Mechanisms of coupling the electromechanical forces of the outer hair cells into the cochlear partition

Electrically induced length changes of outer hair cells (OHC) of the mammalian cochlea are thought to be the main energy source for cochlear amplification. While vibration measurements on isolated outer hair cells of the guinea pig have shown that OHCs can produce constant force and displacement over the entire hearing range, it is not clear if OHCs in vivo, embedded in the Organ of Cord (OC), can still produce enough force to enhance mechanical tuning up to high frequencies. Therefore, we further investigated the role of cochlear OHCs in vivo by measuring the mechanical impedance, the electromechanical force and the displacement of the OC using an in vitro preparation. Our data clearly show that OHCs in situ are able to generate mechanical forces over the entire hearing range with almost constant displacement amplitudes up to 20 kHz. Amplitude values seem to be sufficiently large to account for the role of the OHCs as electromechanical elements in a cochlear amplifier. The displacement data with and without tectorial membrane support the view that underlying the exquisite sensitivity and frequency selectivity of the cochlea, is a complex radial vibration pattern of the OC with different vibration modes.