Efferent control of hair cells mechanically coupled by artificial membranes

The efferent system has been proposed to play a vital function in auditory and vestibular systems, by protecting the sensory hair cells from injury and preserving signal detection sensitivity. We report that the activation of efferents has strong modulatory effects on systems of coupled hair cell bundles. In this study, we use direct electrical stimulation of efferent neurons to probe its effects on the hair cells' internal dynamics, by means of optically tracking the hair bundle motility. In vivo, hair bundles of auditory and vestibular epithelia are connected via overlying membranes; to address this physiological characteristic, we investigate the impact of efferent activity on the collective response of coupled hair bundles. We use a preparation from the American bullfrog sacculus which preserves the active motility of hair bundles, and achieve inter-cell coupling by connecting the cells to artificial mica structures. We found that efferent stimulation impacts hair-bundle dynamics, affecting the amplitude, frequency, and temporal profile of spontaneous oscillations, and altering the dynamical state of the system. Furthermore, efferent activation decreased synchronization among coupled hair bundles, suggesting a mechanism by which neural modulation may reduce the overall sensitivity of the system.