New insights in the evaluation of cochlear impairment

A major new insight into cochlear mechanisms is the recognition of an active process, the cochlear amplifier, that contributes energy to the mechanical movement of the basilar membrane at low sound intensities. The extra energy makes possible the great sensitivity of hearing and also the very sharp tuning of individual neural units, ie, the tips of their tuning curves. The inner hair cells are the final receptor cells, but the outer hair cells are essential for the amplifier action. The amplifier is much more sensitive to anoxia, ototoxic drugs, noise, etc, than the inner hair cell system alone. Severe impairment of the amplifier causes a sensorineural hearing loss with recruitment. Only the tails of tuning curves remain. Thus the cochlea is a dual sensory system, a robust but insensitive broadly tuned passive component sided by a vulnerable, sensitive sharply tuned active component.