Cochlea

The cochlea is the hearing organ of the inner ear. It is a fluid-filled, snail-shaped structure coiled approximately two and a half turns, housed within the temporal bone of the skull. Inside the cochlea, the basilar membrane runs the length of the spiral and is lined with thousands of sensory hair cells that are responsible for converting mechanical sound vibrations into electrical signals the auditory nerve can carry to the brain.

The cochlea performs a sophisticated frequency analysis: different regions of the basilar membrane respond to different frequencies of sound. High-frequency sounds cause maximum displacement at the base of the cochlea, near the oval window. Low-frequency sounds produce maximum movement at the apex, the innermost coil. This spatial organization is called tonotopy, and it is preserved throughout the auditory pathway from the cochlea to the auditory cortex.

Cochlear hair cells are divided into two types: outer hair cells (which amplify and fine-tune incoming sound) and inner hair cells (which are the primary sensory cells that transmit signals to the auditory nerve). Cochlear hair cells in humans do not regenerate once damaged. Damage from noise, aging, ototoxic medications, or genetic factors results in permanent sensorineural hearing loss. Cochlear implants restore hearing by electrically stimulating the surviving auditory nerve fibers when cochlear hair cell function is too reduced for amplification to be effective.