Motility of cochlear outer hair cells.

A recent exciting discovery in the physiology of hearing has been that fine tuning and regulatory properties of the mammalian auditory system reside in the micromechanics of the basilar membrane. While the inner hair cells are considered primary afferent transducers, the outer hair cells are postulated to modulate transduction based on their motile properties. These new insights have impacted on our view of auditory processing and are also leading to improved diagnostic procedures (such as those based on otoacoustic emissions) and a better understanding of sensorineural pathology. Outer hair cell motility may be separated into two categories: fast and slow. Fast motility is voltage-driven and frequency-following and provides positive feedback to the motion of the basilar membrane. Slow motility (shape changes measured in milliseconds to seconds) can be triggered through depolarization by potassium, osmotic effects, mechanical stimulation, efferent neurotransmitters, and the elevation of intracellular second messengers and calcium ions. These slow changes may be superimposed upon the fine-tuning provided by the fast motility. This paper discusses the mechanism underlying the shape changes induced by different stimuli, and assesses the role of fast and slow motility in the physiology and pathology of auditory transduction.