Micromechanics of the Mammalian Cochlea

Northwestern University, Evanston IL

Investigators

Abstract

The sense of hearing results from a series of complex events that transform acoustic pressure waves into the perception of sound. During normal hearing, sound energy is converted to mechanical energy by the middle ear, which then is converted to mechanical motion in the structures of the inner ear, or cochlea. Within the cochlea, the sensory cells-the inner and outer hair cells-are sensitive transducers that convert mechanical vibrations into electrical impulses in the auditory nerve. The resulting nerve impulses are sent to the central auditory nervous system, where they are interpreted and experienced as sound. The sensory hair cells play a critical role in the hearing process. Both inner and outer hair cells have stereocilia located on their apical membranes. Bending of these stereocilia results in a voltage change-or receptor potential-across the cell membrane. The receptor potential in an inner hair cell results in transmitter release from the basal end of the cell. In contrast, the receptor potential in an outer hair cell produces a somatic length change proportional to the membrane voltage change. These length changes in outer hair cells are thought to amplify the sound-induced movements of the cochlear partition. The macromechanics of the hair cell/basilar membrane/organ of Corti system are well described. What remains to be understood is the micromechanics, that is, the mechanical events that take place between displacement of the basilar membrane and deflection of the inner hair cell stereocilia. Using the recently developed hemicochlea preparation, it now is possible to study the micromechanics of the passive cochlea in a radial cross-section. The experiments proposed here are aimed at (1) clarifying the micromechanical processes that transform basilar membrane vibration into deflection of the inner hair cell stereocilia, (2) describing the micromechanical events that result solely from somatic length changes in outer hair cells, (3) investigating the effect of electrical stimulation on different cell types in the cochlea, (4) examining the interactions between somatic length changes of outer hair cells and the vibration of the basilar membrane known to occur in vivo, and (5) providing empirical data for finite-element models of the cochlea.