Efferent desensitization of auditory nerve fibre responses in the cochlea of the turtle Pseudemys scripta elegans.

Abstract

Extracellular recordings were made from single auditory afferents in the isolated half-head of the turtle, and changes in their acoustic sensitivity was examined following electrical stimulation of the efferent fibers to the basilar papilla. Short trains of efferent shocks caused a prolonged elevation of the pure tone thresholds of the auditory afferents and an abolition of their spontaneous activity. These changes could be demonstrated in a majority of recordings and without antidromic firing of the afferent. The amount of desensitization increased steeply with shock number and a train of 10 closely spaced shocks could elevate the threshold at the most sensitive or characteristic frequency by 4 orders of magnitude. Desensitization also occurred with single efferent shocks at repetition frequencies exceeding 25/s. Discharge rate vs. sound pressure functions were constructed for a number of afferents. The maximum slope of the functions, and the saturated firing rates were both reduced by efferent stimulation; there was also an over-all shift of the rate-intensity function to higher stimulus levels. Such effects would enable the afferent to signal a wider range of sound pressures. Efferent stimulation caused a broadening of the afferent frequency-threshold curves by removal of the narrowly-tuned region around the characteristic frequency. The loss in tuning and concomitant improvement in temporal resolution may be a functionally important consequence of efferent action.