Evidence of tectorial membrane radial motion in a propagating mode of a complex cochlear model

Abstract

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...

Altered Traveling Wave Propagation and Reduced Endocochlear Potential Associated with Cochlear Dysplasia in the BETA2/NeuroD1 Null Mouse

“…Recent modeling studies suggest that the cochlear coiling can impact cochlear micromechanics, particularly at the apex, by redistributing energy wave density (Cai et al 2005;Cai et al 2004;Manoussaki et al 2006). However, the effect of the altered cochlear geometry of the BETA2/NeuroD1 null in these models is unknown.…”

Organ of Corti Potentials and the Motion of the Basilar Membrane

“…Complex interactions between stereocilia and the tectorial membrane or fast structural alterations within the organ may also be important (Zwislocki and Kletsky, 1979;cf. Ulfendahl et al, 1995;Gummer et al, 1996;Fridberger et al, 2002a;Fridberger and Boutet de Monvel, 2003;Cai et al, 2004). Because of the lateral interference shown in Figure 8, comparisons between the BM and electric potential phases should be performed with caution.…”

Finite-element model of the active organ of Corti

“…Similarly, Nam & Fettiplace [22] developed an elastic micromechanical model of the organ of Corti to study force transmission and elastic wave propagation [23], in which OHC somatic and hair bundle active forces were both considered. Developments of the finite-element method and in computational power have allowed newer models to study wave propagation in the cochlear partition [23][24][25], mechanical effects of OHC somatic and hair bundle motility [22,26], fluid-solid interaction [27,28] and detailed motion patterns within the organ of Corti in response to static pressure loading [29]. The active amplification process within the cochlea has also been studied using either lumped-parameter models [30 -32], or simplified three-dimensional models [21,[33][34][35][36].…”