Exploring the Spontaneous Oscillations of the Bullfrog Sacculus

The remarkable sensitivity of auditory detection is still not fully explained. In particular, under in vitro conditions, free-standing hair bundles of the bullfrog (Lithobates catesbeianus) sacculus have been known to exhibit spontaneous oscillations indicative of an energy-consuming amplifier. To study active motility in greater detail, we developed methods to simultaneously measure multiple hair bundles in parallel. We used a high-speed Complementary Metal Oxide Semiconductor (CMOS) camera to obtain the required temporal resolution (∼1 ms), on ∼10-20 hair cells in a single field of view. Simultaneous measurement allowed us to introduce two new capabilities in comparison to traditional methods in the field. Firstly, we could probe for correlations between pairs of cells. Secondly, we could acquire statistically significant, sample sizes.;We measured the statistical distribution of the oscillation periods of cells from different areas within the sacculus. and on different epithelia. Spontaneous oscillations exhibited a broad peak period 33 ms and showed a uniform spatial distribution across the sacculus.;We found that, innate bundle movements exhibit a. complex profile with multiple periodicities. As this complexity cannot be explained with the current theoretical models, we hypothesized it to result from an interaction between two coupled oscillators. In particular we posited that the profile was due to the interaction between mechanical oscillations and the electrical circuit composed of the ion channels in the hair cell soma. We examined the effect of modifying the somatic electrical circuit upon frequency and profile of active hair bundle movement. Inhibition of somatic ion channels using targeted neurotoxins and modified physiological solutions strongly affected the bundles' mechanical behavior, modifying the amplitude and the temporal characteristics of the oscillation profile.;These experiments revealed further unexpected effects. First, modifying ionic conditions in the solutions bathing the hair bundles vastly changed the oscillation profile of the bundles, further corroborating tight coupling between membrane potential of the cell and active motility of the stereocilia. Second, mechanical loading qualitatively changed the oscillation profiles, typically removing the multiple periodicities. These new characteristics of the spontaneous oscillations indicate a complex feedback system that needs to be incorporated into current models, to the enrich our understanding of the active process.