Outer hair cells as fast and slow cochlear amplifiers with a bidirectional transduction cycle.

Cochlear hair cells fulfil the key function in hearing. However, possible differences in functions of inner and outer hair cells are the subject of a current scientific discussion. In the present paper, we present steps of the sensory transduction in living mammalian hair cells. The transduction is a mechano-electrical process based upon ionic channels in the outer cell membrane. These mechanisms are suggested to be similar in inner and outer hair cells, allowing inner hair cells the actual sound perception. By contrast, the outer hair cells seem to control actively the micromechanics of the inner ear on the basis of fast and slow motor properties. Our results revealed that in these cells, electro-mechanical mechanisms can induce fast and slow motile events. Together with the above-mentioned mechano-electrical transduction, the electro-mechanical mechanisms are assumed to establish a bidirectional transduction cycle in outer hair cells. These active motor properties of outer hair cells are obviously disturbed in acute attacks of Meniére's disease or in aminoglycoside intoxication, and may thus partly explain the resulting deafness.