Multiple myosin motors and mechanoelectrical transduction by hair cells.

Hair cells are exquisitely specialized mechanoreceptors, responding only to specific frequencies of sound or to distinct head movements (reviewed in Hudspeth, 1989, 1992). A hair cell carries out mechanoelectrical transduction with its mechanically sensitive hair bundle, a beveled collection of stereocilia and one solitary kinocilium. Although the kinocilium is a true cilium, with the familiar 9 + 2 arrangement of microtubule doublets, stereocilia are actin-based. Stereocilia contain several hundred cross-linked actin filaments, so they are quite rigid. A stereocilium pivots at its flexible basal insertion point, where only a few dozen actin filaments penetrate the cell. Deflection of the bundle causes adjacent stereocilia to slide along each other, stretching elastic gating springs that tug open transduction channels located at the top of the bundle. Hair cells adapt to sustained stimuli: for example, hair cells in the bullfrog’s sacculus can detect transient vertical accelerations of less than lop5 g, despite a constant 1 g stimulus from gravity (Koyama et al., 1982). Gatingspring tension, and hence channel open probability, is controlled by adaptation motors, which likely contain myosin molecules (reviewed in Hudspeth and Gillespie, 1994; Gillespie, 1995). During an excitatory stimulus, when gating-spring tension is high, Ca2+ entering through open transduction channels triggers the adaptation motors to slip down the cytoskeleton and reduce tension (Fig. 1). By contrast, during inhibitory bundle