Transduction and adaptation in sensory hair cells of the mammalian vestibular system.

The human vestibular apparatus detects head movements and gravitational stimuli which impinge upon the mechanosensory hair cells of the inner ear. The hair cells, in turn, transduce these stimuli into electrical signals which are transmitted to the brain. These sensory cells are exquisitely responsive, signaling deflections of their mechanosensitive organelles as small as 1-2 nanometers. Remarkably, they are able to preserve this level of sensitivity even when confronted with large tonic stimuli, such as gravity. To accomplish this feat hair cells have devised a novel adaptation process that repositions the mechanotransduction apparatus on a millisecond time scale to allow high sensitivity over a broad operating range. Mechanotransduction in hair cells occurs via a direct gating mechanism in which hair bundle deflection focuses tension onto membrane-bound, cation-selective ion channels located near the tips of the hair bundle. Increased tension favors an open conformation of the channel and allows calcium to enter the cell. Elevated intracellular calcium promotes adaptation which has been hypothesized to result from the activity of a cluster of molecular motors that continually adjust the tension in the transduction apparatus. Although the transduction channel itself remains elusive, myosin Ic has recently been identified as a molecular component of the "adaptation" motor.