An analysis of the micromechanics of the organ of Corti: significance of microfluid flow

We exploit novel modeling techniques to investigate the micromechanics of the organ of Corti (OC). Our first aim was to confirm that the tunnel of Corti (ToC) can sustain fluid wave propagation, as this may provide physiological grounds for the explanation of the cochlear amplifier by non-classical cochlear models. The experimental evidence is that OHC contraction induces oscillatory flow in the tunnel of Corti. The question we address is whether this oscillatory flow is produced by an actual fluid wave traveling in the ToC or is merely an oscillating flow with no spatial phase change. We hypothesize that the pillar cells must not present a significant barrier to flow into the tunnel of Corti if the latter can support sustainable traveling fluid waves in response to outer hair cell motion. We use both analytical and numerical models to investigate this hypothesis. The numerical model consists of a realistic three dimensional finite element model of ToC in the middle turn of the gerbil cochlea. The analytical estimates and numerical calculations give similar estimates for the impedance of the pillar cells to fluid flow into the tunnel of Corti. We conclude that the row of pillar cells does not significantly impede fluid exchange between ToC and the space of Nuel. The wavelength of the resulting fluid wave launched into the tunnel is 0.9 mm, which is similar, but somewhat larger, than the wavelength estimated for the classical traveling wave. We also found that this fluid wave propagates at least 1 wavelength before being significantly attenuated. Our results support the hypothesis that there is an additional source of longitudinal coupling, provided by the tunnel of Corti, as required in non-classical models of the cochlear amplifier. Our second aim was to assess the influence of the interstitial microfluid flow on the micromechanics of the organ of Corti. For this purpose, a finely resolved short section of the cochlea was simulated to study the fluid-elastic interaction. A modal analysis of the section was performed with and without cochlear fluid and the modal results were interpreted as the limiting case of wave propagation. The analysis results suggest that: (1) The long wave response is similar to the classical OC motion, with both arcuate and pectinate regions of the basilar membrane moving in phase and a pivoting of arch of Corti about the inner pillar foot. In this mode, however, the two inner rows of OHCs bend radially in phase while moving out of phase with the outermost row of OHCs. (2) The resonant response of the short cochlea section is characterized by a complex fluid-structure interaction mode, where the two regions of the basilar membrane move out of phase and fluid is moved between the tunnel of Corti, the interstitial spaces between the OHCs, and the outer tunnel. The outer hair cell rows move all in phase following the radial flow direction. A significant fluid motion was observed between the cylindrical cellular structures of the OC as the result of the structural displacement. This indicates that the flow of interstitial fluid avoids overpressuring the OC, and is responsible for driving cellular movements. (3) In both the long and short wave cases, fluid is squeezed radially, back and forth in the subtectorial space as a result of the bending motion of the reticular lamina in the region above the OHC heads. This causes a compression and expansion of the subtectorial space. This motion is consistent with experimentally observed motion during electrical stimulation experiments. Finally, we have developed a method to analyze periodic fluid-elastic waveguides with complex geometries, such as the cochlea. The method is a hybrid numerical-analytical based on the Floquet theory. We apply the method to two and three dimensional waveguides. We present and discuss the details of the method