The membrane-based mechanism of cell motility in cochlear outer hair cells

The sensitivity of the mammalian inner ear is extraordinary. At the threshold of hearing, the incoming sound produces vibrations inside the organ of Corti that are of the same order of magnitude or less than the thermal noise motion (Hudspeth, 1989,1997; Dallos, 1996). Such sensitivity is achieved through an energy-dependent process commonly referred to as "cochlear amplifier," i.e., the cycle-by-cycle amplification of the intracochlear vibrations that neutralizes viscous losses by positive mechanical feedback (Patuzzi and Robertson, 1988;Ashmore and Kolston, 1994;Kolston, 1995). The mechanical feedback force that makes the amplification possible is thought to be provided by cochlear outer hair cells (OHCs), a class of specialized sensorimotor cells hosted within the organ of Corti (Lim and Kalinec, 1998; Nobili et al., 1998). Direct support to the feedback hypothesis comes from the fact that OHCs posses a unique ability to change significantly their shape in response to electrical stimulation (Brownell et al., 1985; Kachar et al., 1986), a phenomenon called electromotility.