Understanding the micromechanics of the cochlea

Abstract

The function of the cochlea is to convert the sound waves that reach our ears into neural signals that can be interpreted by the brain. Its mechanics can be modelled as two fluid chambers separated by the basilar membrane (BM), on which the Organ of Corti (OoC) sits. The OoC is the cochlear sensory organ composed of different cells and tissues, including the reticular lamina, the tectorial membrane (TM), the outer and inner hair cells and other supporting cells. The process of transduction is achieved through two mechanisms: a passive and an active one. The passive mechanism is responsible for the tonotopic map of the cochlea, while the active mechanism determines the high sensitivity and frequency selectivity of our hearing. This thesis contributes to the understanding of the passive and the active mechanism of the cochlea. First an analytical solution to the equations governing the cochlear mechanics is derived, in the case of a passive, locally reacting BM, including fluid compressibility and viscosity. The solution is expressed in terms of only a few nondimensional parameters and it is shown that one of these, a phase-shift parameter, has the greatest influence on the cochlear response, as it determines the form of coupling between the fluid and the BM. In terms of the active mechanism, an existing elemental model is extended (e.g. Elliott and Ni 2018) to include the micromechanical structure of the OoC, as described in a detailed Finite Element Model (FEM) of the cochlea developed by Grosh’s laboratory (e.g. Sasmal and Grosh 2019). The use of an elemental method, instead of a detailed FEM, provides insight into the study of the active mechanism by dividing it into two terms; one due to the dynamics of the BM, including longitudinal coupling within the OoC, and one due to various types of fluid coupling. The effects of the various type of longitudinal coupling are discussed and it is shown that the most important one in determining the amplification and stability of the cochlea is the longitudinal coupling in the TM. To better understand the effect of longitudinal coupling, a method is developed to derive, from the elemental model, the wavenumber distribution associated with the different types of waves that can propagate in the cochlea. In particular, it is shown that, for a model with TM longitudinal coupling only, the wavenumber distribution associated with the main travelling wave is characterized by an imaginary part which is positive in a small region just before the frequency at which the BM peaks, indicating a distributed amplification of the response in this region.

More information

Identifiers

ORCID for Riccardo Marrocchio: orcid.org/0000-0001-5269-4620

ORCID for Ben Lineton: orcid.org/0000-0003-4784-7762

Catalogue record

Export record