Cochlear basal and apical differences reflected in the effects of cooling on responses of single auditory nerve fibers.

Responses of single auditory nerve fibers in the Mongolian gerbil were examined before and during rapid, moderate cooling of the cochlea. Reducing cochlear temperature from 35-39 degrees C to 29-32 degrees C led to stable, reversible changes in spontaneous firing rates (SRs), and responses to tonebursts, as characterized by frequency tuning curves and rate-versus-intensity curves. The nature and extent of effects of cooling were strongly linked to characteristic frequency (CF). Rate thresholds at the CF were increased by 0-15 dB for fibers with CFs below 8 kHz, and by 10-30 dB for higher CFs. Although SRs were generally reduced, the percent reduction in SR was striking CF dependent. For fibers with CFs below 4 kHz, the reduction did not exceed 50% of the initial SR. For higher CFs, the reduction was always greater than 50%. The effects of cooling on intensity curve shape differed qualitatively for fibers with CFs below and above 3-4 kHz. The slope of the curve was reduced by an average of 50% for lower CFs, but on average was unchanged for higher CFs. Cooling-related increases in CF threshold probably reflect impairment of active mechanical processes. The CF dependence of these increases suggests either that active mechanical processes are more impaired in the cochlear base relative to the apex, or that they play a more crucial role in determining sensitivity in the base. The CF-dependent changes in SR and in the shape of rate-intensity curves caused by cooling correspond to an enhancement of basal/apical differences seen at normal temperatures. They are best explained by longitudinal gradients in the properties of the inner hair cells and their afferent synapses. Basal and apical differences in the distribution of SRs and in supra-threshold response properties suggest that stimulus coding strategies differ between low and high frequency regions of the cochlea.