The sensory and motor roles of auditory hair cells
Robert Fettiplace* and Carole M. Hackney‡
Abstract | Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their hair bundle. Outer hair cells are not only detectors, but also generate force to augment auditory sensitivity and frequency selectivity. Two mechanismsof force production have been proposed: contractions of the cell body or active motion of the hair bundle. Here, we describe recently identified proteins involved in the sensory and motor functions of auditory hair cells and present evidence for each force generator. Both motor mechanisms are probably needed to provide the high sensitivity and frequency discrimination of the mammalian cochlea.Auditory sensing in mammals occurs in a subdivision of the inner ear known as the cochlea, a fluid-filled tube coiled up like a snail’s shell to fit into the temporal bone at the base of the skull. Sound pressure fluctuations are transmitted to the cochlea in several steps: vibrations of each eardrum, conduction through three small middle ear bones that act in a lever-like fashion, and production ofpressure waves within the cochlea to displace the basilar membrane (FIG. 1). This complex mechanical coupling ensures that sound energy is efficiently transferred from air to cochlear fluid over a wide frequency range1,2. Detection of the sound stimulus and its conversion to an equivalent electrical waveform, termed mechanoelectrical transduction, occurs in hair cells riding on the elongatedbasilar membrane. Sound-induced motion of the basilar membrane excites the hair cells by deflecting their hair bundles to activate mechanoelectrical transduction (MET) ion channels3. Owing to gradients in the size and stiffness of the cochlear partition (the basilar membrane and associated hair cells and supporting cells), the place of maximal vibration varies systematically with the sound frequencylike resonances in a musical instrument1. As a consequence, each hair cell produces responses that, near the threshold of hearing, are tuned to a characteristic frequency (CF) (BOX 1). The mammalian cochlea contains two classes of hair cell, inner and outer, with distinct functions (FIG. 2). Information about the acoustic environment — speech, music or other sounds in the outside world — is relayedprimarily by the electrical signals of inner hair cells (IHCs) (BOX 2), whereas the main task of outer hair cells (OHCs) is to boost the stimulus by electromechanical feedback4. The OHC contribution is known as the ‘cochlear amplifier’, a mechanism that increases both the amplitude and frequency selectivity of basilar membrane vibration for low-level sounds. The unique features of mammalianhearing — the middle ear bones, the extended basilar membrane and the separation of function between IHCs and OHCs — have all evolved because of selective pressure to extend the upper frequency limit of hearing5. These changes enabled the first nocturnal mammals with small heads to spatially localize the sounds made by other animals, especially calls of their young, on the basis of interaural intensitydifferences. Here, we summarize recent discoveries of hair cell mechanisms underlying mechanoelectrical transduction and frequency tuning. We describe the newly discovered proteins responsible for the precise organization of the hair bundle, many of which have been revealed by studying the genetics of hereditary deafness6 (BOX 3). We present recent evidence about the performance and molecularidentity of the MET channel. Finally, we describe the basis of reverse transduction, which enables OHCs to act as force generators and contribute to cochlear frequency selectivity. A fuller understanding of hair cell mechanisms may settle the long-standing question of how the mammalian cochlea achieves its remarkable sensitivity and frequency discrimination.