Transmission of radial shear forces to cochlear hair cells

Abstract

The radial shear forces transmitted to the cuticular plates of cochlear sensory cells are investigated by means of a mathematical model. The model is based on the fine anatomy of the hair cells and their supporting structures. It predicts that the inner hair cells are stimulated by a viscous force which is linearly proportional to, and in phase with, basilar membrane velocity. The outer hair cells are stimulated by a shear force which is linearly proportional to, and in phase with, basilar membrane displacement at low frequencies (e.g., less than 700 Hz for a midbasal turn cell). At higher frequencies, the shear force transmitted to the outer hair cells is a function of both displacement and velocity. This outer hair-cell shear force is at least an order of magnitude greater than the corresponding inner hair cell force for the frequencies and cochlear positions studied. These results compare favorably to recent cochlear microphonic data from normal and kanamycin-treated guinea pigs for frequencies less than 4000 Hz. Shear force amplitude envelopes are presented for 800- and 1600-Hz pure tones. The results suggest that the “velocity-sensitive” inner hair cells are candidates for “place-mechanism” receptors while the outer hail cells are candidates for “frequency-mechanism” receptors. The model predicts no mechanical fine tuning in the force transmission.