cochlea

The spiral cavity in the internal ear, which is concerned with hearing. It consists of a bony part with a central pillar (the modiolus) and a spiral duct (part of the membranous labyrinth). The space between the bony and membranous parts is filled with perilymph (a fluid similar to cerebrospinal fluid). A thin spiral shelf projects from the modiolus, on which lies the basilar membrane. Sound waves in the air are transmitted from the middle ear to the cochlea through the oval window, making the basilar membrane vibrate. The basal part of the basilar membrane responds with nerve impulses to both high and low frequencies, while the apical part responds to low frequencies only. Vibrations of the membrane stimulate a set of hair cells, which trigger impulses in the cochlear part of the eighth cranial nerve. These signals are then transmitted to the auditory areas of the brain for interpretation.

Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea. Structures include:

the scala vestibuli (containing perilymph), which lies superior to the cochlea duct and abuts the oval window. the scala tympani (containing perilymph), which lies inferior to the scala media and terminates at the round window. the scala media (containing endolymph), which is the membraneous cochlea duct containing the organ of Corti. the helicotrema is the location where the scala tympani and the scala vestibuli merge Reissner's membrane separates the scala vestibuli from the scala media. The basilar membrane separates the scala media from the scala tympani. It is lined with hair cells — sensory cells topped with hair-like structures called stereocilia. As the fluid moves, thousand of "hair cells" are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells.

The stapes of the middle ear transmits to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph (fluid) in the scala vestibuli (upper chamber of the cochlea).

This motion of perilymph in turn vibrates the endolymph in the scala media, thus causing movements of the hair bundles of the hair cells, which are acoustic sensor cells that convert vibration into electrical potentials. The hair cells in the organ of Corti are tuned to certain sound frequencies, being responsive to high frequencies near the oval window and to low frequencies near the apex of the cochlea.

The hair cells are arranged in four rows in the Organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly receive neural input from the brain , which influences their motility as part of the cochlea’s mechanical pre-amplifier.

For very low frequencies (below 20Hz), the pressure waves propagate along the complete route of the cochlea - up scala vestibuli, around helicotrema and down scala tympani to the round window. Higher frequencies do not propagate to the helicotrema but are transmitted through the endolymph in the cochlea duct to the perilymph in the scala tympani.

A very strong movement of the endolymph due to very loud noise may cause hair cells to die. In birds and in other non-mammalian vertebrates the compartment containing the sensory cells for hearing is occasionally also called “cochlea”, although it is not coiled up. The superior frequency resolution in mammals is due to their unique mechanism of pre-amplification of sound by active cell-body vibrations of outer hair cells.