Spontaneous Hair Bundle Oscillations in the Bullfrog Amphibian Papilla

Hair bundles, the mechanosensitive organelles of the inner ear, are known to behave as active oscillators. We investigated the nonlinear dynamics of spontaneous hair bundle oscillations in the bullfrog's amphibian papilla (AP), its primary auditory organ, whose mechanical behavior had not been previously characterized. Using a novel ex vivo preparation and high-speed imaging, we analyzed 238 bundles. The observed dynamics were highly complex, with oscillations classified into three distinct regimes: regular, bursting, and spiking. These behaviors suggest a rich underlying dynamical system with signatures of bifurcations and potential for chaotic dynamics. Oscillation frequencies ranged from 1 to 90 Hz, with most cells oscillating below 40 Hz. Phase-space analysis revealed circulating probability fluxes, demonstrating a violation of detailed balance and confirming the active nature of these oscillations. These results establish the amphibian papilla as a valuable system for exploring nonlinear and nonequilibrium dynamics in auditory mechanics, complementing studies of the more extensively characterized saccular hair cells.