The role of the tectorial membrane in cochlear mechanics

The tectorial membrane (TM) is strategically located in the mammalian cochlea, anchoring the apical pole of the outer hair cell stereocilia and hovering just above the sensory inner hair cell stereocilia. Genetic mutations of TM-specific proteins cause nonsyndromic deafness in humans while experiments in mutant mice clearly demonstrate the impact that manipulations of TM structural proteins have on hearing thresholds and cochlear sensitivity. This direct evidence shows the importance of this cochlear structure. Mathematical models (with Zwsilocki as an early proponent) have long incorporated the TM but have not included a complete electrical and fluidic coupling. We show that an electromechanical model of the active processes in the cochlea combined with a fluid-structure model that strongly couples the basilar membrane and the TM to the cochlear fluids is crucial in understanding the low and high frequency response in a base-to-apex model of the cochlea. Specifically, we show that the onset of nonlinearity at high frequencies is due to a radial resonance of the uncoupled TM and investigate the influence of fluid-loading on the TM and TM viscoelasticity on the low-pass behavior seen in the apex. A connection of this new wave bearing mechanism to otoacoustic emissions will be discussed.