The motion of the tectorial membrane (TM) with respect to the reticular lamina subserves auditory function by bending the outer hair cell bundles and inducing fluid flows that shear the inner hair bundles in response to sound energy. Little is currently known about its intrinsic elasticity or about the relation between the mechanical properties and function of the membrane. Here we subdivide the TM into three longitudinal regions and five radial zones and map the shear modulus of the TM using atomic force microscopy, and present evidence that the TM elasticity varies radially, after the distribution of type A collagen fibrils. This is seen most dramatically as a decrease in shear modulus in the neighborhood of the sensory hair cells; we argue that this inhomogeneity of properties not only protects the hair bundles but also increases the energy efficiency of the vibrational shearing during sound transduction.

We develop a hybrid approach for modeling the cochlea, in which we let the WKB method determine the axial propagation of waves and restrict the numerics to transverse planes, where we solve a fluid-solid interaction eigenvalue problem. The cochlear fluid is treated as viscous and incompressible. Viscous effects are confined to oscillatory boundary layers and the thin gap between the reticular lamina (RL) and the lower surface of the tectorial membrane (TM). Our model includes axial fluid coupling and also axial elastic coupling via a basilar membrane (BM) modeled as an orthotropic clamped plate. Three-dimensional (3D) flow is solved in two-dimensional (2D) domains, with interactions with 2D elastic domains representing the organ of Corti (OC) and the TM. The OC contains inhomogeneities representing discrete cellular structures. We have computed the interaction between the BM, TM, OC, and the cochlear fluid to find the complex-valued wavenumberfrequency relation and vibrational modes. The details of the cochlear fluid flow and pressure fields are calculated, along with displacements of the elastic structures. Simulation of passive radial modes in the apical region of a guinea pig cochlea for frequencies less than 1 kHz indicates monophasic vibration of the BM and a synchronous rotation of three rows of outer hair cell stereocilia induced by a shearing motion between the RL and TM.

Knowledge of vibratory patterns in the cochlea is crucial to understanding the stimulation of mechanosensory cells. Experiments to determine the motion of the cochlear partition and surrounding fluid are extremely challenging. As a result, the motion data are incomplete and often contradictory. The bending mechanism of hair bundles, thought to be related to the shear motion and endolymphatic flow between the tectorial membrane (TM) and reticular lamina (RL), is controversial. We, therefore, extend the frequency range of our previous hybrid analytical-finite-element approach to model the basal as well as apical regions of the guinea pig cochlea. We solve the fluid-solid interaction eigenvalue problem for the axial wavenumber, fluid pressure, and vibratory relative motions of the cochlear partition as a function of frequency. A simple monophasic vibratory mode of the basilar membrane is found at both ends of the cochlea. However, this simple movement is associated with a complex frequency-dependent relative deformation between the TM and the RL. We provide evidence of a radial component of TM motion that is out of phase with the RL and that facilitates the bending of outer hair cell stereocilia at appropriate frequencies at both the cochlear base and apex.A dynamic fluid-structure interaction occurs when the cochlea is stimulated by environmental sounds. This interaction elicits a transverse wave on the cochlear partition (CP), which travels from the base to the apex of the spiral-shaped hearing organ. Details of the vibratory mode shape of this traveling wave are directly related to the bending of hair bundles and, therefore, are very important to the understanding of the stimulation of inner hair cells (IHCs) and outer hair cells (OHCs), which lies at the very core of hearing transduction (1, 2). The tectorial membrane (TM) radial resonance is thought to be one potential bending mechanism of the stereocilia. The origin and evolution of ideas concerning the radial motion of the TM is discussed fully in Zwislocki's recent monograph (3). In short, new ideas were needed to resolve discrepancies in both amplitude and phase of the tuning of mechanical, neural, and cellular responses. TM radial motion, consequently, was incorporated in mathematical models of cochlear micromechanics by using the lumped-parameter approach (1, 3-9). Although indirect evidence inferred from frequency tuning of otoacoustic emissions (10, 11) and cochlear microphonics (12) lent support to the idea of radial TM motion, direct evidence awaited the measurements of Gummer et al. (13) and Hemmert et al. (14). These latter findings contrast those of Ulfendahl et al. (15), who reported almost no radial motion of the TM. Preliminary observations of TM motion in the hemicochlea preparation also do not yet show a significant TM radial motion (16,17). It seems that the local excitation in that preparation has difficulty in exciting a significantly propagating mode, which could affect the TM motion.Another controversy involves the transverse mode shape...