Central vestibular system: vestibular nuclei and posterior cerebellum

Abstract

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between “active” and “passive” movements.

The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and β-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

Introduction

The vestibular nerve branches as it enters the brain stem into two fiber bundles containing axons of unequal thickness (Fig. 1) [61]. The bundle containing thicker axons enters the medulla between the ventral aspect of the inferior cerebellar peduncle and the dorsal aspect of the spinal tract of the trigeminal nucleus. It turns caudally and then passes into the vestibular complex. The bundle with thinner axons ascends to the cerebellum by passing through the superior vestibular and lateral vestibular nuclei (SVN, LVN). The thin axons then distribute to several folia within the cerebellum, but primarily to the ipsilateral uvula-nodulus. The distribution of vestibular primary afferents within the cerebellum is the subject of a chapter in this volume by Newlands and Perachio. The vestibular complex and uvula-nodulus are responsible for the initial processing of vestibular information by the central nervous system. Their separate and combined contributions are the subject of this review.