FIGURE 25.9 • Electron micrographs of the kinocilium and stereocilia of a vestibular sensory hair cell. a. Scanning electron micrograph of the apical surface of a sensory hair cell from the macula of the utricle. Note the relationship of the kinocilium (K) to the stereocilia (S). 47,500. (Reprinted with permission from Rzadzinska AK, Schneider ME, Davies C, Riordan GP, Kachar B. An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal. J Cell Biol 2004;164:887-897.) b. Transmission electron micrograph of the kinocilium (K) and stereocilia (S) of a vestibular hair cell in cross section. The kinocilium has a larger diameter than the stereocilia. 47,500. (Reprinted with permission from Hunter-Duvar IM, Hinojosa R. Vestibule: sensory epithelia. In: Friedmann I, Ballantyne J, eds. Ultrastructural Atlas of the Inner Ear. London: Butterworth, 1984.) occurs in stereocilia that are deflected toward its tallest edge (toward the kinocilium if present). This movement exerts tension on the fibrillar tip links, and the generated force is used to open mechanically gated ion channels near the tip of the stereocilium. This allows for an influx of K, causing depolarization of the receptor cell. This depolarization results in the opening of voltage-gated Ca2 channels in the basolateral surface of the hair cells and the secretion of a neurotransmitter that generates an action potential in afferent nerve endings. Movement in the opposite direction (away from the kinocilium) closes the MET channels, causing hyperpolarization of the receptor cell. The means by which stereocilia are deflected varies from receptor to receptor; these are discussed in the sections describing each receptor area.
Two types of hair cells are present in the vestibular labyrinth.
Both hair cell types are associated with afferent and efferent nerve endings (Fig. 25.10). Type I hair cells are flask-shaped, with a rounded base and thin neck, and are surrounded by an afferent nerve chalice and a few efferent nerve fibers. Type II hair cells are cylindrical and have afferent and efferent bouton nerve endings at the base of the cell (see Fig. 25.10).
Sensory Receptors of the Membranous Labyrinth
Cristae ampullaris are sensors of angular movements of the head.
Each ampulla of the semicircular duct contains a crista ampullaris, which is a sensory receptor for angular movements of the head (see Figs. 25.8c and 25.11). The crista ampullaris is a thickened transverse epithelial ridge that is oriented perpendicularly to the long axis of the semicircular canal and consists of the epithelial hair cells and supporting cells (Plate 108, page 946).
A gelatinous protein–polysaccharide mass, known as the cupula, is attached to the hair cells of each crista (see Fig. 25.11). The cupula projects into the lumen and is surrounded by endolymph. During rotational movement of the head, the walls of the semicircular canal and the membranous semicircular ducts move, but the endolymph contained within the ducts tends to lag behind because of inertia. The cupula, projecting into the endolymph, is swayed by the