Distributed Impedance Model of Tectorial Membrane Traveling Waves

The mammalian cochlea is a remarkable sensor that can detect motions smaller than the diameter of a hydrogen atom and can perform high-quality spectral analysis to discriminate as many as 30 frequencies in the interval of a single semitone (Kossl and Russell, 1995; Dallos, 1996). These extraordinary properties of the hearing organ depend on traveling waves of motion that propagate along the basilar membrane (BM) (von Bekesy, 1960) and ultimately stimulate the sensory receptors. The strategic location of the TM relative to the hair bundles suggests that the TM plays a key role in stimulating hair cells. Mouse models with genetically modified structural components of the TM have been shown to exhibit severe loss of cochlear sensitivity and altered frequency tuning (McGuirt et al, 1999; Legan et al, 2000; Simmler et al, 2000; Legan et al, 2005; Russell et al, 2007), thereby providing further evidence that the TM is required for normal cochlear function. However, the mechanical processes by which traveling wave motion along the BM leads to hair cell stimulation remain unclear (Guinan et al, 2005), largely because the important mechanical properties of the TM have proved difficult to measure. Consequently, the mechanical function of the TM has been variously described as a rigid pivot, a resonant structure, and a free-floating mass (Davis, 1958; Allen, 1980; Zwislocki, 1980; Mammano and Nobili, 1993) in “classical” cochlear models, which assume that adjacent longitudinal sections of the cochlear are uncoupled except for energy propagation through the fluid (de Boer, 1997).