Micromechanics of the Inner Ear

Fluid-structure models of the mechanics and pathomechanics of the Inner Ear

The projects Za 249/4, Ha 2075/9, Gr 1388/14, Gu 194/7 and Vo 899/6 sponsored by the German Research Foundation (DFG) produced finite-element models of structures of the organ of Corti of the inner ear of mammals at different geometric scales. A biological system exists in each of these scales that exhibits a high level of complexity in its material properties through an inhomogeneous microstructure,multi-phase, anisotropy and high variability. Using the created finite element models, fundamental statements can be made about the mechanical properties of the structures as well as their functions in the hearing process.

Basal membrane

The basal membrane divides the scala media from the scala tympani of the cochlea. Viewed from a morphological aspect, the basal membrane is sectionedin radial direction into the zona arcuata and the zona pectinata. The first one exhibits a random short-fiber actin structure, which macroscopically indicates isotropic material behavior. In contrast, the zona pectinata displays a strongly pronounced collagen fiber structure in radial direction, whose packing density decreases exponentially with the longitudinal coordinates in apical direction. The orientation of the fibers as well as the spatial variability of the packing density indicate a graded, orthotropic anisotropic material behavior. Using the example of the guinea pig cochlea, the coefficients in the elasticity tensor could be determined in the case of linear-elastic gradient material by development of a three-dimensional FE-model. It could be shown that with physiologically relevant pressure excitation by a surrounding fluid in the apical end, an excitation of the Corti organ altered by the basal response occurs due to the spatially altered material parameters in connection with a realistic geometry. The stiffness gradient following the longitudinal coordinate is primarily determined through the effective thickness of the basal membrane, while the curvature is mainly relevant in the apical areas.

The outer sensory hair cell

The outer sensory hair cell plays a central role in the overall behavior of the organ of Corti. This is due to the electromotile properties of the cellular body (soma) and the motility of the stereocilia bundle. Anatomic examinations of the cell wall reveal a multilayer composition, interspersed with an orthogonally oriented actin-spectrin mesh. The harmonic finite element analysis shows that in the cell wall’s viscosity tensor, in the material law a different anisotropy type to the elasticity tensor leads to an adequate mapping of measured values of the axial impedance.

Pillar cells

The pillar cells are a part of the supporting tissue of the organ of Corti. Therefore they influence the vibration behavior of the cell composit substantially. A characteristic of the cytoskeleton of the pillar cells is that the microtubules, which are arranged in bundles, lead to an anisotropic behavior of the cells. With the help of the numerical homogenization calculation, the determination of effective and homogeneous as well as anisotropic replacement material parameters is possible. The geometrical data is obtained from three-dimensional reconstructions of the transmission electron microscopy (TEM).

Publications

• Fleischer, M., Harasztosi, C., Nowotny, M., Zahnert, T., & Gummer, A. W. (2011). Continuum Mechanical Model of the Outer Hair Cell (C. A. Shera & E. S. Olson, Eds.; Vol. 1403, pp. 160–165). AIP Conference Proceedings. http://dx.doi.org/10.1063/1.3658078
• Yarin, Y. M., Lukashkin, A. N., Poznyakovskiy, A. A., Meißner, H., Fleischer, M., Baumgart, J., Richter, C., Kuhlisch, E., & Zahnert, T. (2014). Tonotopic Morphometry of the Lamina Reticularis of the Guinea Pig Cochlea with Associated Microstructures and Related Mechanical Implications. J Assoc Res Otolaryngol, 15(1), 1–11. https://doi.org/10.1007/s10162-013-0420-1