MECHANICS OF THE ORGAN OF CORTI

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The objective is to understand the role of the mechanical activity of the inner ear in the transformation of sound into neural stimulation. This will be a continuation of mathematical studies involving appropriate techniques of wave analysis and the fundamental mechanics of fluids and solids. Fundamental to this work is the use of only physical parameters in the mathematical models. We are not interested in curve-fitting exercises. in the past period, a study of the micromechanics indicated that most popular models contradict the facts of anatomy and reasonable material properties. Strong support was found, however, for the behavior of cilia suggested by Pickles and co-workers, which is consistent with the measurements of ciliary stiffness of Flock and Strelioff, which are, in turn, consistent with known properties of actin. A conclusion is that the onset of buckling of the tip fibers occurs at displacements around 0.1 nm, which provides a strong mechanical nonlinearity at threshold levels of excitation. A model for the electrokinetic activity of the outer hair cells, motivated by measurements and suggestion by Brownell, indicates that this will be effective at high frequencies, well over 20 kHz. Thus a substantial basis for an active mechanism and nonlinearity appears to have been established. In the next period of effort, the OHC electrokinetic model along with the Davis battery model for the general electrical fields, will be incorporated into the three dimensional cochlear model. Very important is the phase relation between the basilar membrane displacement and shear force on the OHC cilia. We have previously developed realistic models for the mechanical distortion of the organ of Corti and for the oscillating and streaming flow of the fluid in the sub-tectorial membrane region. The coupling of these is necessary for the correct phase relations. The expectation is that the OHC electrophoresis will provide significant forces acting on the basilar membrane, and thus a physiologically vulnerable sharpening of the response of the basilar membrane and the IHC cilia. This will be computed for the straight cochlear model with rectangular cross section. Equally puzzling are the reasons for various features of the gross cochlear anatomy, which so far have eluded both simple and elaborate analysis. A substantial extension of the analysis capability to include the effect of curvature and discontinuities of both the elastic tissue and the cochlear fluids will be carried out, using the very large finite element approach.