[The cochlea: modern physiologic insights into sensorineural hearing loss]

The cochlea is organized tonotopically: each frequency between 20 Hz and 20 kHz has its own location on the basilar membrane. In addition the cochlea has a large dynamic range (sound intensity). Active movements of the outer hair cells of the sensory organ of Corti play a crucial part in determining hearing acuity and frequency selectivity. Long-known properties such as combination tones and more recent observations like spontaneous and evoked otoacoustic emissions (the cochlea as sound source) fit in the same concept of a cochlea acting as a non-linear fine-tuned amplifier. Most sensorineural losses in people with diminished hearing are caused by injury to the hair cells, e.g. by medicines. Damage of outer hair cells impairs the cochlear amplifier, leading to loss of sensitivity to soft sounds and loss of frequency selectivity. Damage to the inner hair cells has much the same effect as conductive hearing loss. Tinnitus is not caused by otoacoustic emissions, but probably by denervation hypersensitivity following loss of cochlear input.