J. Anat. (1979), 129, 1, pp. 107-116

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The lateral vestibular nucleus of the rat, with a note on its dorsal extension into the cerebellujm K. M. SHAMBOUL

Anatomy Department, Faculty of Medicine, University of Khartoum

(Accepted 28 June 1978) INTRODUCTION

The vestibular nuclear complex in vertebrates lies deep to the pontomedullary junction and receives terminals of primary afferents from the vestibular ganglion. The complex has been investigated to some extent in monkeys and various other mammals (Fraser, 1901; Mettler, 1944; McMaster, 1966; Tarlov, 1969), but most extensively in cats (e.g. Brodal & Pompeiano, 1957; Carpenter, Brittin & Pines, 1958; Massopust, 1960; Nyberg-Hansen and Mascitti, 1964; Mugnaini, Walberg and Haugli-Hansen, 1967). Most of these workers have divided the mammalian vestibular complex into four main nuclei: medial, lateral, superior and inferior. The lateral vestibular nucleus (LVN) has been defined as that part of the vestibular nuclear complex which contains the 'giant' multipolar neurons of Deiters (1865). It is bounded craniocaudally by the entering fibres of the vestibular nerve as they ascend and descend in the brain stem (Gray, 1928). In man, according to Olszewski & Baxter (1954), this nucleus extends rostrally from the level of the cranial pole of the inferior olive to the level of the caudal pole of the abducent and the principal trigeminal nuclei. It is limited medially by the medial vestibular nucleus and laterally by the inferior cerebellar peduncle, turning dorsally to enter the medullary substance of the cerebellum (Brodal & Angant, 1967). The cytoarchitecture and somatotopical features of the lateral vestibular nucleus in the rat have not been studied. The present work is a cytoarchitectonic study of this nucleus in rats based on Nissl-stained preparations. The somatotopical features of the nucleus will be dealt with in subsequent publications. MATERIAL AND METHODS

Twelve adult male albino rats of Wistar strain (175-250 g body weight) and 8 neonates (6-12 days old) were used. The animals were anaesthetized with ether, the chest wall was opened, and a plastic cannula was introduced into the aorta through a puncture in the left ventricle. While the heart was still beating the animal was perfused with 150-250 ml of warm 0 9 % saline followed by 10 % formalin in 2 % acetic acid (maintained at 37 °C and injected at a perfusion pressure of 100-120 mm Hg). After perfusion the brain was dissected out and kept for 2 weeks in acidified formalin. The brain stem, with the intact cerebellum, was removed, dehydrated in ascending grades of alcohol, cleared in cedar-wood oil and toluene, and embedded in paraffin wax. 12,m transverse (coronal) and longitudinal (horizontal) sections were cut serially and stained with 0 5 % aqueous cresyl fast violet (CFV) for 3-5 minutes. Some sections were impregnated with silver nitrate (0-2 %) and counterstained with cresyl fast violet to demonstrate nerve fibres and chromatin granules respectively

(Fink-Heimer, 1967). 0021-8782/79/2828-6170 $02.00 © 1979 Anat Soc. G.B. & I.

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In combined Nissl and fibre-stained brain stem preparations the lateral vestibular nucleus was seen in the dorsolateral region of the cranial part of the medulla and the caudal part of the pons. It was in the angle formed by the diverging ascending and descending branches (open arrows) of the vestibular nerve (closed arrow, Fig. 1). Its caudal pole was related medially to the medial vestibular nucleus, which was replaced more rostrally by the genu of the facial nerve. Laterally it was related throughout its caudocranial extent to the descending fibres of the vestibular nerve.

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These fibres divided the nucleus into a large medial part of large multipolar neurons, and a smaller lateral zone containing medium-sized neurons. The nucleus was limited further laterally by the inferior cerebellar peduncle. Caudally it was continuous with the cranial pole of the inferior vestibular nucleus. The cells of the latter were arranged into V-shaped medial and lateral wings embracing the caudal pole of the lateral vestibular nucieus. At mid-levels of its caudocranial axis the dorsal aspect of the lateral vestibular nucleus extended medial to the inferior cerebellar peduncle into the cerebellum between the dentate nucleus interpositus (Figs. 2, 3). This extension was noticed throughout the middle and cranial thirds of the nucleus. It receded, however, towards the rostral pole where lateral vestibular neurons were seen in the dorsal part of the pons lateral to the facial nerve (Fig. 3). The dorsal extension was particularly noticeable in neonatal rats, in which the lateral vestibular neurons formed a well demarcated group in the ventral part of the cerebellum (Fig. 4). Such an observation was also made at even more caudal levels, before the inferior cerebellar peduncle started to spread into the cerebellum (Fig. 5). The rat's lateral vestibular nucleus contained two main types of neurons: (1) large multipolar neurons (50-60 ,um) predominantly found in the dorsolateral part of the nucleus; (2) small spherical or spindle neurons, placed ventromedially (Fig. 1). Cells intermediate in size, having multipolar or fusion shape, were also present throughout the nucleus. However, such cells were more numerous ventrally. The large multipolar neurons were not peculiar to the dorsal part of the lateral vestibular nucleus but were also present in the cranial pole of the inferior vestibular nucleus, thus making it difficult to differentiate these two nuclei (Fig. 6). In contrast, the cranial

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The subdivision of the vestibular nuclear complex is controversial because of difficulties in drawing clear boundaries between adjacent neuronal populations. The lateral vestibular nucleus of the rat is intimately related to, and almost inseparable from, adjacent vestibular nuclei. This is similar to the situation observed by Brodal & Pompeiano (1957) and McMaster (1966) in cats and monkeys respectively. The intermingling of adjacent vestibular neurons is particularly noticeable in the caudal pole of the lateral vestibular nucleus and the cranial part of the inferior vestibular nucleus, where large multipolar neurons are in juxtaposition. This has led some workers to subdivide the nucleus into sub-units. Thus sub-nucleus 'L' was defined by Nyberg-Hansen and Mascitti (1964) as a group of neurons belonging to, and lying in the lateral part of, the cat's lateral vestibular nucleus. In the present investigation such a sub-nucleus was not found. The rat's lateral vestibular nucleus does not differ significantly from that in other mammals as regards its shape and position. The nucleus is a pear-shaped structure having similar relations to those in the cat and monkey. However, in the rat it has been possible to divide the nucleus into a dorsal part composed of large multipolar neurons, and a ventral part made up of intermediate and small neurons. The dorsal extension of the lateral vestibular nucleus into the ventral part of the cerebellum has not been described before. This part demonstrates an intense degree of retrograde response following cordotomies as low as lumbar segments of the spinal cord (Shamboul, 1977). Weston (1936) described, in reptiles, a nucleus vestibularis dorsolateralis as that part of the vestibular nuclear complex lying between

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the nucleus ventralis (the homologue of the lateral vestibular nucleus in mammals) and the cerebellar roof nuclei. It could be surmised that the dorsal part of the lateral vestibular nucleus constitutes in reptiles, rats, and probably in other lower mammals, a neuronal complex continuous with the cerebellar roof nuclei. The components of such a complex are intimately related phylogenetically, structurally and functionally. It seems that in higher mammals this part of the nucleus has diminished in size consequent upon the development and overgrowth of the neocerebellum and its involvement in the regulation of posture. Neurophysiological studies have defined different types of neurons in the lateral vestibular nucleus of cats. Of these, Type I and Type II are prevalent in the dorsal and ventral parts of the nucleus respectively (Precht, Gripo & Wagner, 1967). The former cells demonstrated rapid responses with short latencies, while the latter showed slow responses on antidromic spinal cord stimulation. Since such spike characteristics are correlated with axon diameter and perikaryal size (Allen, Sabah & Toyama, 1971), it would seem that Type I and Type II cells correspond to the large multipolar and the small neurons demonstrated histologically in this study. SUMMARY

The lateral vestibular nucleus of the rat has been studied in Nissl preparations: It is similar, in shape and position, to its counterpart in cats and monkeys. A dorsal extension of the nucleus into the cerebellum has been described for the first time. Two types of neurons, large multipolar and small, have been demonstrated histologically, and their significance correlated with available neurophysiological information.

This work was done as a part of a Ph.D. thesis in the University of London, under the supervision of Professors R. Warwick and P. L. Williams. I am deeply indebted to them for their enthusiastic criticism and unfailing help. I am also grateful to the British Council and the University of Khartoum for financial support, and to Miss Ragaa Sami for typing the manuscript. REFERENCES

ALLEN, G. I., SABAH, N. H. & TOYAMA, H. (1971). Effects of forelimb and hindlimb nerve stimulation on Deiters' neurons. Brain Research 25, 645-650. BRODAL, A. & POMPEIANO, 0. (1957) The vestibular nuclei in the cat. Journal of Anatomy 91. 438-454. BRODAL, A. & ANGANT, P. (1967). The termination of spinovestibular fibres in the cat. Brain Research 5, 494-500. CARPENTER, M. B., BRITrN, G. M. & PiNEs, J. (1958). Isolated lesions of the fastigial nuclei in the cat. Journal of Comparative Neurology 109, 65-90. DEITERS, 0. (1865). Untersuchungen uiber Gehirn und Ruckenmark des Menschen und der Saugetiere. In Nervous System of Vertebrates Including Man, vol. i, (ed. Braunschweig), p. 511. New York: Macmillan. FRASER, E. H. (1901). An experimental research into the posterior longitudinal bundle and Deiters' nucleus. Journal of Physiology 27, 372-397. FINK, R. P. & HEIMER, J. (1967). Two methods for selective impregnation of degenerating axons and their synaptic endings in the central nervous system. Brain Research 4, 369-374. GRAY, L. P. (1928). Some experimental evidence on the connections of the vestibular mechanisms in the cat. Journal of Comparative Neurology 41, 319, 364-369. MASSOPUST, L. C. (1960). Some descending vestibular projections in the medulla and spinal cord. Neurology 10, 697-704.

K. M. SHAMBOUL 116 McMAsTER, R. E. (1966). Vestibular projection to nuclei of extraocular muscles. American Journal of Anatomy 118, 163-194. METTLER, F. A. (1944). Physiologic consequences and anatomic degeneration following lesions of primate brain stem. Journal of Comparative Neurology 80, 69-148. MUGNAINI, E., WALBERG, F. & HAUGLI-HANSEN, E. (1967). Observation on the fine structure of the lateral vestibular nucleus. Experimental Brain Research 4, 146-186. NYBERG-HANSEN, R. & MASCITTI, T. (1964). Sites and mode of termination of fibres of the vestibulospinal tract in the cat. An experimental study with silver impregnation methods. Journal of Comparative Neurology 122, 369-387. OLSZEWSXI, J. & BAXTER, D. (1954). Cytoarchitecture of the Human Brain Stem. Atlas, pp. 74-78. New York: Lippincott Co. PRECHT, W., GRIPO, J. & WAGNER, A. (1967). Contribution of different types of central vestibular neurons to the vestibulospinal system. Brain Research 4, 199-223. SHAMBOUL, K. M. (1977). Vestibulospinal and spinovestibular connexions of the lateral vestibular nucleus in rats. Ph.D. Thesis, University of London. TARLOV, E. (1969). The rostral projections of primate vestibular nuclei. An experimental study in macaque, baboon and chimpanzee. Journal of Comparative Neurology 135, 27-56. WESTON, J. K. (1936). The reptilian vestibular and cerebellar gray with fibre connections. Journal of Comparative Neurology 65, 93-200.