Abstract
Cochlear frequency selectivity plays a key role in our ability to understand speech, and is widely believed to be associated with cochlear amplification. However, genetic studies targeting the tectorial membrane (TM) have demonstrated both sharper and broader tuning with no obvious changes in hair bundle or somatic motility mechanisms. For example, cochlear tuning of Tectb–/– mice is significantly sharper than that of TectaY1870C/+ mice, even though TM stiffnesses are similarly reduced relative to wild-type TMs. Here we show that differences in TM viscosity can account for these differences in tuning. In the basal cochlear turn, nanoscale pores of TectaY1870C/+ TMs are significantly larger than those of Tectb–/– TMs. The larger pore size reduces shear viscosity (by ∼70%), thereby reducing traveling wave speed and increasing spread of excitation. These results demonstrate the previously unrecognized importance of TM porosity in cochlear and neural tuning.
Highlights
The mammalian inner ear separates sounds by their frequency content, and loss of this separation impairs our ability to understand speech in noisy environments in ways that cannot generally be compensated with a hearing aid
tectorial membrane (TM) segments were excised from the basal turn of the mouse cochlea and suspended between two supports immersed in artificial endolymph (Fig. 2 A)
Wild-type TM segments exhibited the highest wave speeds over the measured frequency range, whereas Tectb–/– and TectaY1870C/þ TM speeds were significantly lower by ~20 and ~40%, respectively
Summary
The mammalian inner ear separates sounds by their frequency content, and loss of this separation impairs our ability to understand speech in noisy environments in ways that cannot generally be compensated with a hearing aid. Whereas this problem is well understood, its molecular origins are not. TectaY1870C/þ and Tectb–/– mutations target a- and b-tectorin, respectively Both of these tectorins are structural macromolecules that are thought to contribute to elastic properties of the TM (Fig. 1 A). Both TectaY1870C/þ and Tectb–/– mutants have normal hair bundles and TM attachments. They exhibit distinctly different hearing phenotypes: Tectb–/– mice have sharpened basilar membrane (BM) tuning by a factor of 2–3 at mid to high frequencies [7], whereas TectaY1870C/þ mice have normal BM tuning (Fig. 1 B) and even broader neural tuning [5]
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