Paul A. Fuchs
Dept. of Physiol., Univ. Colorado Sch. Med., 4200 E. 9th Ave., Denver, CO 80262
Efferent neurons of the superior olivary complex synapse with auditory hair cells of vertebrates, in particular on outer hair cells (OHCs) of the mammalian cochlea. Efferent activity reduces the sensitivity and frequency selectivity of cochlear afferent fibers, presumably by inhibition of the OHCs. This inhibition is thought to be produced by the release of acetylcholine (ACh) from the efferent neurons. Electrical recordings from hair cells in various vertebrate species have shown that efferent activity, or exogeneously applied ACh, hyperpolarizes the cell by increasing the potassium conductance of the basolateral membrane. This potassium conductance is calcium dependent and so appears to be secondary to an initial effect of ACh to increase the cytoplasmic calcium activity. The increase in calcium-dependent potassium conductance moves the membrane potential toward the potassium equilibrium potential, near -80 mV. That hyperpolarization, in combination with the shunting effect of the underlying conductance increase, will minimize sound-evoked depolarization. OHCs are thought to enhance cochlear vibration by a voltage-dependent mobility elicited by depolarization. The effect of efferent inhibition may be to prevent the participation of OHCs as cellular amplifiers of the traveling wave, and thereby reduce cochlear sensitivity.