There are three rows of OUTER HAIRCELLS (OHC) and one row of INNER HAIR CELLS (IHC) located on the BASILAR MEMBRANE. Ninety-five per cent of AUDITORY NERVE fibres innervates the IHC with up to 20 fibres per IHC. The IHCs are sensory receptors that convert basilar membrane movement into activation of nerve fibres. This process comprises opening of POTASSIUM channels (see ION CHANNEL) in the cilia at the top of the IHC producing a DEPOLARIZATION which allows CALCIUM ions to enter at the base of the IHC and transmitter VESICLES to be released.
In contrast, OHCs respond to depolarization by fast length changes that, for low sound levels, provide positive mechanical feedback to the basilar membrane thereby increasing its movement amplitude by up to 100-fold. This permits low-level sounds to depolarize the IHCs. Malfunctioning of OHCs in a given TONOTOPIC REPRESENTATION range increases the hearing threshold by up to 40dB for the corresponding frequencies. OHCs receive an important EFFERENT nerve supply from the brainstem that is affected by the sound level at the same or the other ear. Activation of these nerve fibres causes slow length changes of the OHC which regulate overall movement of the basilar membrane.
