When sound enters the ear canal, it vibrates the tympanic membrane, or eardrum. These vibrations are passed through the inner ear via three small bones called ossicles: the malleus, the incus, and the stapes. Finally, vibrations of the stapes stimulate the movement of a fluid called perilymph within the bony labyrinth of the inner ear.
Perilymph fills the both the vestibular and tympanic ducts of the cochlea. Between these two channels lies the cochlear duct, which is home to the organ of Corti. There, the soundinduced movement of perilymph in the cochlea is translated to an electrical signal that is sent to the brain for processing.
An electrical signal is generated by inner hair cells that sit above the basilar membrane, which separates the cochlear duct from the tympanic duct. As the basilar membrane vibrates in response to fluid movement, it pushes the hair cells along another membrane, known as the tectorial membrane, which shifts laterally to bend projections at the tips of the cells, called stereocilia.
The bending of the stereocilia results in the depolarization of the inner hair cell and initiates a nerve impulse through the spiral ganglion neuron at the base of the cell. A series of outer hair cells serves to mechanically amplify the vibrations that trigger the inner hair cells to fire. High-frequency sounds stimulate hair cells at the base of the cochlea, while low-frequency sounds stimulate hair cells at the apex.