RESONANT TECTORIAL MEMBRANE MOTION IN THE INNER-EAR - ITS CRUCIAL ROLE IN FREQUENCY TUNING

The tectorial membrane has long been postulated as playing a role in t he exquisite sensitivity of the cochlea. In particular, it has been pr oposed that the tectorial membrane provides a second resonant system, in addition to that of the basilar membrane, which contributes to the amplification of the motion of the cochlear partition. Until now, tech nical difficulties had prevented vibration measurements of the tectori al membrane and, therefore, precluded direct evidence of a mechanical resonance, In the study reported here, the vibration of the tectorial membrane was measured in two orthogonal directions by using a novel me thod of combining laser interferometry with a photodiode technique, It is shown experimentally that the motion of the tectorial membrane is resonant at a frequency of 0.5 octave (oct) below the resonant frequen cy of the basilar membrane and polarized parallel to the reticular lam ina, It is concluded that the resonant motion of the tectorial membran e is due to a parallel resonance between the mass of the tectorial mem brane and the compliance of the stereocilia of the outer hair cells, M oreover, in combination with the contractile force of outer hair cells , it is proposed that inertial motion of the tectorial membrane provid es the necessary conditions to allow positive feedback of mechanical e nergy into the cochlear partition, thereby amplifying and tuning the c ochlear response.