Power Dissipation in the Mammalian Cochlea and Consequences on the Frequency Selectivity

Thesis (Ph. D.)--University of Rochester. Department of Mechanical Engineering, 2017.The dimensions of the structures in the cochlea, vibrating in viscous fluid, range from a few hundred micrometers to just a few micrometers. Flow on such a small scale is expected to be dominated by the viscosity. Because the viscous dissipation is considered to be detrimental to the function of sound amplification and frequency tuning, the cochlea is believed to use cellular actuators to overcome the dissipation. Compared to the extensive investigations on the cellular actuators, the dissipating mechanisms have not been given appropriate attention. Many theoretical studies used an inviscid fluid approximation, and lumped the viscous effect to viscous damping of the cochlear partition. Others completely neglected viscous dissipation in the cochlear partition, but considered fluid viscosity. In this work, power dissipation in the mammalian cochlea was investigated at a tissue and a whole-cochlear level. Tissue level investigation was performed using a two-dimensional fluid-structure interaction model of the subtectorial space. The inner hair cell hair bundle stiffness and the size of the gap between the inner hair cell stereocilia tip and the tectorial membrane were identified as two main parameters affecting the power dissipation. A closed-form equation for the hydrodynamic forces from the subtectorial space was suggested to facilitate implementation into a full cochlear model. Whole-cochlear level power dissipation was investigated using a computational model of the cochlea incorporating viscous fluid dynamics, organ of Corti detailed micro-structural mechanics, and electro-physiology of the outer hair cells. The modelling results suggest that the major energy dissipation occurs within the organ of Corti complex, as opposed to within the viscous fluid in the cochlear scalae. A novel claim resulting from the investigation is that power dissipation in the cochlea can be beneficial for high quality tuning. The dissipation enhances the tuning quality by limiting the spread of energy from the amplification site in the cochlear partition