Chaotic Dynamics Enhance the Sensitivity of Inner Ear Hair Cells

Hair cells of the auditory and vestibular systems can detect sounds that induce sub-nanometer vibrations of the hair bundle, below the stochastic noise levels of the surrounding fluid. Hair cells of certain species are known to oscillate without external stimulation. These spontaneous oscillations are believed to be a manifestation of an underlying active mechanism and may play a role in signal detection. We previously demonstrated experimentally that the spontaneous oscillations exhibit chaotic dynamics. We propose that the instabilities giving rise to chaotic dynamics are responsible for the extreme sensitivity of hair cells. We will present experimental measurements of spontaneous and driven hair bundle oscillations. By varying the conditions of the surrounding fluid, we were able to modulate the degree of chaos observed in the hair cell dynamics. We found that the hair bundle is most sensitive to small-amplitude stimulus when it is poised in the weakly chaotic regime. Further, we found that the response time to a force step decreases as the level of chaos is increased. These results agree well with our numerical simulations of a chaotic Hopf oscillator and suggest that chaos may be responsible for the extreme sensitivity and temporal resolution of hair cells.