The anisotropy of the tectorial membrane guides stereocilia deflection

The extreme performance of mammalian hearing is due to the cochlea's remarkable amplification of sound-induced mechanical stimuli. The mechanical coupling between the tectorial membrane (TM) and outer-hair cells' stereocilia leads to a crucial step in hearing mechanics: the conversion of outer-hair cells (OHC's) vertical displacement motion to the lateral motion of the stereocilia. Nonetheless, the mechanism which governs stereocilia motion is not known. Here we present a mechanism for this motion conversion by demonstrating that the relations between material properties of the TM along its three axes act as a “mechanical guide”. By using lateral force spectroscopy measurements combined with finite-element simulations, we show that along the entire length of the TM, the normal stiffness modulus (Ez) is significantly larger than the lateral modulus values (Ex, Ey). Our “mechanical guiding” model is supported by finite-element simulations of the interactions between the TM and the stereocilia. When the TM is simulated as an isotropic material (Ex=Ey=Ez), no lateral deflection of the stereocilia is generated. In contrast, when our experimentally obtained mechanical properties (Ez>Ex,Ey) were incorporated into the simulated TM, a clear lateral motion is observed. Our results provide a mechanism for cochlear amplification induced by TM-stereocilia coupling.