro (n = 3) resulted in a similar high density of silver grains within the contralateral oculomotor nucleus, but the background level of silver grains was significantly higher after in vitro (264 +/- 38/2,500 microns2) than after in vivo injections (195 +/- 17/2,500 microns2). HRP injection in the principal abducens nucleus (n = 9) resulted in retrograde labeling of cell bodies in the medial rectus subdivisions of the bilateral oculomotor nuclei. Ipsilateral projections predominated, with about 10 (+/- 8) labeled cells over contralateral projections (about 3 +/- 2). Average diameters of these oculomotor internuclear neurons (OcINT) were again smaller (10.8 +/- 2 microns; n = 18) than those of medial rectus motoneurons (14.4 +/- 3 microns; n = 52). In addition, retrogradely labeled cells were consistently encountered in the bilateral vestibular nuclei, the cerebellar nuclei, the dorsal brainstem caudal to the abducens nuclei, and ipsilaterally in the pretectum. Most of the vestibular neurons were located in the rostral part of the vestibular nuclear complex. These neurons might constitute part of the three-neuronal arc of the vestibulo-ocular reflex in the frog. Labeled cells in the pretectum were restricted to the ipsilateral posterior thalamic nucleus (P).(ABSTRACT TRUNCATED AT 400 WORDS)     

A comparative study of the tangential-oculomotor projection.

Most of the neurons of the tangential vestibular nucleus of birds project to the controlateral oculomotor complex, but it is not known whether there is a homologous projection in mammals. In this study, horseradish peroxidase (HRP) was injected into the oculomotor nucleus of chicks and rabbits, and the distributions of labelled neurons in the target region of the vestibular complex in the two species were compared. In chicks, a large number of labelled neurons formed a continuous band of neurons located in the contralateral tangential, descending and medial nuclei. In rabbits a similar band of labelled neurons was found in the contralateral descending and medial vestibular nuclei, but most of the neurons were caudal to the incoming vestibular nerve fibers, and only a few rostral neurons were located among these fibers. Our results suggest that the tangential nucleus neurons projecting to the oculomotor nucleus may be homologous to the most lateral neurons of the neuronal band of rabbits.     

Organization of the cerebellum in the pigeon (Columba livia): III. Corticovestibular connections with eye and neck premotor areas.

The connections of the cerebellar cortex with vestibular premotor neurons of the oculomotor and collimotor systems in the pigeon were delineated in experiments using WGA-HRP as an anterograde and retrograde tracer. Putative premotor neuron pools were identified by injections into the oculomotor (mIII) and trochlear nuclei (mIV) and into the most rostral portion of the cervical neck motor nucleus, nucleus supraspinalis (SSp). The retrograde data indicate that ipsilateral projections upon oculomotor neurons arise from the medial portions of the superior (VeS) and tangential (Ta) nuclei. Contralateral projections originate from the infracerebellar nucleus, the interstitial vestibular region including the main (lateral) portion of the tangential nucleus, and from the descending and medial vestibular nuclei (VeD, VeM). These projections were confirmed in anterograde studies that also defined the connections of these vestibular premotor regions with specific subnuclear divisions of the pigeon's "oculomotor" nuclei (mIII, mIV, mVI). The organization of projections from the vestibular nuclei to the pigeon's extraocular motoneurons is similar to that reported in mammals. Projections upon neck premotor neurons arise primarily from neurons in the interstitial region of the vestibular nuclear complex. After injections in SSp, retrogradely labeled neurons were found, contralaterally, in the lateral part of the tangential and superior vestibular nuclei and in the dorsolateral vestibular nucleus (VDL). Ipsilateral labeling was seen in the medial interstitial region (VeM, VeD, and medial Ta). These projections were confirmed in anterograde experiments. With the exception of VDL, vestibular nuclei projecting to neck motoneurons also project to extraocular motoneurons. Thus the infracerebellar nucleus projects exclusively, and the superior vestibular nucleus predominantly, upon oculomotor (mIII, mIV) nuclei; VDL projects predominantly upon the neck motor nucleus, whereas the interstitial vestibular regions (medial Ta, rostral VeD, intermediate VeM) project upon both collimotor and oculomotor neurons. The pattern of retrograde labeling seen in the cerebellar cortex after injections into vestibular premotor nuclei was used to define the projections of specific cerebellar cortical zones upon vestibular eye and neck premotor neurons. Corticovestibular projections upon these regions arise from the auricle and lateral unfoliated cortex, the posterior lobe components of cortical zones B and E, and from the vestibulocerebellum. Each of these cortical zones projects upon components of the vestibular nuclear complex, which are premotor to either oculomotor nuclei or collimotor nuclei. The hodological findings are related to the functional organization of the oculomotor and collimotor systems in the pigeon and compared with the mammalian data.     

Identification of vestibuloocular projection neurons in the developing chicken medial vestibular nucleus

Biocytin was injected into the oculomotor, trochlear, or abducens nucleus on one side using isolated chicken brainstem preparations or brain slices to identify the medial vestibular nucleus (MVN) neurons projecting to these targets. Oculomotor nucleus injections produced retrogradely labeled neurons in the contralateral ventrolateral MVN (MVN_VL), with few labeled neurons in the ipsilateral MVN_VL and rarely in the dorsomedial MVN on either side. Labeled MVN_VL neurons were identified as stellate (95%) and elongate (5%) cells. Trochlear nucleus injections produced a similar pattern of MVN neuron labeling. Abducens nucleus injections resulted in retrogradely labeled stellate (87%) and elongate (13%) neurons in the MVN_VL, which had smaller cell bodies than those projecting to the oculomotor nucleus. Anteroposteriorly, labeled MVN_VL neurons were coextensive with the tangential nucleus, with neurons projecting to the oculomotor nucleus distributed lateral to and intermixed with the more medially situated neurons projecting to the abducens nucleus. The fundamental pattern of vestibuloocular projecting neurons was similar at both embryonic ages studied, E16 and E13. In contrast to the case in mammals, where most vestibuloocular projection neurons reside within the MVN, most retrogradely labeled neurons in these chicken preparations were found within the ventrolateral vestibular, descending vestibular, and tangential nuclei. The morphological identification and mapping of vestibuloocular projection neurons in the chicken MVN described here represents the first step in a systematic evaluation of the relationship between avian vestibuloocular neuron structure and function. © 2009 Wiley‐Liss, Inc.     

Afferents to the oculomotor nucleus in the goldfish (Carassius auratus) as revealed by retrograde labeling with horseradish peroxidase.

The goal of this work was to compare the distribution and morphology of neurons projecting to the oculomotor nucleus in goldfish with those previously described in other vertebrate groups. Afferent neurons were revealed by retrograde labeling with horseradish peroxidase. The tracer was electrophoretically injected into the oculomotor nucleus. The location of the injection site was determined by the antidromic field potential elicited in the oculomotor nucleus by electrical stimulation of the oculomotor nerve. Labeled axons whose trajectories could be reconstructed were restricted to the medial longitudinal fasciculus. In order of quantitative importance, the afferent areas to the oculomotor nucleus were: (1) the ipsilateral anterior nucleus and the contralateral tangential and descending nuclei of the octaval column. Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site. Specifically, stained cells were found in the caudal pole of the superior reticular nucleus, throughout the medial reticular nucleus and in the rostral area of the inferior reticular nucleus; (4) eurydendroid cells of the cerebellum, located close to the contralateral eminentia granularis pars lateralis, were also labeled; and (5) a small and primarily ipsilateral group of labeled cells was located at the mesencephalic nucleus of the medial longitudinal fasciculus. The similarity in the structures projecting to the oculomotor nucleus in goldfish to those in other vertebrates suggests that the neural network involved in the oculomotor system is quite conservative throughout phylogeny. Nevertheless, in goldfish these projections appeared with some specific peculiarities, such as the cerebellar and mesencephalic afferents to the oculomotor nucleus.     

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.     

On the projections from the vestibular and perihypoglossal nuclei to the spinal trigeminal and lateral reticular nuclei in the cat

The projection from the vestibular and perihypoglossal nuclei to the spinal trigeminal and lateral reticular nuclei has been studied in cats where the wheat germ agglutinin-horseradish peroxidase complex has been used as a retrograde tracer. All injections were made at the level of the caudal pole of the inferior olive. The medial and descending vestibular, and the perihypoglossal nuclei were found to project to the spinal trigeminal nucleus. The projection to the lateral reticular nucleus reaches its medial-most part only, and originates in the lateral vestibular nucleus. The lateral part of the reticular formation also appears to be the target for some vestibular efferent fibres, mainly from the descending vestibular nucleus. The retrogradely labelled cells within the medial and descending vestibular nuclei are of all sizes and distributed throughout their entire territory. Certain observations furthermore indicate that the fibres reaching the lateral reticular nucleus are collaterals only from the vestibulospinal tract. The projections are bilateral. The observations confirm and extend previous observations on the afferent projections to the spinal trigeminal and lateral reticular nuclei.     

The vestibular nuclei in the domestic hen (Gallus domesticus). IV. The projection to the spinal cord

Horseradish peroxidase was injected at various levels of the spinal cord of the hen and the cells in the brain stem, labeled due to retrograde transport of the tracer enzyme, were mapped with particular reference to the vestibular nuclear complex. The Deiters' nuclei project ipsilaterally to the spinal cord. The projection from the nucleus Deiters ventralis reaches down to the lumbosacral spinal cord. The spinal projection from the nucleus Deiters dorsalis is somatotopically organized. Cells localized rostrally within the nucleus project to upper parts of the spinal cord cells localized caudally project to the lumbosacral spinal cord. The medial and the descending nucleus contain labeled cells in two compartments of the nuclei. In each of them a rostral one projects to the upper cervical cord, a caudal one projects to the thoracical spinal cord. In addition, labeled cells are observed in the reticular formation, mainly in the pons and the medulla oblongata, in the red nucleus, in the raphe nuclei and in the periaqueductal grey. Very few labeled cells are observed in the locus ceruleus, the nucleus intercalatus and in the nucleus of the solitary tract. In some cases, the number of labeled cells in the various nuclei in the brain stem projecting to the spinal cord was counted to get an impression of the quantitative importance of the vestibulospinal projection relative to afferents to the spinal cord from the other nuclei. The findings are discussed in the light of what is known of the vestibulospinal projection in mammals.     

Afferent and efferent connections of the oculomotor region of the fastigial nucleus in the macaque monkey.

Afferent and efferent connections of the fastigial oculomotor region (FOR) were studied in macaque monkeys by using axonal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). When injected HRP is confined to the FOR, retrogradely labeled cells appear in lobules VIc and VII of the ipsilateral vermis and in group b of the contralateral medial accessory olive (MAO). In reference to the maps of topographical organization, the extent of the effective site in the fastigial nucleus (FN) could be assessed from the distributions of labeled Purkinje cells (P cells) in the vermis and labeled olivary neurons in the MAO. In contrast to the unilateral nature of the P-cell and climbing-fiber projections, those from the other brainstem regions to the FOR were bilateral. Following the injection of HRP into the FOR, the largest number of retrogradely labeled cells appeared in the pontine nuclei. Although the number of labeled cells was greater on the contralateral side in both the peduncular and dorsomedial pontine nuclei (DMPN), the number of each side was virtually identical in the dorsolateral pontine nucleus (DLPN). In the nucleus reticularis tegmenti pontis (NRTP), labeled cells were located only in its medial and dorsolateral portions bilaterally. In the vestibular complex, labeled cells appeared in the superior (SVN), medial (MVN), and inferior vestibular nuclei (IVN) bilaterally. The lateral vestibular nucleus (LVN), including y group and the ventrolateral vestibular nucleus, were free of labeled cells. Labeled cells appeared also in the perihypoglossal nucleus (PHN) bilaterally. In the pontine raphe (PR) and paramedian pontine reticular formation (PPRF), labeled cells appeared bilaterally in the caudal third of the area between the oculomotor and abducens nuclei. Labeled cells appeared also in the mesencephalic and medullary reticular formation. Tracing of anterogradely labeled axons demonstrated that most fibers from the FOR decussated within the cerebellum and entered the brainstem via the contralateral uncinate fasciculus. Some crossed fibers ascended with the contralateral brachium conjunctivum and terminated in the midbrain tegmentum. A small contingent of fibers advanced further to the thalamus. In the mesodiencephalic junction, labeled terminals were found contralaterally in the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) and a medial portion of FOrel's H Field. They appeared also in the central mesencephalic reticular formation (cMRF), the periaqueductal gray (PAG), the posterior commissure nucleus, and the superior colliculus. The oculomotor and trochlear nuclei, the red nucleus, and the interstitial nucleus of Cajal were free of labeled terminals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Afferents to the flocculus of the cerebellum in the rhesus macaque as revealed by retrograde transport of horseradish peroxidase

To investigate the afferent projections to the flocculus in a nonhuman primate, we injected horseradish peroxidase into one flocculus of six rhesus macaques (Macaca mulatta) and processed their brains according to the tetramethylbenzidine protocol to reveal retrogradely labeled neurons. Labeled neurons were found in a large set of nuclei within the rostral medulla and the pons. The greatest numbers of labeled neurons were in the vestibular complex and the nucleus prepositus hypoglossi. There were neurons labeled bilaterally throughout all the vestibular nuclei except the lateral vestibular nucleus, but most of the labeled neurons were in the caudal parts of the medial and inferior vestibular nuclei and in the central part of the superior vestibular nucleus; the nucleus prepositus was also labeled bilaterally, primarily caudally. Modest numbers of labeled neurons were found in the y-group, most ipsilaterally, and many neurons were labeled in the interstitial nucleus of the vestibular nerve. No labeled neurons were found in the vestibular ganglion following a large injection into the flocculus. A second large source of afferents to the flocculus was the medial, paramedial, and raphe reticular formation. Dense aggregates of labeled neurons were located in several pararaphe nuclei of the rostral medulla and the rostral pons and in the nucleus reticularis paramedianus of the medulla and several component nuclei of the nucleus reticularis tegmenti pontis bilaterally. Several groups of cells within and abutting upon the medial and rostral aspects of the abducens nucleus were labeled bilaterally. There was a modest projection from two parts of the pontine nuclei. Both a dorsal midline nucleus ventral to the nucleus reticularis tegmenti pontis and a collection of nuclei in a laminar region adjacent to the contralateral middle cerebellar peduncle contained labeled neurons whose numbers, while modest, were large compared to the projections to the flocculus in other animals. This generic difference may be due to the greater development of the smooth pursuit system in monkeys and the consequent need for a more substantial input from the cerebral cortex. As in other genera, the inferior olive projected to the flocculus via the dorsal cap of Kooy and the contiguous ventrolateral outgrowth. The projection was completely crossed and large injections labeled virtually every neuron in the dorsal cap, suggesting that the dorsal cap is the principal source of climbing fiber afferents.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projections of the vestibular and cerebellar nuclei in Rana pipiens

The efferent projections and cytoarchitecture of the vestibulocerebellar region were examined to determine the nuclear boundaries and potential homologies. The anterior portion of the vestibular complex projects to the ipsilateral oculomotor and trochlear nuclei and is the major source of commissural fibers. Neurons in the rostromedial portions of the complex project to the contralateral trochlear nucleus. Large neurons in the ventrolateral portion of the complex give rise to a bilateral vestibulospinal pathway. Medium-sized neurons in the neuropil and small neurons in the central gray giving rise to bilateral projections to the spinal cord and oculomotor nuclei as well as commissural and ipsilateral cerebellar efferents. Projections from the nucleus of the cerebellum reach the contralateral spinal cord and cerebellar nucleus and there is also a bilateral projection to the ventral rhombencephalic and mesencephalic basal plates. The medial portion of the nucleus gives rise to commissural, ipsilateral mesencephalic and contralateral spinal projections. The lateral portion of the nucleus projects to the contralateral ventral mesencephalon. On the whole, the results of this investigation substantiate the division of the anuran vestibular complex in anurans into nuclei which may be homologous to the superior nucleus and nucleus of Deiters in mammals. The case for distinct descending and medial nuclei is less compelling. Further, it appears possible to divide the nucleus of the cerebellum into medial and lateral components whose connectivity is similar to that of reptiles and to a lesser extent mammals.     

Cerebellar efferents in the lizard Varanus exanthematicus. II. Projections of the cerebellar nuclei

The projections of the cerebellar nuclei have been studied in the lizard Varanus exanthematicus with various experimental anatomical techniques. In anterograde degeneration experiments (lesions of the cerebellar peduncle) both ascending and decending contralateral projections were found. Ascending fibers which could be traced from the cerebellar commissure ventralward decussated at the level of the trochlear and oculomotor nuclei. These fibers coursed rostralward to the mesodiencephalic junction. With anterograde tracing techniques (3H-leucine and HRP) this tract was found to terminate in the nucleus ruber and the interstitial nucleus of the fasciculus longitudinalis medialis. Moreover, retrograde tracer studies (HRP, "Fast Blue") showed that this tract appeared to arise mainly in the lateral cerebellar nucleus. With both anterograde degeneration and tracing techniques (3H-leucine and HRP) a bundle of fibers could be followed, which decussates in the basal part of the cerebellum and passes dorsally around the contralateral medial cerebellar nucleus to the lateral side of the brainstem. This contralaterally descending projection system was found, lateral to the vestibular nuclear complex, and as far caudally as the descending vestibular nucleus, to terminate on various vestibular nuclei. Horseradish peroxidase studies showed that this contralaterally descending projection system originates mainly in the medial cerebellar nucleus, but ipsilaterally descending projections were also found. With the fluorescent double labeling technique ("Fast Blue" and "Nuclear Yellow") the projections of the cerebellar nuclei described above were confirmed. Furthermore, double labeling revealed neurons in both cerebellar nuclei (especially the medial nucleus) that project to both the mesencephalon and the cervical spinal cord. The present results indicate that the efferent connections of the cerebellar nuclei in the lizard Varanus exanthematicus are organized as two main projections, an ascending projection comparable to the mammalian brachium conjunctivum arising in the lateral cerebellar nucleus, and a descending projection comparable to the mammalian hook bundle (fasciculus uncinatus), originating mainly in the medial cerebellar nucleus. Such projections are common for terrestrial vertebrates.     

Vestibulo-oculomotor connections in an elasmobranch fish,the atlantic stingray,Dasyatis sabina

In elasmobranch fishes, including the Atlantic stingray, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. This is different from most vertebrates, in which the medial rectus is innervated by the ipsilateral oculomotor nucleus. This observation led to the prediction that the excitatory vestibulo-extraocular motoneuron projections connecting each semicircular canal to the appropriate muscle should use a contralateral projection from the vestibular nuclei to the motoneurons. This hypothesis was examined in the Atlantic stingray by injecting horseradish peroxidase unilaterally into the oculomotor nucleus. It was found that vestibulo-oculomotor projections arise from the ipsilateral anterior octaval nucleus and the contralateral descending octaval nucleus. The same pattern was observed when the trochlear nucleus was involved in the injection. There were no cells labeled in the region of the abducens nucleus, and no candidate for a nucleus prepositus hypoglossus was identified. The presence of compensatory eye movements, the directional sensitivity of the semicircular canals, the location of the motoneurons innervating each eye muscle, and our results indicate that the excitatory input to the extraocular motoneurons is derived from the contralateral descending octaval nucleus, and the inhibitory input is derived from the ipsilateral anterior octaval nucleus. The absence of both abducens internuclear interneurons and a nucleus prepositus hypoglossus suggests that eye movements, particularly those in the horizontal plane, are controlled differently in elasmobranchs than in other vertebrates examined to date. © 1994 Wiley-Liss, Inc.     

Rhombomeric organization of vestibular pathways in larval frogs

Rhombencephalic subnuclei and projection pathways related to vestibular function were mapped in larval ranid frogs. The retention of overt postembryonic rhombomeres (r) allowed direct visualization of the locations of neurons retrogradely labeled with fluorescent dextran amines from the midbrain oculomotor complex, cerebellum, vestibular nuclei, and spinal cord. Oculomotor projecting vestibular neurons were mainly located in bilateral r1/2, ipsilateral r3, and contralateral r5-8, and spinal projecting vestibular neurons mainly in ipsilateral r4 and contralateral r5. Vestibular commissural neurons were located in r1-3 and r5-7 and were largely excluded from r4. Cerebellar projecting neurons included contralateral inferior olivary neurons in r8 and vestibular neurons in bilateral r6/7 and contralateral r1/2. Mapping these results onto adult anuran vestibular organization indicates that the superior vestibular nucleus derives from larval r1/2, the lateral vestibular nucleus from r3/4, and the major portions of the medial and descending vestibular nuclei from r5-8. The lateral vestibulospinal tract projects from an origin in r4, whereas a possible ascending tract of Deiters arises in r3. Rhombomere 5 contains a nuclear group that appears homologous to the tangential nucleus of fish, reptiles, and birds and thus likely serves gravistatic and linear vestibulomotor reflexes. Comparisons between frogs and other vertebrates suggest that vestibular neurons performing similar computational roles during head movements originate from the same segmental locations in different species.     

Cortical projections to nuclei adjacent to the oculomotor complex in the medial dien-mesencephalic tegmentum in the monkey

Cortical projections to cell groups surrounding the oculomotor complex were studied by using the retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique in old and new world monkeys. Fluid HRP injections or transcannular solid polyacrylamide HRP gel implants were made into the oculomotor nucleus (OMN) and adjacent nuclei to label retrogradely corticofugal neurons that project to this region, and cortical HRP gel implants were made in various areas of the frontal lobe to label anterogradely the trajectories and terminations of cortico-paraoculomotor projections and thus to confirm the retrograde findings. Projections to the paraoculomotor cell groups in the medial dien-mesencephalic tegmentum originate almost exclusively from the frontal lobe. Both retrograde and anterograde studies confirmed that the prearcuate cortex in the concavity of the arcuate sulcus, including the frontal eye field, and, to a lesser extent, suprarcuate rostral dorsal area 6 cortex and the dorsomedial convexity (area 9), project to the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the dorsal region of the prerubral field, nucleus of Darkschewitsch (ND), medial accessory nucleus of Bechterew (NB) and dorsomedial parvocellular red nucleus (dmPRN). The premotor area 6 and motor area 4 cortex, on the other hand, give rise to projections that target a larger portion of the parvocellular red nucleus, extending rostrally into the ventral region of the prerubral field, and a rather intense projection to the ND. The interstitial nucleus of Cajal (IC) was distinguished more by its light, or lack of, projections from the frontal cortex. The inferior parietal lobule (IPL, area 7) which has certain common physiological properties with the frontal eye field (FEF area 8) related to the oculomotor system, lacked retrogradely labeled neurons in all cases where transcannular gel implants into the OMN eliminated the possibility of HRP uptake in the corpus callosum or other structures traversed in needle injections, suggesting that the IPL affects eye movement primarily through its rostrally directed corticocortical associational connections with the FEF. In additional cases, the ND-NB-dmPRN configuration of cells that receives FEF input is shown to project to the inferior olivary complex (i.e., is pre-olivo-cerebellar), whereas riMLF and IC give rise to descending projections in the MLF, which target extraocular muscle motor nuclei, vestibular complex, and spinal cord. The results are discussed in terms of the potential role of the cerebral cortex in eye movement mechanisms.