Anisotropic Material Properties of Wild-Type and Tectb−/− Tectorial Membranes

Abstract

The tectorial membrane (TM) is an extracellular matrix that is directly coupled with the mechanoelectrical receptors responsible for sensory transduction and amplification. As such, the TM is often hypothesized to play a key role in the remarkable sensory abilities of the mammalian cochlea. Genetic studies targeting TM proteins have shown that changes in TM structure dramatically affect cochlear function in mice. Precise information about the mechanical properties of the TMs of wild-type and mutant mice at audio frequencies is required to elucidate the role of the TM and to understand how these genetic mutations affect cochlear mechanics. In this study, images of isolated TM segments are used to determine both the radial and longitudinal motions of the TM in response to a harmonic radial excitation. The resulting longitudinally propagating radial displacement and highly spatially dependent longitudinal displacement are modeled using finite-element models that take into account the anisotropy and finite dimensions of TMs. An automated, least-square fitting algorithm is used to find the anisotropic material properties of wild-type and Tectb À/À mice at audio frequencies. Within the auditory frequency range, it is found that the TM is a highly viscoelastic and anisotropic structure with significantly higher stiffness in the direction of the collagen fibers. Although no decrease in the stiffness in the fiber direction is observed, the stiffness of the TM in shear and in the transverse direction is found to be significantly reduced in Tectb À/À mice. As a result, TMs of the mutant mice tend to be significantly more anisotropic within the frequency range examined in this study. The effects of the Tectb À/À mutation on the TM's anisotropic material properties may be responsible for the changes in cochlear tuning and sensitivity that have been previously reported for these mice.

Reducing tectorial membrane viscoelasticity enhances spontaneous otoacoustic emissions and compromises the detection of low level sound

“…G tm and η tm are material properties that have been directly reported by Lemons et al . 55 and other previous studies 17,42,56 for the TM of the mouse cochlea. These parameters have values ( G tm ≈ 5 kPa at the 20 kHz location and η tm = 0.03 Pa · s) that are lower than reported by Lemons et al .…”

“…Similarly, the occurrence of SOAEs is significantly increased in the Ceacam16 KO mice 36 ; while the mechanical properties of the TM of Ceacam16 KO mice have not been characterized, our theoretical results suggest that the enhancement of SOAEs in Ceacam16 KO might also be due to a decreased viscosity and/or stiffness of the TM. Previous work has shown that the Tectb KO mutation primarily affects the shear viscosity of the TM, which is about 50% lower than its value in WT TMs 42,55 . Our model predicts that if the mutation only affects the TM shear modulus, Tectb KO mice should emit slightly more SOAEs than WT mice, which is consistent with measurements by Cheatham et al .…”

“…However, longitudinal coupling by the Y-shaped elements, which play an essential role in cochlear amplification in some models 52–54 , might have a more significant influence on SOAE generation. Because some genetic mutations are known to affect TM longitudinal coupling 41–43,55 , longitudinal coupling of the TM is particularly interesting and was the focus of this work.…”

“…Previous work has characterized the material properties of isolated segments of murine TMs based on a transversely isotropic and viscoelastic model within the auditory frequency range 55 . In that study, Lemons et al .…”

“…In that study, Lemons et al . 55 reported that the TM stiffness is about 25 times higher in the radial direction than in the longitudinal direction. In the current model, the TM is assumed to vibrate rigidly within each cross-section, which is consistent with the high stiffness in the radial direction.…”