Problems in Coclear Micromechanics

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As part of a long-standing collaboration with NIDCD's Dr. Richard Chadwick and his group, we have been instrumental in a number of projects during the past years. Using primarily the atomic force microscope (AFM), we completed the measurements of the elastic modulus of the tectorial membrane (TM) whose role in hearing is critical, but whose exact mechanism of action remains controversial. It was found that the TM is radially inhomogeneous and that such a property is favorable to the shearing efficiency of the hair cells (the stimulatory action on the sensory cells). In addition, we participated in the final stage of the project on the effects of the cochlear spiral curvature. It was demonstrated that there is a strong correlation between the ratios of outer to inner radii of spiral curvatures and the low frequency hearing limit for all the mammals for which there is geometrical data available. This validated the hypothesis proposed in our previous theoretical work. Currently, our attention has been turned to the properties of the basilar membrane (BM). The BM divides the cochlear duct into two chambers with distinct ionic contents. In addition, the organ of Corti (OC) where the sensory cells reside is supported atop the BM. The pressure fluctuations caused by the incoming sound waves cause the BM to vibrate along with the sensory cells (inner and outer hair cells) which sense the motion and respond accordingly. The mechanics of the cochlear partition (BM, OC and bonny wall supports) vary slowly along the longitudinal cochlear axis. Some of this variation is due to the changing width of the BM along the cochlear axis, but it also known that the collagen fiber content and distribution also vary along the same axis. In addition, it is hypothesized that these collagen fibers are radially pre-stressed. The actual in-situ mechanics are probably determined by all these factors. We want to measure the material intrinsic mechanical properties of the BM at different radial and longitudinal locations. We excise BM's from guinea pigs and attach them to appropriately modified glass substrates that facilitate firm attachment. The AFM is used to image the topography and to measure the Young's moduli by analyzing force-distance data from indentation experiments.