Receptor activity in the utriculus of the sprat

Abstract

Electrical responses were obtained from the utriculus of the sprat (Sprattus sprattusL). during stimulation by bursts of oscillatory pressures ranging in frequency from 2·5 to 880 Hz and in amplitude from 0·12 to 220 N m−2(28–93 dB above threshold). These responses had double the frequency of the stimulus and we argue that they were given by two major groups of utricular receptors, one group (decompression or d-receptors) responding to movements of the utricular floor away from the fenestra of the auditory bulla, the other group (compression or c-receptors) responding to movements of the utricular floor towards the fenestra. These groups were identified with oppositely oriented groups of utricular hair cells coupled mechanically by the rigid base on which they lie. Responses grew with increasing stimulus strength over a range of over 100 dB, the slope of the log-log relation decreasing with the larger stimuli. Above 40 Hz the delays between responses and mechanical displacements were very small and the amplitude of response depended only on the amplitude of displacement and not on frequency. Below 40 Hz the sensitivity declined more steeply with reduction of frequency than would be expected from the mechanical responses of the utricular floor. When stimulating frequencies were mixed the responses were not those expected on the hypothesis that different groups of receptors were excited by the different frequencies but those expected if all displacements of the utriculus towards or away from the fenestra from a rest position gave responses in either one or other of two major groups of receptors. The response to mixed frequency pressure waves was such that high peaks were exaggerated and information about polarity preserved. The response of the utriculus to the first cycle of a burst of sinusoidal pressure stimuli was usually larger than that to subsequent cycles.