Hair Cells of the Inner Ear

A scanning electron micrograph of a hair bundle protruding abouit 10 µm from the apical surface of a hair cell in the bullfrog's sacculus. In this instance, the bundle includes some 60 stereocilia, each of which contains a core of cross-linked actin filaments. At the tall edge of the bundle stands the kinocilium, which is endowed with a spherical swelling at its tip. The supporting cells that surround the hair cell are studded with microvilli.

A hair cell is an epithelial cell derived from the embryonic ectoderm. Its apical surface is characterized by the hair bundle, a cluster of 20-300 mechanoreceptive "feelers" termed stereocilia. At the tall edge of the beveled hair bundle stands a single kinocilium, a microtubule-containing true cilium. When the cell experiences an auditory or accelerational stimulus, deflection of the hair bundle is transduced into an electrical response, the receptor potential, across the cell's plasma membrane. Our earlier experiments have shown that transduction involves the direct opening of cation-selective ion channels as a result of mechanical stimuli applied to the hair bundle. A positive stimulus, which moves the bundle towards its kinocilum-bearing tall edge, depolarizes the membrane. This response is then transmitted to an afferent fiber of the eighth cranial nerve by synapses that stud the cell's basolateral membrane surface. The resultant signal travels to the brain, where it is interpreted as a sound or an acceleration.