Outer hair cell function
|Outer hair cells use their receptor potential to contract and exert force on the basilar membrane ---thereby generating a POSITIVE FEEDBACK MECHANISM which amplifies the vibration of the membrane in a nonlinear, highly frequency specific manner. This force produces its own fluid wave, which is conducted back through the perilymph, vibrating the middle ear apparatus and generating sounds that are emitted from the ear (OTOACOUSTIC EMISSIONS).|
- think of a swing: if a person sitting on a swing (basilar membrane) pumps his legs (OHC), the amplitude of the swing motion in response to a push (sound stimulus) is increased
- Sound induces the travelling wave on the basilar membrane.
- Basilar membrane vibration causes stereocilia deflection.
- In both types of hair cells, the mechanical stimulus is transduced and receptor potentials are generated. In IHCs, this leads to neurotransmitter release and AP generation in underlying auditory nerve fibres.
- However, in OHCs, receptor potentials result in contraction, which amplifies the basilar membrane motion.
- This amplification increases the movement of the basilar membrane (positive feedback).
OHCs have been shown to be capable of contracting at rates up to 70 kHz
- OHC contraction adds energy to the cochlea, which can be transmitted back out through the middle ear as otoacoustic emissions (a normal function of the cochlea that is exploited in some hearing tests)
- efferent innervation provided to the OHC from the superior olivary complex is thought to adjust the resting membrane potential of the OHCs, thereby adjusting the amount of feedback provided to the basilar membrane (i.e., the CNS can change the sensitivity and frequency selectivity of the cochlea)