Force on inner hair cell cilia

The cochlea of the inner ear transforms the incoming sound pressure into neural excitation. Despite extensive experiments and modeling for the past century, understanding the behavior of the cochlea is far from complete. With an efficient program (Fast4) for shell of revolution structures, all mechanical (elastic) details of the curved cochlear cross-section and the organ of Corti can be computed. Based on the known values for the elastic moduli of the protein fibers and estimates for the geometry, the responses to point and pressure loads have been calculated which are reasonably close to the direct measurements. In the present work, the details of the inner hair cell are included, with a fluid gap between the tip of the cilia and the Hensen stripe of the tectorial membrane. A simple model for the near contact indicates nonlinear response similar to the intracellular recordings. This near contact is included into a more complete elastic model for the organ of Corti that includes three rows of cilia and tip links. The phase of the maximum tension of the tip link, which causes excitation of the cell, is computed for low frequencies for comparison to measurements. For low frequencies the fluid motion in the organ of Corti is approximately two-dimensional. The phase is found to be affected by: (1) geometrical difference between basal and upper turns of the cochlea, (2) initial gap spacing between the tip of the cilium and the Hensen stripe, (3) initial gap spacing between the tip of the cilium and the tectorial membrane, (4) presence of an electrode probe constraint on the motion of the inner hair cell, and (5) the stiffness of the tectorial membrane. The latter is the most significant. For a soft tectorial membrane, the excitation is generally between maximum velocity and displacement of the basilar membrane toward scala vestibuli. However, for a stiff tectorial membrane, the phase changes to an excitation with velocity and displacement toward scala tympani. Thus a possible mechanical reason is offered for the auditory nerve excitation in the base of the chinchilla cochlea for basilar membrane velocity toward scala tympani and in the middle and upper turns of the guinea pig cochlea, excitation for velocity toward scala vestibuli.