Sound amplification by the outer hair cells
Most of the neurons (about 95% of the spiral ganglion cells) that form the auditory nerve form synapses with inner hair cells. In other words, inner hair cells are much more heavily innervated than the outer hair cells. This suggests that most of the cochlear output to the central auditory system is derived from inner hair cells. However, the number of outer hair cells is surprisingly about 3 times greater than that of inner hair cells. What could be the significance of outer hair cells?
The role of outer hair cells is thought to amplify the movement of the basilar membrane, especially when the sound stimuli are low intensity. The keys to the function of outer hair cells as so-called cochlear amplifier are motor proteins found in their plasma membranes. One of these proteins may be prestin.
Motor proteins in the outer hair cells amplify the response of the basilar membrane
When the basilar membrane is deflected in response to sound wave, bending of the stereocilia causes depolarization and triggers shortening of the hair cell by the action of activated motor proteins. Bending towards the other direction causes hyperpolarization, which inactivates the motor proteins and causes lengthening of the cell. The shortening or lengthening of the hair cell thereby enhances the movement of the basilar membrane. By amplifying the response of basilar membrane, bending of the stereocilia on the inner hair cells is increased. This significantly increases the transduction process and therefore the cochlear output to the brain. In fact, administration of a chemical furosemide, which probably inactivates outer hair cell motor proteins, is known to reduce movement of the basilar membrane by about 100-fold due to loss of the cochlear amplifier.
Figure 1: The role of outer hair cells in sound amplification. (a) Motor proteins in the plasma membranes of outer hair cells. (b) Bending of the stereocilia causes potassium ion influx, polarizing the hair cell and triggering motor proteins to shorten the cell length. (c) The shortening ( or lengthening) of the hair cell increases the deflection of the basilar membrane. (d) Drug treatment (eg. furosemide) reduces hair cell tranduction and thus the flexing of basilar membrane.
Image courtesy of https://openlearn.open.ac.uk/mod/resource/view.php?id=263164 under a Creative Commons licence.