Efferent Control of Hair Cell and Afferent Responses

40 The sensations of sound and motion generated by the inner ear are controlled by the brain 41 through extensive centripetal innervation originating within the brainstem. In the semicircular 42 canals, brainstem efferent neurons make synaptic contacts with mechano-sensory hair cells and 43 with the dendrites of afferent neurons. Here, we examine the relative contributions of efferent 44 action on hair cells and afferents. Experiments were performed in-vivo in the oyster toadfish, 45 Opsanus tau. The efferent system was activated via electrical pulses to the brainstem, and 46 sensory responses to motion stimuli were quantified by simultaneous voltage recording from 47 afferents and intracellular current and/or voltage-clamp recordings from hair cells. Results 48 showed synaptic inputs to both afferents and hair cells leading to relatively long latency 49 intracellular signaling responses: excitatory in afferents and inhibitory in hair cells. Generally, 50 the net effect of efferent action was an increase in afferent background discharge and a 51 simultaneous decrease in gain to angular motion stimuli. Inhibition of hair cells was likely the 52 result of a ligand-gated opening of a major basolateral conductance. The reversal potential of the 53 efferent-evoked current was just below the hair cell resting potential thus resulting in a small 54 hyperpolarization. The onset latency averaged ~90ms and latency to peak response was 15055 400ms. Hair cell inhibition often outlasted afferent excitation and in some cases latched hair cells 56 in the “off” condition for over a second following cessation of stimulus. These features endow 57 the animal with a powerful means to adjust the sensitivity and dynamic range of motion 58 sensation. 59 60 61 62 63 64 65 66 Efferent Control of Hair Cell and Afferent Responses