The role of tectorial membrane stiffness and viscosity on traveling waves and resonance

Classical cochlear models have long suggested that the tectorial membrane (TM) is a resonant system, which contributes to both sensitivity and frequency selectivity. However, these models do not consider longitudinal coupling through the TM and assume damping in the subtectorial space controls sharpness of tuning. Recent TM wave motion results show that the TM can couple activity over several hundred rows of hair cells, suggesting that TM inertial and viscoelastic properties may overcome the dissipative effects of subtectorial damping. Here we manipulate the dynamic shear modulus (G′) and shear viscosity (η) of the TM by altering pH and adding polyethylene glycol (PEG) to the bath surrounding the TM. Analysis of a distributed impedance model shows that increasing TM shear viscosity (via addition of PEG) and decreasing shear modulus (via altering bath pH) both decrease wave decay constants by 35-42% over a broad range of frequencies. This reduction in spatial extent of TM waves is consistent with sharpened cochlear tuning (i.e. reduced spread of excitation), suggesting that TM wave and resonance models differ significantly. While both suggest that the TM plays an important role in determining cochlear tuning, the mechanism by which sharpened tuning is achieved in TM wave models is driven by spatial extent of TM waves.