EFFERENT CONTROL OF COCHLEAR INNER HAIR CELL RESPONSES IN THE GUINEA-PIG

The efferent crossed olivocochlear bundle (COCB) was electrically stimulated during intracellular recordings from cochlear inner hair cells in anesthetized guinea-pigs. The effect of COCB stimulation was to decrease the magnitude of the inner hair cell depolarizing component (DC) and alternating component (AC) receptor potentials evoked by tone bursts at the characteristic frequency. At low pressure levels, the decrease in receptor potentials caused by COCB stimulation was equivalent to decreasing the sound intensity by 9-24 dB. COCB stimulation usually had a similar effect on the compound action potential of the auditory nerve. The change in inner hair cell membrane resistance during moderate-level sound was measured for sound alone and when sound was accompanied by COCB stimulation. Sound alone produced a greater membrane resistance change than sound with COCB stimulation, in proportion to the DC receptor potential during the same conditions. The time course of the COCB effect was slow, with 50-250 ms required for a full effect and for recovery. The effects of varying the frequency and voltage of electrical stimulation were similar for the DC receptor potential and for the compound action potential. For sounds of high level and for frequencies well below the characteristic frequency, COCB stimulation was less effective in reducing receptor potentials. Frequency tuning curves for the DC receptor potential taken during intervals of continuous COCB stimulation showed decreases in sensitivity primarily in the tip segment of the tuning curve. When no sound stimulus was present, the resting membrane potential was relatively unaltered during COCB stimulation. The resting membrane resistance did not change during COCB stimulation. Since the COCB innervates mainly the outer hair cells, these results strongly suggest that changes in outer hair cell activity can influence the receptor potentials of inner hair cells and thus alter the transmission of acoustic responses to the CNS.