Recoveries of whole-nerve AP thresholds, amplitudes and tuning curves in gerbils following noise exposure

Abstract

The recoveries of whole-nerve action potential (AP) thresholds, AP amplitudes and AP tuning curves in gerbils were monitored following two weeks of exposure to band-pass noise at 85 dBA. Recordings were made by means of electrodes chronically implanted in the subjects' bullas. The noise exposure caused threshold elevations in all of our subjects, with the greatest shifts occurring an octave or more above the 2 kHz upper cutoff frequency of the noise. The magnitudes of the shifts varied greatly (up to 26 dB) across subjects. Thresholds of animals with the smallest initial loss of sensitivity returned to pre-exposure values within 16 days, while those of animals with greater initial losses remained elevated beyond 16 days. AP amplitudes and AP tuning curves (APTCs) were most affected at frequencies where the initial threshold shifts were greatest. At these frequencies AP amplitudes were reduced, and APTCs showed broadened tips, reduced tip-to-tail ratios, and in some cases a shift in the frequency of the tip. These effects did not necessarily reverse with threshold recovery, suggesting that AP amplitudes and AP tuning curves are more sensitive indices of acoustic injury than are AP thresholds.