Physiology of the Sense of Equilibrium
Receptors of the equilibrium organ, and the stimuli that excite them. Within the temporal bone, next to the cochlea, is the vestibular organ — a transducer intimately involved in the maintenance of equilibrium. The relative orientation of these parts of the labyrinth was shown in Figure 5-1 (cf. also Fig. 6-5). The vestibular organ is phylogenetically related to the ear, particularly with respect to the receptors of the two systems. In both cases, the receptors are hair cells. The adequate stimulus of the hair cells in the organ of Corti, as described in Chapter 5, is shearing of the cilia. This is also true in the vestibular organ, but here the shearing forces arise in a different way. Figure 6-1 illustrates the operation of this mechanism in one kind of sensory subunit of the vestibular organ, the macula. The cilia of the hair cells project into a gelatinous mass in which small granules of high specific gravity are embedded. The mass and its contents together are called the otolith membrane. There is a stepwise gradation in length of the cilia, the largest (called the kinocilium) being differently constructed than the rest. This differentiation distinguishes these hair cells from those of the cochlea, which lack a kinocilium and have only stereocilia.
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