Review the key structures of the organ of Corti.
Inner hair cell
Each ear contains a single row of about 3500 inner hair cells. While fewer in number, the inner hair cells account for 90 to 95% of the afferent auditory fibers, suggesting that the inner hair cells convey most of the information about sounds to the brain.
Outer hair cell
Each ear contains about 12,000 outer hair cells, arranged in three rows. The outer hair cells are involved in modulating acoustic stimulation—sharpening the tuning to different frequencies.
The stereocilia of the hair cells extend into indentations in the bottom of the tectorial membrane. Fluid vibrations in the cochlea bend the stereocilia, producing a change in the response rate of the hair cells.
Efferent nerve fibers from the superior olivary nucleus
IHC efferents lead from the brain to the IHCs—through which the brain can control the responsiveness of IHCs. OHC efferents lead from the brain to the OHCs. OHCs change their length almost instantaneously in response to commands from the brain. Through this electromechanical action, OHCs continually modify the stiffness of regions of the basilar membrane, resulting in both sharpened tuning and pronounced amplification.
The basilar membrane separates the tympanic canal from the middle canal. The basilar membrane vibrates in response to sound, causing the hair cell stereocilia that are inserted into the tectorial membrane to bend, triggering excitation of the hair cells.
Afferent nerve fibers to the cochlear nucleus of the brain stem
IHC afferents convey to the brain the action potentials that provide the perception of sounds. IHC afferents make up about 95% of the fibers leading to the brain. OHC afferents are thought to convey information to the brain about the mechanical state of the basilar membrane, but not the perception of sounds themselves.