Efferent desensitization of the responses of turtle single auditory nerve fibers

Abstract

Extracellular recordings were made from single auditory afferents and frequency-threshold curves determined in the isolated half-head of Pseudemys scripta. Constant current shocks, 20–500 μA, in trains at 250 s1, delivered at the junction between the anterior and posterior roots of the statoacoustic nerve, resulted in a prolonged elevation of the thresholds to pure tones, and a broadening of the frequency-threshold curves of the auditory afferents. As an example, in one sharply tuned afferent, a threshold elevation of 41 dB at the characteristic frequency (CF, 700 Hz) was accompanied by a reduction in Q10 dB from 4.7 to 0.9. The extent of desensitization increased steeply with number of shocks, and a train of seven shocks could elevate the threshold at the CF by 80 dB. The desensitization could be demonstrated in a majority of recordings, and observed in the absence of antidromic firing of the afferent. We suggest that a component of this efferent effect may result from a detuning of the hair cell's electrical filter [A. C. Crawford and R. Fettiplace, J. Physiol. 312, 377–412 (1981)].