High-frequency vibration of the organ of corti in vitro

The mechanism by which the electromechanical force generated by the outer hair cells (OHC) produces the exquisite sensitivity, frequency selectivity and dynamic range of the cochlea is unknown. To address this question, we measured the electrically induced radial vibration pattern at different levels within the organ of Corti of the guinea pig. Two in vitro preparations were used: 1) a half turn including modiolar bone and cochlear partition, without tectorial membrane (TM); the basilar membrane (BM) was supported from its tympanal side. 2) A temporal bone preparation, where the bony wall was removed above and below the measurement location to permit introduction of electrodes. In the latter case, the cochlear partition was in its normal mechanical environment, with free swinging BM and with TM. Velocity of BM, reticular lamina (PL), and upper and lower sides of the TM in response to broadband electrical stimulation of the OHCs was measured with a laser Doppler vibrometer. The interferometer was sensitive enough to permit measurement without reflective beads or the like. The frequency range of the stimulation was 480 Hz - 70 kHz. Displacement amplitudes were constant up to 10 kHz, after which they dropped with -14 to -17 dB/oct. Moving across the RL in the radial direction, phase reversals characteristic of pivoting points occurred above the pillar cells and the outer tunnel. No phase reversals were observed on the BM and TM.