Cell and molecular basis of hearing

The mammalian auditory organ is a specialized sensory epithelium capable of detecting subnanometer movements produced by sound and transducing them into electrical signals. It is a dual system consisting of two types of sensory cells: the inner hair cells, which provide afferent input to the central nervous system, and the outer hair cells, which provide frequency resolution and signal amplification. The critical element in mechano-reception is the hair bundle, a cluster of stereocilia located in the apical end of the sensory cells. Mechanical stimulation causes deflection of ciliary bundles that leads to the opening or closing of transduction channels located in the apical part of the plasma membrane of the stereocilia. The current concept of auditory function requires an active process of signal amplification within the cochlea. It is generally believed that outer hair cells, functioning as both sensor and motor elements, are responsible for this amplification process. The motor function consists of changes in cell length produced by the concerted action of a large number of independent molecular motors distributed along the length of the outer hair cell lateral plasma membrane. Although experimental evidence suggests its association with conformational changes in transmembrane proteins, the exact nature of the outer hair cell force generation mechanism on a molecular level is still unknown.