Vestibular Nerve

Introduction

The vestibulocochlear nerve is primarily composed of the centrally directed axons of bipolar neurons housed in the petrous portion of the temporal bone ( Fig. 19.1 ). The peripheral processes project to neuroepithelial cells in the vestibular labyrinth and cochlea. The nerve enters the brainstem at the junctional region of the pons and medulla oblongata.

Vestibular System

The bony labyrinth of the inner ear is a very dense bony shell containing perilymph , which resembles extracellular fluid in general. The perilymph provides a water jacket for the membranous labyrinth , which encloses the sense organs of balance and of hearing. The sense organs are bathed in endolymph . The endolymph resembles intracellular fluid, being potassium rich and sodium poor.

The vestibular labyrinth comprises the utricle , the saccule , and three semicircular ducts ( Fig. 19.2 ). The utricle and saccule contain a 3 × 2-mm ² macula . Each semicircular duct contains an ampulla at one end, and the ampulla houses a crista. (It should be pointed out that clinicians commonly speak of ‘canals’ where ‘ducts’ would be strictly more appropriate.)

The two maculae are the sensory organs of the static labyrinth , which signals head position. The three cristae are the organs of the kinetic or dynamic labyrinth , which signals head movement.

The bipolar cells of the vestibular (Scarpa) ganglion occupy the internal acoustic meatus. Their peripheral processes are applied to the five sensory end organs. Their central processes, which constitute the vestibular nerve , cross the subarachnoid space and synapse in the vestibular nuclei previously seen in Figure 17.14, Figure 17.15 .