The vestibular complex in a prosimian primate (Galago senegalensis): morphology and spinovestibular connections

Compensation following bilateral vestibular damage

Bilateral loss of vestibular inputs affects far fewer patients than unilateral inner ear damage, and thus has been understudied. In both animal subjects and human patients, bilateral vestibular hypofunction (BVH) produces a variety of clinical problems, including impaired balance control, inability to maintain stable blood pressure during postural changes, difficulty in visual targeting of images, and disturbances in spatial memory and navigational performance. Experiments in animals have shown that non-labyrinthine inputs to the vestibular nuclei are rapidly amplified following the onset of BVH, which may explain the recovery of postural stability and orthostatic tolerance that occurs within 10 days. However, the loss of the vestibulo-ocular reflex and degraded spatial cognition appear to be permanent in animals with BVH. Current concepts of the compensatory mechanisms in humans with BVH are largely inferential, as there is a lack of data from patients early in the disease process. Translation of animal studies of compensation for BVH into therapeutic strategies and subsequent application in the clinic is the most likely route to improve treatment. In addition to physical therapy, two types of prosthetic devices have been proposed to treat individuals with bilateral loss of vestibular inputs: those that provide tactile stimulation to indicate body position in space, and those that deliver electrical stimuli to branches of the vestibular nerve in accordance with head movements. The relative efficacy of these two treatment paradigms, and whether they can be combined to facilitate recovery, is yet to be ascertained.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Central projections of the saccular and utricular nerves in macaques

The central projections of the utricular and saccular nerve in macaques were examined using transganglionic labeling of vestibular afferent neurons. In these experiments, biotinylated dextran amine was injected directly into the saccular or utricular neuroepithelium of fascicularis (Macaca fascicularis) or rhesus (Macaca mulatta) monkeys. Two to 5 weeks later, the animals were killed and the peripheral vestibular sensory organs, brainstem, and cerebellum were collected for analysis. The principal brainstem areas of saccular nerve termination were lateral, particularly the spinal vestibular nucleus, the lateral portion of the superior vestibular nucleus, ventral nucleus y, the external cuneate nucleus, and cell group l. The principal cerebellar projection was to the uvula with a less dense projection to the nodulus. Principle brainstem areas of termination of the utricular nerve were the lateral/dorsal medial vestibular nucleus, ventral and lateral portions of the superior vestibular nucleus, and rostral portion of the spinal vestibular nucleus. In the cerebellum, a strong projection was observed to the nodulus and weak projections were present in the flocculus, ventral paraflocculus, bilateral fastigial nuclei, and uvula. Although there is extensive overlap of saccular and utricular projections, saccular inputs to the lateral portions of the vestibular nuclear complex suggest that saccular afferents contribute to the vestibulospinal system. In contrast, the utricular nerve projects more rostrally into areas of known concentration of vestibulo-ocular related cells. Although sparse, the projections of the utricle to the flocculus/ventral paraflocculus suggest a potential convergence with floccular projection inputs from the vestibular brainstem that have been implicated in vestibulo-ocular motor learning.

Central projections of the vestibular nerve: a review and single fiber study in the Mongolian gerbil

The primary purpose of this article is to review the anatomy of central projections of the vestibular nerve in amniotes. We also report primary data regarding the central projections of individual horseradish peroxidase (HRP)-filled afferents innervating the saccular macula, horizontal semicircular canal ampulla, and anterior semicircular canal ampulla of the gerbil. In total, 52 characterized primary vestibular afferent axons were intraaxonally injected with HRP and traced centrally to terminations. Lateral and anterior canal afferents projected most heavily to the medial and superior vestibular nuclei. Saccular afferents projected strongly to the spinal vestibular nucleus, weakly to other vestibular nuclei, to the interstitial nucleus of the eighth nerve, the cochlear nuclei, the external cuneate nucleus, and nucleus y. The current findings reinforce the preponderance of literature. The central distribution of vestibular afferents is not homogeneous. We review the distribution of primary afferent terminations described for a variety of mammalian and avian species. The tremendous overlap of the distributions of terminals from the specific vestibular nerve branches with one another and with other sensory inputs provides a rich environment for sensory integration.

Aging and the human vestibular nuclei: morphometric analysis

The data concerning the effects of age on the brainstem are scarce and few works are devoted to the human vestibular nuclear complex. The study of the effects of aging in the vestibular nuclei could have clinical interest due to the high prevalence of balance control and gait problems in the elderly. We have used in this work eight human brainstems of different ages sectioned and stained by the formaldehyde-thionin technique. The neuron's profiles were drawn with a camera lucida and Abercrombie's method was used to estimate the total number of neurons. The test of Kolmogorov-Smirnov with the correction of Lilliefors was used to evaluate the fit of our data to a normal distribution and a regression analysis was done to determine if the variation of our data with age was statistically significant. Aging does not affect the volume or length of the vestibular nuclear complex. Our results clearly show that neuronal loss occurs with aging in the descending (DVN), medial (MVN), and lateral (LVN) vestibular nuclei, but not in the superior (SVN). There are changes in the proportions of neurons of different sizes but they are not statistically significant. The neuronal loss could be related with the problems that elderly people have to compensate unilateral vestibular lesions and the alterations of the vestibulospinal reflexes. The preservation of SVN neurons can explain why vestibulo-ocular reflexes are compensated after unilateral vestibular injuries.

Parasolitary nucleus: a source of GABAergic vestibular information to the inferior olive of rat and rabbit

At least two subnuclei of the inferior olive, the beta-nucleus, and the dorsomedial cell column (dmcc), contain vestibularly responsive neurons that receive a dense descending projection that uses gamma-aminobutyric acid (GABA) as the transmitter. In contrast to the GABAergic innervation of other olivary subnuclei, the terminal boutons that terminate on neurons in the beta-nucleus and the dorsomedial cell column remain intact after cerebellectomy, ruling out both the cerebellum and the cerebellar nuclei as afferent sources. By using both immunohistochemical as well as orthograde and retrograde tracer methods, we have identified the source of the GABAergic pathway to the beta-nucleus and dmcc in both rat and rabbit. Under physiologic recording of single olivary neurons to guide electrode placement, we injected the bidirectional tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the beta-nucleus and dmcc of the inferior olive. These injections retrogradely labeled neurons in the parasolitary nucleus (Psol) near the vestibular complex. Psol neurons were identified as GABAergic with an antibody to glutamic acid decarboxylase (GAD). In the rat, Psol neurons are small (5-7 microm in diameter) and number approximately 1,800. In the rabbit, they are slightly larger (6-9 microm in diameter) and number approximately 2,200. WGA-HRP injections in conjunction with GAD immunohistochemistry double labeled a high percentage of neurons in both the rat and rabbit Psol. Injection of the orthograde tracer Phaseolus vulgaris-leucoagglutinin into the area of the Psol revealed a projection from this region to both the beta-nucleus and dmcc. Subtotal electrolytic lesions of this division of the Psol caused a substantial reduction in GAD-positive synaptic terminals in both the ipsilateral beta-nucleus and dmcc. The location of these GABAergic neurons, bordering both the nucleus solitarius and caudal vestibular complex, emphasizes the importance of the Psol in the processing of both vestibular and autonomic information pertinent to postural control.

Morphometric analysis of the human vestibular nuclei

BACKGROUND

Cytoarchitectural investigations of the vestibular nuclei have been undertaken in different species of mammals. These data provide a description of the general architecture of the nuclei but limited information about quantitative characteristics of their cell population. We have recently obtained data about the morphometric parameters of the vestibular nuclei neurons in some species. The application of quantitative image analysis techniques to the research of the cellular morphology in the vestibular area of humans might provide basic information to compare with data from animal studies, taking into account the observed correlation between physiological and morphological properties of vestibular neurons.

METHODS

The characteristics of the major vestibular nuclei in humans have been studied with light microscopic techniques in serially cut sections. Camera lucida drawings of the vestibular nuclei and their neurons were made and subjected to computerized image analysis. For each vestibular nucleus, information was obtained about topography, morphological characteristics (i.e., location, volume, and length), and the number and morphometric parameters of their neurons (cross-sectional areas, maximum and minimum diameters). Morphometric data about cell parameters were statistically analyzed by comparing the populations within different parts of each nucleus and from different nuclei.

RESULTS

Among the vestibular nuclei, the medial, which is the largest, has the greatest number of neurons, and the interstitial, the least. The lateral and interstitial nuclei contain the largest cells, and the descending nucleus has the smallest cells. The superior nucleus contains cells of intermediate size. The size of cells decreases in a rostrocaudal direction in the medial, lateral, and descending nuclei, the opposite trend being observed in the superior nucleus. Within the superior and medial nuclei, there are discrete areas with cells with distinctive characteristics.

CONCLUSIONS

These results suggest that, just as most of the anatomical characteristics of the second-order neurons found in animals have been preserved in humans, so the physiological mechanisms observed in the vestibular system of animals should apply to humans.

Rostrocaudal and ventrodorsal change in neuronal cell size in human medial vestibular nucleus

BACKGROUND

The present paper describes the cytoarchitectonic, morphometric, and three-dimensional characteristics of the human medial vestibular nucleus (MVN). We also studied the regional distribution, in size, of the different neurons and its possible relationship with a functional polarization of the different regions of the nucleus.

METHODS

Nine adult human brainstems (30-50 years of age) without neurological problems were used. Specimens were obtained from necropsy and fixed in 4% paraformaldehyde and 5% acetic acid in distilled water. After fixation, blocks were washed, dehydrated, and embedded in paraffin and serial sectioned at 20 microns. Sections were stained with formaldehydethionin, dehydrated, cleared in eucalyptol, and mounted with Eukitt. MVN neurons were drawn with the aid of a camera lucida at 200-micron intervals at 390 x magnification. Serial 50-micron frozen sections were used to determine the volume of the MVN. The three-dimensional reconstruction of MVN was accomplished with a drawing program in a Macinthosh II computer and an AVS on a Stardent workstation computer.

RESULTS

In the three-dimensional reconstruction, the human MVN shows a pyramidal form. The base of this pyramid constitutes the rostral limit, and its vertex forms the caudal border of the MVN. The estimated volume is 30.44 +/- 0.85 mm3, with a neuronal population of 127,737 cells and 4,136 neurons/mm3 in density. The average neuronal cross-section changes from one minimum at caudal level (212.46 +/- 2.04 microns 2) to one maximum at rostral level (491.47 +/- 5.08 microns 2). Four cell types, small (< 200 microns 2), medium (200-500 microns 2), large (500-1000 microns 2), and giant (> 1,000 microns 2) cells, were observed. Medium cells constitute 66%, small cells 18%, and large and giant cells 15% and 1% of the neuronal population.

CONCLUSIONS

The MVN shows a variation in neuronal size, and it has the highest neuronal density of all the human vestibular nuclei. Large cells predominate in rostral regions of the MVN, with significant differences in the area and diameter of the cells among rostral, central, and caudal regions. Furthermore, the largest cells are grouped in the ventrolateral part of the nucleus, close to its boundaries with the inferior and the lateral vestibular nuclei. The morphological polarization, with respect to the neuronal size of the MVN, can be related to a functional polarization of rostral and caudal regions of this nucleus.

Spinovestibular projections in the rat, with particular reference to projections from the central cervical nucleus to the lateral vestibular nucleus

Projections from the spinal cord to the vestibular nuclei were examined following injections of Phaseolus vulgaris-leucoagglutinin, cholera toxin subunit B, or biotinylated dextran at various levels of the spinal cord in the rat. Labeled terminals were abundant after injections of the tracers into the C2 and C3 segments containing the central cervical nucleus. Labeled terminals were seen in the descending vestibular nucleus and the parvocellular, magnocellular, and caudal parts of the medial vestibular nucleus throughout its rostrocaudal extent. Labeled terminals were most numerous in the lateral vestibular nucleus throughout its rostrocaudal extent. The projections from the central cervical nucleus to the vestibular nuclei were exclusively contralateral to the cells of origin because the axons of the central cervical nucleus neurons cross in the spinal cord. Following tracer injections in the cervical enlargement, many labeled terminals were seen in the magnocellular part of the medial vestibular nucleus, but a few were seen in the lateral and the descending vestibular nucleus. Injections into more caudal segments resulted in sporadic terminal labeling in the magnocellular part of the medial vestibular nucleus, the descending vestibular nucleus, and the caudal part of the lateral vestibular nucleus. The results indicate that primary neck afferent input relayed at the central cervical nucleus is mediated directly to the contralateral vestibular nuclei. It is suggested that this projection serves as an important linkage from the upper cervical segments to the lateral vestibulospinal tract in the tonic neck reflex.

Projections of the individual vestibular end-organs in the brain stem of the squirrel monkey

The central nervous system (CNS) projections of primary afferent neurons from individual vestibular receptors were studied using horseradish peroxidase (HRP) or biocytin labeling in 14 ears from 7 adult squirrel monkeys using the technique developed in the chinchilla (Lee et al., 1989, 1992). The specificity of labeling was verified by examining the location of the labeled fibers and cell bodies in the vestibular nerve and Scarpa's ganglion. Labeled fibers and cells were restricted to nerves and areas belonging to groups of cells in either the superior or the inferior ganglion of the vestibular nerve. In the vestibular nerve root, labeled primary afferent fibers also exhibited a receptor-dependent segregation at the entrance to the medulla. Fibers from the HSC and the SSC were found rostrally and those from the PSC and the SAC were found in the caudal area. The UTR fibers were situated intermediate between these two groups of fibers. (A bundle of fibers, probably vestibular efferents, was identified immediately rostrally and ventromedially to the UTR fibers.) The primary afferent fibers bifurcated into secondary ascending and descending fibers at the lateral border of the vestibular nuclei, forming a longitudinal rostrocaudal vestibular tract. The secondary fibers from individual end-organs occupied specific locations in the tract: the UTR fibers were dorsal to the SSC and the HSC fibers, PSC fibers were found most medially, and the SAC fibers occupied the lateralmost area. The secondary UTR fibers overlapped considerably with those of the SSC and the HSC. The orderly receptor-dependent segregation of fibers was more prominent in the descending tracts than in the ascending tracts. In the vestibular nuclei complex the location of the tertiary branches of various end-organs exhibited considerable overlap within the major vestibular nuclei (SN, superior nucleus; LN, lateral nucleus; MN, medial nucleus; DN, descending nucleus). There were still differences, however, in the projection pattern. Fibers from the SAC ran primarily in the lateral area, fibers from the SSC and the UTR were found ventromedially to the SAC fibers, and the HSC projected slightly medially to the fibers from the SSC. The PSC fibers projected most medially. The UTR and SAC sent numerous fibers to the cerebellum. Fibers from the semicircular canals projected through the rostrodorsal region of the SN and presumably also projected to the cerebellum. The precise termination of fibers was evaluated by studying the location of labeled boutons, which were identified in all major vestibular nuclei. Labeled boutons from all the receptors were in the rostral and central areas of the SN, and in the MN mainly in the rostral two-thirds. In the LN, boutons from all the receptors were in the rostroventral part, most of which were from the UTR and SAC. No labeled boutons were in the caudodorsal part of this nucleus. Labeled boutons in the DN primarily surrounded the descending tract fibers and were particularly prominent medially. In specimens in which superior vestibular nerve receptor organs were scratched vestibular efferent fibers were also labeled. These fibers traveled in the most ventral part of the vestibular nerve root and projected in the ventral aspect of the LN to labeled soma in the ipsilateral and contralateral brain stem. Specificity the in projection patterns of efferent fibers from different end-organs could not be ascertained.

Rostrocaudal changes in neuronal cell size in human lateral vestibular nucleus

A cytoarchitectonic and morphometric study of the human lateral vestibular nucleus (LVN) is presented. In sagittal sections, the LVN appears as a triangular cell group rostrally located near the motor trigeminal nucleus and caudally near the vestibular root. The estimated volume is 13.49 mm3 with a neuronal population of 25,046 cells and 1855 neurons/mm3 in density. The average neuronal cross-sectional area changes from a minimum caudally (380.02 +/- 7.23 microns 2) to a maximum rostrally (825.16 +/- 25.10 microns 2). Four types of neurons can be observed: small (< 200 microns 2), medium (200-500 microns 2), large (500-100 microns 2) and giant or Deiter's cells (> 1000 microns 2). The small and medium cells constitute 62%, large cells 26% and the giant cells only 12% of the neuronal population.

Afferent innervation of the vestibular nuclei in the chinchilla. II. Description of the vestibular nerve and nuclei

The morphological characteristics of the vestibular nuclei of the chinchilla were studied in horizontally cut serial sections of the brain stem. Horseradish peroxidase labeling allowed unambiguous delineation of the vestibular nuclei and areas of innervation by the vestibular afferent fibers. The cytoarchitecture of the vestibular nuclei was documented with the aid of camera lucida drawings and quantitatively evaluated with computerized methodology. The cellular groups identified in other species were found in the chinchilla. The superior vestibular nucleus (SN) originated ventromedial to the mesencephalic tract and nuclei of the trigeminal nerve. This nucleus contained medium-sized cells with a central group of larger cells (20-34 microns in diameter). It received its maximum vestibular innervation caudally in the ventrolateral and dorsal aspects of the nucleus. Fibers projected to the SN in bundles with thick fibers surrounded by thin ones. The lateral vestibular nucleus (LN) originated 0.9-1.2 mm below the rostral aspect of the vestibular area. It was ventrocaudal to the SN and contained many large cells with diameters of 45-60 microns. The LN was innervated mainly in the ventrocaudal aspect by oblique and transverse fibers that formed a dense mesh. The medial vestibular nucleus (MN) originated 0.3-0.6 mm caudal to the beginning of the SN, adjacent to the floor of the IVth ventricle. It extended for 3-4 mm along the SN, LN and descending vestibular nucleus (DN). The MN contained the densest and most homogeneous cells, which had diameters of 10-20 microns. This nucleus received its greatest innervation at the level of the vestibular root. Thin fibers traveled to the MN through the SN and LN. The caudal pole of the nucleus did not receive fibers. The DN originated 1.8-2.5 mm caudal to the origination of the SN, between the caudal LN and the MN. Caudally it replaced the LN. Most of the cells of the DVN were medium-sized, with diameters of 10-20 microns. The main vestibular innervation of the DN was in the lateral aspect of the nucleus. Tertiary fibers projected in small, separate bundles of uniform-sized thick fibers. The interstitial nucleus originated 1.1-1.4 mm from the beginning of the SN. It occupied the center of the vestibular root, 0.8-0.9 mm medial to the root entry zone. It contained a few large cells (greater than 20 microns in diameter), many medium-sized cells, and some small cells.(ABSTRACT TRUNCATED AT 400 WORDS)

A direct projection from the medial vestibular nucleus to the cervical spinal dorsal horn of the rat, as demonstrated by anterograde and retrograde tracing

Phaseolus vulgaris leucoagglutinin and wheat germ agglutinin-horseradish peroxidase were iontophoretically injected into different parts of the vestibular nuclear complex (VNC) of the rat. Injections centered into the caudal part of the medial vestibular nucleus revealed a vestibulospinal projection predominantly to the dorsal horn of the cervical spinal cord, besides the expected projection to the intermediate zone (IZ) and ventral horn (VH). While most of the anterogradely labelled fibres could be localized in laminae III to V, some scattered fibres were also seen in laminae I and VI. Lamina II remained free of labelling. The dorsal horn (DH) area with detectable anterograde labelling showed a rostrocaudal extension from C1-C6. Injections into other parts of the VNC labelled fibres and terminals in the IZ and VH while the DH remained almost free of labelling. Additionally, fluorogold and wheat germ agglutinin-horseradish peroxidase were pressure- or iontophoretically injected at different levels into the spinal cord to confirm the projection to the dorsal horn by means of retrograde tracing. Labelled neurons in the area of the medial vestibular nucleus (MVN), from which anterograde labelling in the DH was obtained, were only detectable after fluorogold and wheat germ agglutinin-horseradish peroxidase injections into the cervical spinal cord, in particular its DH. This projection from the caudal medial vestibular nucleus to the dorsal horn of the cervical spinal cord probably enables the VNC to influence sensory processing in the DH, in addition to its well-established influence on posture and locomotion via projections to the intermediate zone and ventral horn.

Primary vestibular projections in the chinchilla

The central projections of fibers from the vestibular nerve were studied in 19 chinchillas after horseradish peroxidase labelling. In addition, the limits of the vestibular nuclei and the anatomical characteristics of their neurons were also studied. All five vestibular nuclei received primary afferents, but there were extensive areas of them that received very little or no projections at all, such as the rostral part of the superior vestibular nucleus, the dorsocaudal part of the lateral vestibular nucleus, the caudal half of the medial vestibular nucleus and the caudalmost aspect of the dorsal vestibular nucleus.

A reinvestigation of the spinovestibular projection in the cat using axonal transport techniques

There are numerous discrepancies within the literature concerning the sources of spinovestibular fibers and their distribution in the vestibular complex. Sources of afferents from all spinal levels were sought using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Following injections of this tracer in all portions of the vestibular complex retrograde labelling was densest at upper cervical levels, especially within the contralateral central cervical nucleus. Labelling was also observed in laminae VI (ipsilaterally), IV, V, VII, and VIII (bilaterally). At progressively more caudal levels, numbers of labelled cells decreased but were similarly distributed in these laminae. The terminal distribution of spinal efferent fibers within the vestibular complex was revealed by injecting wheat germ agglutinin conjugated to horseradish peroxidase or tritiated amino acids into various levels of the spinal cord. These studies showed that all spinal levels project to the descending vestibular nucleus and group x. The f-tail of the descending vestibular complex receives projections from upper cervical and thoracic levels. Terminations within the medial vestibular nucleus arise from both upper cervical and lumbar levels. No conclusive evidence was found supporting the presence of substantial direct spinal projections to the lateral vestibular nucleus, superior vestibular nucleus, or group z. Possible functional roles for the spinovestibular projection in posture and gaze are discussed.

Anatomic connections of inferior parietal cortex (area 7) with subcortical structures related to vestibulo-ocular function in a monkey (Macaca fascicularis)

Connections of the posterior parietal cortex (area 7) with subcortical structures related to the vestibulo-ocular function were studied on four macaque monkeys by using anterograde and retrograde tracer. Wheat germ agglutinin (WGA)-horseradish peroxidase (HRP) or tritiated amino acids were injected into the posterior part of area 7, including the caudal end of the superior bank of both the superior temporal sulcus and the lateral sulcus. The posterior parietal cortex was found to be reciprocally connected with three different ipsilateral thalamic nuclei: the nucleus ventralis posterior inferior, the magnocellular part of the medial geniculate nucleus, and some intralaminar nuclei. Through these connections, area 7 might control the vestibulo-ocular response (VOR) by modulating the ascending vestibular information. This cortical area 7 also projects to the ipsilateral intermediate and deep layers of the superior colliculus and to several ipsilateral pontine nuclei. The dorsolateral pontine nucleus is of particular interest because it is known to be related to smooth pursuit eye movements. Cortical area 7 also was seen to project to the accessory nucleus of Darkschewitsch, to all the vestibular nuclei, and to the nucleus propositus hypoglossi; the last two projections were found to be bilateral with a greater ipsilateral contribution. Efferents from posterior parietal cortex are directed to precise regions within the vestibular nuclei that are specifically involved in vestibulo-ocular reflex, or that are in turn connected with brainstem structures implicated in smooth pursuit eye movements. These connections are consistent with the posterior parietal cortex exerting a multilevel influence on the different systems dealing with eye-head movement coordination.

Comparisons of the immunocytochemical localization of choline acetyltransferase in the vestibular nuclei of the monkey and rat

Immunocytochemical studies of the brainstem were done in the squirrel monkey and rat using the same polyclonal antisera for choline acetyltransferase (ChAT). Cells immunoreactive for ChAT (ChATir) were evident in large numbers in visceral and motor cranial nerve nuclei in both species, but virtually no ChATir cells were seen in the vestibular nuclear complex of the rat. In the monkey ChATir cells were distributed in caudal parts of the medial (MVN) and in dorsal parts of the inferior (IVN) vestibular nuclei. Only a few immunoreactive cells were seen in the rostral MVN and none were found in cell group f of the IVN. Nearly all cells of group z and x, which do not receive primary vestibular afferents, were immunoreactive to ChAT. None of the cells in the superior and lateral vestibular nuclei, cell group y, the infracerebellar nucleus or the interstitial nucleus of the vestibular nerve were immunoreactive for ChAT. Cells immunoreactive to ChAT were present in large numbers in the rostral part of the nucleus prepositus in the monkey, but not in the rat. The relatively small number and distribution of ChATir cells in the MVN suggested they could constitute only a small fraction of the MVN neurons that contribute to a massive commissural system. Significant differences in cholinergic vestibular neurons appear to exist between the rat and the monkey.

Vestibular nuclear complex in the guinea pig: a cytoarchitectonic study and map in three planes

Cytoarchitectonic and fiberarchitectonic criteria were used in the preparation of a detailed map illustrating the vestibular nuclear complex of the guinea pig. The brainstems used for this study were serially cut at 16 micron in the transverse, the sagittal, or the horizontal plane. The sections were studied after being stained alternately with a combined cell and fiber staining method and a Nissl stain. The basic cytoarchitectonic features of the four main vestibular nuclei, their extent, as well as their relationship to the surrounding structures are described. Additionally, the location, topographical features, and the cytoarchitecture of the small groups (f,g,l,x,y,z) associated with the vestibular nuclei are reported. Group f is especially well developed and easily distinguishable in the guinea pig. Furthermore, a hitherto undescribed cell cluster found dorsal to the dorsal border of the superior vestibular nucleus is presented. The results and especially the differences from the descriptions of other species are discussed.

Cytoarchitecture and saccular innervation of nucleus y in the mouse

The cytoarchitecture and saccular innervation of the mouse nucleus y were investigated by using Golgi, Nissl, and myelin stains and anterograde axonal transport of horseradish peroxidase. Nucleus y was found to be a compact group of cells in a small fiber-free region dorsal to the restiform body. Qualitative and morphometric analyses showed that most (75%) of the nucleus y neurons could not be reliably subdivided into morphologic subgroups, but varied continuously in soma size (15-25 microns), shape (fusiform to stellate), and number of dendrites (two to four), and had sparsely branched dendrites with an average of 3 to 4 spines per 10 microns of length. Three groups of cells that were identified morphometrically accounted for 10% (type I: large stellate cells), 9% (type II: long-dendrite cells), and 6% (type III: elongated soma cells) of the y neurons. Vestibular nerve axons transporting horseradish peroxidase after injury at their origin in the saccular neuroepithelium were found to form a dense terminal meshwork that was virtually co-extensive with the cytoarchitectonic boundaries of nucleus y. Nucleus y was distinguished from the overlying infracerebellar nucleus on the basis of anatomical, cytoarchitectural, and hodological features.

Observations on the secondary vestibulocerebellar projections in the macaque monkey

The distribution of retrogradely labeled cells in the nuclei of the vestibular nuclear complex following injections of horseradish peroxidase in various parts of the cerebellar cortex (except the nodulus and paraflocculus) has been mapped in the macacus rhesus monkey. In the main the findings correspond to those made in other mammalian species (cf. Table 1). The flocculus receives afferents bilaterally from the superior, medial and descending vestibular nucleus, group y, the interstitial nucleus of the vestibular nerve and also from the abducent nucleus. The projection to the posterior vermis (lobules VIII and IX), especially to lobule IX, is more abundant than that to lobules VI-VII. The projection to the anterior lobe vermis appears to be modest. Evidence for projections to the cerebellar hemispheres was not obtained. Whether the lateral vestibular nucleus projects to the cerebellum in the macaque is uncertain. The regular occurrence of weakly labeled cells among heavily labeled ones suggests that many of the cerebellar projecting cells may have axonal branches passing to other destinations. The findings lend support to the notion that there are precise topical relations within the entire secondary vestibulocerebellar projection. For example, in the medial nucleus the sites of origin of fibers to the flocculus and uvula are different. Surprisingly, many cells in group z were found to project to the uvula and - to a lesser extent - to lobule VIII. The group z may, therefore, not be a pure relay nucleus in a spinothalamic pathway, as generally assumed. The rather marked cerebellar projection of the abducent nucleus, especially to the flocculus, is of interest for the analysis of cerebellar control of eye movements in the macaque.

The vestibular nuclei in the macaque monkey

The topography and the main features of the cytoarchitecture of the vestibular nuclear complex in the macaque monkey have been studied in serially cut, Nissl-stained, transverse, parasagittal and horizontal sections. In addition to the four main vestibular nuclei, the topographically closely related small cell groups (f, l, x, y, and z), distinguished by Brodal and Pompeiano ('57) in the cat, have been considered and illustrated. The vestibular nuclear complex in the macaque in general corresponds in topography and architecture to the situation described in some other mammals on which information is available, such as opossum, rabbit, cat, Galago, and man. Some dissimilarities in detail are found. For example, in man the lateral vestibular nucleus differs somewhat from the general pattern, especially in its position, and the small group f, fusing with the descending nucleus, appears to be indistinct; likewise the group y. The latter and the group z appear to be particularly well developed and easily distinguished in the macaque. The question of whether cytoarchitectonic areal differences within the vestibular nuclear complex can be correlated with differences in connections is discussed. Also in this respect there appears to be a general similarity between observations in the macaque and in other mammals. A correlation is most evident in the superior vestibular nucleus, and is rather clear in the medial and lateral vestibular nuclei and for the groups f,x,y, and z, whereas no such correlation can be found in the descending (inferior) nucleus. For several reasons it is difficult to draw reliable conclusions about comparative anatomical trends in the phylogenesis of the vestibular nuclear complex.

Distribution of primary vestibular fibers in the brainstem and cerebellum of the monkey

Attempts were made to determine the central projections of ganglion cells innervating individual semicircular ducts in the monkey by implanting or injecting tritiated amino acids (leucine and/or proline), or horseradish peroxidase (HRP), selectively into a single ampulla. Central transport via the vestibular ganglion in animals receiving isotope implants or injections fell into three categories: (1) transport from ganglion cells innervating all receptive elements of the labyrinth, (2) transport from ganglion cells innervating the three semicircular ducts, and (3) transport from cells of the inferior vestibular ganglion innervating the posterior semicircular duct. Transneuronal transport of isotope was observed in secondary vestibular fibers in animals where proline was used and survival exceeded 12 days. Transneuronal labeling of secondary auditory fibers was independent on the [3H]amino acid used, and occurred with survivals of 10 or more days. HRP implanted into the ampulla of the lateral semicircular duct in several animals produced retrograde transport to efferent vestibular and cochlear neurons, but did not result in transganglionic labeling of primary vestibular or auditory fibers. Primary vestibular fibers terminate throughout the superior (SVN) and medial vestibular nuclei (MVN). Within SVN, terminals are most pronounced in its central large-celled portion, but extend into peripheral parts of the nucleus, except for a small medial area near its junction with the oral pole of MVN. Primary projections to MVN are homogenously distributed throughout the nucleus excepting a small circular area of sparse terminals along its ventral margin. Primary vestibular afferents terminate mainly in rostral and caudal portions of the inferior vestibular nucleus (IVN), but do not reach cell group 'f'. Projections to the lateral vestibular nucleus (LVN) are restricted to its ventral part. Primary projections to the accessory vestibular nuclei reach the interstitial nucleus of the vestibular nerve (NIVN) and cell group 'y'. Fibers project beyond the vestibular nuclei (VN) to terminate ipsilaterally in the accessory cuneate nucleus (ACN), the subtrigeminal lateral reticular nucleus (SLRN), and well-defined portions of the reticular formation (RF). Projections to SVN and MVN are derived primarily from ganglion cells innervating the semicircular ducts, while projections to caudal IVN, cell group 'y' and ACN are related mainly to macular portions of the vestibular ganglion. NIVN receives both macular and duct afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for thalamic projections from external cuneate nucleus, cell groups Z and X, and the mesencephalic nucleus of the trigeminal nerve in squirrel monkey

Horseradish peroxidase (HRP) was injected into the thalamic ventral tier of squirrel monkey and the brainstem was examined for the presence of HRP-reactive cells in the external cuneate nucleus (ECN), nuclei Z and X, and the mesencephalic nucleus of the trigeminal nerve (MNV). Results indicate that the ECN, Z and X have a direct projection to contralateral ventral tier. Thalamic projecting cells in the ECN are located predominantly in the medial part of this nucleus. The MNV projects bilaterally to the ventral tier but predominantly to the ipsilateral side. Consequently, the present report establishes evidence that the above nuclei, which are known to be extensively interconnected with cerebellum, spinal cord and/or other brainstem nuclei, also have a thalamic projection in a New World primate, (Saimiri sciureus).