Nonlinear Dynamics of Sound Detection in the Auditory System

The auditory system can detect displacements as small as 3 Å and frequencies as high as 200 kHz. Significant evidence indicates that it achieves this sensitivity by active amplification performed by the sensory cells. Furthermore, its highly nonlinear response has characteristics that are well captured by equations based on the Hopf bifurcation. The study of auditory detection therefore poses a problem at the intersection of nonlinear dynamics with out-of-equilibrium physics. To understand the mechanisms behind this remarkable sensitivity, we are exploring the dynamics of hair cells – the sensory receptors of the auditory system. In this study, we develop a robust in vitro preparation of an auditory organ (amphibian papilla) and observe spontaneous and driven oscillations of hair bundles. The preliminary data has shown a large variation in the character of spontaneous oscillations in amphibian papilla, including a range of amplitudes and frequencies, varying levels of noise, bursting behavior, and other phenomena, indicating that multiple bifurcations may characterize the underlying dynamics. Measurements obtained from individual hair cells show they phase lock to extremely weak signals and exhibit a compressive nonlinearity similar to in vivo observations. In the future, we plan to further explore the nonlinear dynamics of the auditory system, as well as to address the frequency tuning and tonotopy of amphibian papilla.