Sound-evoked vibrations transmitted into the mammalian cochlea produce traveling waves that provide the mechanical tuning necessary for spectral decomposition of sound. These traveling waves of motion that have been observed to propagate longitudinally along the basilar membrane (BM) ultimately stimulate the mechano-sensory receptors. The tectorial membrane (TM) plays a key role in this process, but its mechanical function remains unclear. Here we show that the TM supports traveling waves that are an intrinsic feature of its visco-elastic structure. Radial forces applied at audio frequencies (2-20 kHz) to isolated TM segments generate longitudinally propagating waves on the TM with velocities similar to those of the BM traveling wave near its best frequency place. We compute the dynamic shear storage modulus and shear viscosity of the TM from the propagation velocity of the waves and show that segments of the TM from the basal turn are stiffer than apical segments are. Analysis of loading effects of hair bundle stiffness, the limbal attachment of the TM, and viscous damping in the subtectorial space suggests that TM traveling waves can occur in vivo. Our results show the presence of a traveling wave mechanism through the TM that can functionally couple a significant longitudinal extent of the cochlea and may interact with the BM wave to greatly enhance cochlear sensitivity and tuning.cochlear mechanics ͉ dynamic mechanical properties ͉ longitudinal mechanical coupling T he 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 (1, 2). These extraordinary properties of the hearing organ depend on traveling waves of motion that propagate along the basilar membrane (BM) (3) and ultimately stimulate the mechanosensory receptors. There are two types of cochlear receptors: the inner and outer hair cells (OHCs). Both types of hair cells contain densely packed arrays of stereocilia called hair bundles that transduce mechanical energy into electrical signals (4). These hair bundles project from the apical surface of hair cells toward an overlying gelatinous matrix called the tectorial membrane (TM).The strategic anatomical configuration 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 (5-9), 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 (10), 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-flo...

Remarkable sensitivity and exquisite frequency selectivity are hallmarks of mammalian hearing, but their underlying mechanisms remain unclear. Cochlear insults and hearing disorders that decrease sensitivity also tend to broaden tuning, suggesting that these properties are linked. However, a recently developed mouse model of genetically altered hearing (Tectb−/−) shows decreased sensitivity and sharper frequency selectivity. In this paper, we show that the Tectb mutation reduces the spatial extent and propagation velocity of tectorial membrane (TM) travelling waves and that these changes in wave propagation are likely to account for all of the hearing abnormalities associated with the mutation. By reducing the spatial extent of TM waves, the Tectb mutation decreases the spread of excitation and thereby increases frequency selectivity. Furthermore, the change in TM wave velocity reduces the number of hair cells that effectively couple energy to the basilar membrane, which reduces sensitivity. These results highlight the importance of TM waves in hearing.