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By Carl Sherman



The auditory system transforms mechanical energy—sound waves—into nerve impulses.


Sounds enter the system via the inch-long ear canal, which terminates at the tympanic membrane (eardrum) whose vibrations represent an exquisitely complex response to pressure variations. A series of tiny bones (ossicles) in the middle ear amplify and transmit these vibrations to the cochlea of the inner ear, a fluid-filled tube rolled up like a snail shell.


Hair cells within the cochlea are the receptors of the auditory system. Movement of the membrane lining the cochlea stretches the tips of hair cells to allow an influx of K+ and Ca+2 ions—charged particles that build up to generate electrical signals in acoustic nerve fibers. The frequencies to which each hair cell responds depends on its position in the cochlea.

Auditory signals pass through intermediate brain regions to the primary auditory cortex in the temporal lobe.


Direct connections facilitate quick reaction to loud sounds. Another pathway permits processing en route—coordinating signals from both ears, for example.

Past the primary auditory cortex, signals divide into streams that locate and analyze what we hear. The latter include Wernicke’s area in the dominant hemisphere, essential for language comprehension, and a right-hemisphere area that processes emotional aspects of speech.

Hearing is the first sense for which a device was developed to replace non-working receptors. By 2010, some 219,000 people had received cochlear implants, which restore functional hearing by directly stimulating the auditory nerve.