The Impact of the Fluid-Solid Coupling Behavior of Macro- and Micro-Structures in the Spiral Cochlea on Hearing.

The cilia of the outer hair cells (OHCs) are the key micro-structures involved in cochlear acoustic function, and their interactions with lymph in the cochlea involve complex, highly nonlinear, coupled motion and energy conversions, including macroscopic fluid?solid coupling. Recent optical measurements have shown that the frequency selectivity of the cochlea at high sound levels is entirely mechanical and is determined by the interactions of the hair bundles with the surrounding fluid. In this paper, an analytical mathematical model of the spiral cochlea containing macro- and micro-measurements was developed to investigate how the phonosensitive function of OHCs' motions is influenced by the macro- structural and micro-structural fluid?solid coupling in the spiral cochlea. The results showed that the macro-structural and micro-structural fluid?solid coupling exerted the radial forces of OHCs through the flow field, deflecting the cilia and generating frequency-selective properties of the micro-structures. This finding showed that micro-structural frequency selectivity arises from the radial motions of stereocilia hair bundles and enhances the hearing of sound signals at specific frequencies. It also implied that the macro-structural and micro-structural fluid?solid couplings influence the OHCs' radial forces and that this is a key factor in the excitation of ion channels that enables their activity in helping the brain to detect sound.