MECHANICS OF THE COCHLEAR OUTER HAIR CELL

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The goal of this research is to investigate the mechanics of outer hair cell (OHCs) and to determine the contribution of specific organelles (subsurface cisterna, cortical lattice and cytoplasmic membrane of the lateral wall) and the cytoplasm to the mechanical properties of the cell. This knowledge is necessary for understanding the role of the OHC in cochlear mechanics, specifically in passive and active mechanical filtering. A specific goal of the project is to characterize the OHC axial force-displacement function; this function is crucial for evaluating the magnitude of the forces that outer hair cells exert on the surrounding structures as the result of OHC electromotility. Coordinated experimental and theoretical studies will be directed towards investigating the elastic and viscoelastic properties of the cell. High resolution video microscopy will be used to record the response of isolated guinea pig OHCs to micropipet aspiration and cell inflation through a patch pipet, and to observe thermally driven movement of beads in the cytoplasm. Displacements from each stimulus will be measured from time-lapse analysis of video images. Much of this research focuses on the micromechanics of the lateral wall. A variety of pharmacological treatments will be applied to modify the subcellular components that make up the lateral wall. The mechanical properties of untreated OHCs will be compared with those of pharmacologically treated cells. A theoretical model of the OHC as a fluid-filled viscoelastic cylindrical shell that is capped at both ends will be developed to analyze the experimental results. The model will relate the radial stiffness of the cell and its pressure-inflation response to the axial force-displacement function. The theoretical analysis will also be applied to analyze the results of direct measurements of dynamic force-displacement function during axial loading of isolated cells. The results will provide the axial force-displacement function under static and dynamic conditions and permit a precise determination of the OHC driving force on cochlear mechanics, which will help delineate the mechanical contribution of the OHC to both passive and active mechanical filtering. The results of these studies will quantify the mechanics of the OHC at the cellular and subcellular level and serve as a basis for research on the molecular mechanisms of OHC electromotility. Knowledge of the microstructural elements and their functional significance in OHC mechanics will enhance our understanding of how the OHC contributes to mammalian hearing in health and disease.