Nanoscale Poroelasticity of the Tectorial Membrane Determines Hair Bundle Deflections

Radial Flow Field of Spiral Cochlea and Its Effect On Stereocilia

The opening of the ion channels ultimately depends on the movement and energy conversion of the microstructural organization. It has not been clear how active sound amplification is generated by the microstructure of the cochlea's characteristic spiral shape. In this paper, an analytical model of the spiral cochlea is developed to investigate the radial flow field generated by the spiral shape of the cochlea and its effect on the outer hair cell stereocilia, and to analyze the effect of the spiral shape on the micromechanics of the cochlea. The results show that the spiral shape of the cochlea exerts a radial shear force on the hair cell stereocilia by generating a radial flow field. This causes the stereocilia to deflect in the radial flow field, with the maximum deflection occurring at the apex of the cochlea. This finding explains the microscopic mechanism that causes the cochlea's spiral shape to enhance low-frequency hearing in humans, and it provides a basis for further studies on the contribution of the movement of stereocilia in the radial flow field of the lymphatic fluid to activate ion channels for auditory production.

Age-related degradation of tectorial membrane dynamics with loss of CEACAM16

Studies of genetic disorders of sensorineural hearing loss have been instrumental in delineating mechanisms that underlie the remarkable sensitivity and selectivity that are hallmarks of mammalian hearing. For example, genetic modifications of TECTA and TECTB, which are principal proteins that comprise the tectorial membrane (TM), have been shown to alter auditory thresholds and frequency tuning in ways that can be understood in terms of changes in the mechanical properties of the TM. Here, we investigate effects of genetic modification targeting CEACAM16, a third important TM protein. Loss of CEACAM16 has been recently shown to lead to progressive reductions in sensitivity. Whereas age-related hearing losses have previously been linked to changes in sensory receptor cells, the role of the TM in progressive hearing loss is largely unknown. Here, we show that TM stiffness and viscosity are significantly reduced in adult mice that lack functional CEACAM16 relative to age-matched wild-type controls. By contrast, these same mechanical properties of TMs from juvenile mice that lack functional CEACAM16 are more similar to those of wild-type mice. Thus, changes in hearing phenotype align with changes in TM material properties and can be understood in terms of the same TM wave properties that were previously used to characterize modifications of TECTA and TECTB. These results demonstrate that CEACAM16 is essential for maintaining TM mechanical and wave properties, which in turn are necessary for sustaining the remarkable sensitivity and selectivity of mammalian hearing with increasing age.

Dirac cones and chiral selection of elastic waves in a soft strip

We study the propagation of in-plane elastic waves in a soft thin strip, a specific geometrical and mechanical hybrid framework which we expect to exhibit a Dirac-like cone. We separate the low frequencies guided modes (typically 100 Hz for a 1-cm-wide strip) and obtain experimentally the full dispersion diagram. Dirac cones are evidenced together with other remarkable wave phenomena such as negative wave velocity or pseudo-zero group velocity (ZGV). Our measurements are convincingly supported by a model (and numerical simulation) for both Neumann and Dirichlet boundary conditions. Finally, we perform one-way chiral selection by carefully setting the source position and polarization. Therefore, we show that soft materials support atypical wave-based phenomena, which is all of the more interesting as they make most of the biological tissues.