Nonlinear Dynamics of Auditory Hair Cells and Efferent Neurons

The first level of processing of auditory information in the inner ear is performed by hair cells – specialized cells that detect sound and transmit information to the brain. These cells show extremely high sensitivity, responding to movements smaller than a nanometer. Further, they show a broad dynamic range, for they are able to withstand much larger deflections without permanent damage.

It is still not understood how the auditory system achieves this sensitivity and how it can rapidly reduce it when exposed to a loud environment. It has been proposed that efferent neurons, which transmit information from the brain to the hair cells, serve a protective role that reduces hair cell damage induced by loud sound. However, thus far there is very little understanding of the physics behind this mechanism.

The investigators in this project will measure directly how stimulation of efferent neurons affects the sensitivity of the hair cell, both in terms of its ability to detect weak signals and to withstand strong stimuli. Their goal is to compare the experimental findings to models based on nonlinear dynamics to explain how efference may provide a dynamic feedback system that controls the sensitivity of hearing.

The inner ear contains an extremely sensitive mechanical detector, which routinely responds to nanoscale deflections. At the lowest perceptible levels, sound induces movements in the internal auditory organs that are as small as few Å. Meanwhile, the applied pressures that the auditory systems can withstand range over six orders of magnitude.

A dynamic gain control mechanism which would modulate the sensitivity of detection by a hair cell would serve to explain how a system could simultaneously achieve such remarkable sensitivity while maintaining the requisite robustness. The goal of this project is to determine whether efferent neurons constitute the dynamic feedback mechanism which tunes the nonlinear properties of the hair cell.

The investigators aim to explore how efferent activity affects the mechanical responsiveness of the sensory system by direct measurements performed in vitro on live and active hair cells under different levels of efferent activity. The investigators will compare the findings to theoretical predictions based on nonlinear dynamics theory, which has proposed that an internal control parameter self-tunes the system to the vicinity of a Hopf bifurcation.

This study is therefore complementary to prior work in the field, and will serve to bridge areas previously studied in vivo and in vitro, as well as in theoretical models. The project will also provide important mentoring opportunities, as the PI aims to recruit graduate students from underrepresented groups through participation in the APS Bridge program.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.