Hair cells (ear)
Hair cells, found within the organ of Corti, are responsible for transferring the information in the fluid waves of the cochlea into nerve potentials that are then transmitted to the auditory cortex via the auditory nerve. Hair cells are attached to both the tectorial membrane and the basilar membrane; this makes the hairs, or stereocilia, be pulled in one of two directions depending on the angular displacement of the basilar membrane caused by fluid waves in the cochlea. Research suggests that a complex of cadherin 23 and myosin-1c in stereocilia tips regulates the activity of hair cells' mechanically gated ion channels.
The ear is able to separate frequencies by the combination of three methods. The varying thickness of the basilar membrane allows the location of the activated hair cell to be one indicator of frequency; however the hair cells themselves are also adapted to a particular frequency. Where the basilar membrane is thin and stiff (near the stapes) the hair cells are short and stiff in order to favor high frequencies. Likewise, where the membrane is thick and flexible (near the apex of the cochlea) the hair cells are adapted to low frequencies by being both long and flexible.
Apart from this form of mechanical resonance hair cells are also electrically tuned to a frequency. Hair cells go through a cycle involving two different ions, which is much faster in the hair cells that are adapted to higher frequencies. The ear detects the amplitude of a sound wave by how much the basilar membrane is displaced by the fluid, and, consequently, how strongly the hair cells are pulled. In this manner the hair cells are able to code both the frequency and the amplitude (pitch and loudness) of a sound.
