Transduction and tuning by vertebrate hair cells

The internal ear, or labyrinth, displays extraordinary sensitivity: an acoustical signal at the human auditory threshold or a just-perceptible acceleration imparts to the ear less than an attowatt of power, the power presented to an eye by the absorption of one photon per second. The detectors for such miniscule stimuli are hair cells, specialized mechanoreceptors that transduce inputs into membrane-potential changes. In addition to endowing animals with great sensitivity to sounds, vibrations and accelerations, the organs of the internal ear are selectively responsive to inputs of behavioral importance to each species. The hair cells of the vestibular apparatus, for example, are sensitive to accelerations of particular orientations, while the receptors in the auditory system characteristically respond best to sounds of certain frequencies. Hair cells are accordingly not passive transducers; through their electrical and mechanical properties they also contribute to the selection of stimuli. I discuss here the current evidence bearing on both these roles of the vertebrate hair cell, that of transduction and that of filtering stimuli.