Vestibular Receptors

The vestibular labyrinth receives dual innervation. The distal axonal processes of the bipolar vestibular afferent neurons have cell bodies housed in the vestibular ganglion of the internal acoustic meatus. The afferent axons terminate on the mechanoreceptive vestibular hair cells that serve as the sensory transducers. The vestibular efferent fibers originate in the brainstem.

Hair cells are specialized epithelial cells that have ciliary tufts protruding from their apical surface. Type I cells are goblet shaped and are enclosed in a nerve chalice. Synaptic terminals packed with vesicles are in contact with the base of the chalice and are likely presynaptic terminals. Type II hair cells are more common and have small terminal synaptic boutons. They are innervated by thin nerve branches that form synaptic contact with the bottom of the cell. The efferent endings are presynaptic to the hair cell and filled with vesicles. Type I hair cells are thought to be more sensitive than those of type II. Efferent fibers form typical chemical synapses with hair cells or with afferent terminals, which act to increase the discharge rate of afferent fibers and to modulate their response to mechanical stimuli.

The apical ends of both types of hair cells bear a tuft of 40 or more sensory hairs, or stereocilia, whose bases are embedded in a stiff cuticle, and a single, lower kinocilium, which originates from a basal body and has a structure similar to that of a motile cilium. The entire group of hairs is joined together at its free end. The stimulus for the sensory hair cells is shearing displacement of the hair cells. Displacement of the sensory hair bundle in the direction in of the kinocilium is excitatory and results in depolarization of the hair cell and increased firing of the vestibular nerve fibers. In the opposite direction, the response is inhibitory and results in hyperpolarization of the hair cell and reduced firing of the vestibular nerve. Signal transduction in hair cells occurs via a direct gating mechanism in which the hair bundle deflection puts tension on membrane-bound, cation-selective ion channels located near the tip of the hair bundle. This increased tension opens the channel and allows calcium to enter the cell. The increased intracellular calcium promotes adaptation, which may activate molecular motors that adjust the tension of the transduction apparatus.

The cristae and maculae are especially sensitive to angular and linear acceleration and convert head movements to bending forces on the sensory hairs. The hair cells, the mechanoreceptors in the cristae, are embedded in a gelatinous mass called the cupula, which extends across the ampulla. During angular acceleration, there is displacement of the cupula and resultant bending of the sensory hairs. Because all hair cells in the cristae are oriented in the same direction as their kinocilia, this bending either increases or decreases the discharge rate of all the afferent fibers.

The hairs of the sensory cells found in the maculae of the saccule and utricle are embedded in a gelatinous otolithic membrane, which contains concretions of calcium carbonate called otoconia or otoliths. Because the otoconia are denser than the surrounding fluid, the otolithic membrane tends to move under the influence of linear acceleration. For instance, when the normally horizontal utricular macula is tilted, the pull of gravity tends to make the otolithic membrane slide downward, thus bending the sensory hairs. Because the macula contains hair cells that have two different orientations, this bending increases the discharge rate of some utricular afferent fibers and slows the discharge rate of others. These signals are analyzed by the central nervous system (CNS) for information on the position of the head. The macula of the saccule is in a vertical position and is therefore sensitive to vertical acceleration. The saccule may also contribute to the sensing of head position when the head is oriented with one ear down.

The vestibulospinal tracts are discussed in the spinal cord section.

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