Micro-fluidics in the Inner Ear
The hair bundles in situ are covered by an overlying acellular matrix. In most in vitro experiments, the overlying matrix is removed to gain an access to individual hair bundles. Then a fluid-jet or glass probe is used to stimulate the hair bundle and electrical and mechanical responses are measured. It is unclear how such different stimulus methods affect the response of the hair cell. This project aims to determine how inner hair cells, fluid flow-stimulated receptor cells, respond to realistic fluid stimulation.
There are two distinct types of hair cells in the mammalian cochlea. They are inner hair cells and outer hair cells. Unlike the outer hair cells that are attached to the overlying tectorial membrane, the inner hair cell bundles stand freely in the endolymph fluid. The inner hair cells are stimulated by fluid flow. As a result, the inner hair cell bundles are subject to Brownian noise which must be detrimental to sensing faint sounds. Surprisingly, the inner hair cells detect small signals below thermal noise level (< 1 nm basilar membrane vibration). Understanding the fluid mechanics of inner hair cells is important in figuring out why mammals began to utilize the noisy fluid as a stimulator. Nonetheless, measured data on how the inner hair cell bundle interacts with its ambient fluid is very rare. This has led to contradictory argumentsâsome people think that the inner ear overcomes the thermal noise, while other people argue that the thermal noise helps enhance the sensitivity of hearing. These arguments might be resolved by combining experiments with theoretical studies. Prior theoretical studies (e.g., Nam et al., 2005) tended to extrapolate their fluid mechanics from macro mechanics, which invites further validations. We investigate the micro-fluidics of the inner hair cell bundle through experimental and computational methods.