Introduction

The vestibulocochlear nerve is primarily composed of the centrally directed axons of bipolar neurons housed in the petrous temporal bone (Figure 19.1). The peripheral processes are applied to neuroepithelial cells in the vestibular labyrinth and cochlea. The nerve enters the brainstem at the junctional region of pons and medulla oblongata. The functional anatomy of the vestibular division of the nerve is described in this chapter.

Figure 19.1 Bony labyrinth, viewed from above.

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 the cristae within three semicircular ducts (Figure 19.2). The utricle and saccule contain a 3 × 2 mm² 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.)

Figure 19.2 Locations of the five vestibular sense organs.

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

The bipolar cells of the vestibular (Scarpa’s) 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 Figures 17.14 and 17.15.

Static labyrinth: anatomy and actions

The position and structure of the maculae are shown in Figure 19.3. The utricular macula is relatively horizontal, the saccular macula is relatively vertical. The cuboidal cells lining the membranous labyrinth become columnar supporting cells in the maculae. Among the supporting cells are so-called hair cells, to which vestibular nerve endings are applied. Some hair cells are almost completely enclosed by large nerve endings, whereas others (phylogenetically older) receive only small contacts. At the cell bases are ribbon synapses, the synaptic vesicles being lined up along synaptic bars. Projecting from the free surface of each hair cell are about 100 stereocilia and, close to the cell margin, a single, long kinocilium. The hair cells discharge continuously, the resting rate being about 100 Hz.

Figure 19.3 Static labyrinth.

The cilia are embedded in a gelatinous matrix containing protein-bound calcium carbonate crystals called otoconia (‘ear sand’). (The term ‘otoliths’, when used, refers to the larger, ‘ear stones’ of reptiles.) The otoconia exert gravitational drag on the hair cells. Whenever kinocilia are dragged away from stereocilia, depolarization is facilitated. The macula has a central groove (striola) and the hair cell orientations have a mirror arrangement in relation to the groove. Electrical activity of hair cells is facilitated on one side of the groove by a given gravitational vector, and disfacilitated on the other side.

The maculae also respond to linear acceleration of the head in the horizontal plane (e.g. during walking) or in the vertical (gravitational) plane. Also, when the tilted head is stationary in a flexed or extended position, the facilitated half of the utricular macula discharges intensely in both ears. The saccular ones are more responsive when the head is held to the side.

The primary function of the static labyrinth is to signal the position of the head relative to the trunk. In response to this signal, the vestibular nuclei initiate compensatory movements, with the effect of maintaining the center of gravity between the feet (in standing) or just in front of the feet (during locomotion), and of keeping the head horizontal. These effects are mediated by the vestibulospinal tracts.

The lateral vestibulospinal tract, seen earlier in sections of medulla oblongata in Chapter 17, arises from large neurons in the lateral vestibular nucleus (of Deiters). The fibers descend in the anterior funiculus on the same side of the spinal cord and synapse upon extensor (antigravity) motor neurons. Both α and γ motor neurons are excited, and a significant part of the increased muscle tone is exerted by way of the gamma loop (Ch. 16). During standing, the tract is tonically active on both sides of the spinal cord. During walking, activity is selective for the quadriceps motor neurons of the leading leg; this commences following heel strike and continues during the stance phase (when the other leg is off the ground). Deiters’ nucleus is somatotopically organized, and the functionally appropriate neurons are selected by the flocculonodular lobe of the cerebellum. The flocculonodular lobe (Ch. 25) has two-way connections with all four vestibular nuclei.

Antigravity action is triggered mainly from the horizontal macula of the utricle. The vertical macula of the saccule, on the other hand, is maximally activated by a free fall. The shearing effect on the macula produces a powerful extensor thrust in anticipation of a hard landing.

A small, medial vestibulospinal tract

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