Analysis of the microphonic potential of the bullfrog's sacculus

The sensory epithelium from the bullfrog's sacculus was mounted between two chambers and stimulated by moving the otolithic membrane with a piezoelectric stimulator. The evoked response was measured as the transepithelial “microphonic” potential or, when the epithelium was voltage clamped, as the microphonic current. Microphonic responses were similar to those recorded in other preparations: the whole organ produced a “2f” response (i.e., a response of a frequency twice that of the stimulus) which could be changed to a single-polarity response by stimulating cells of a single polarity; the response saturated asymmetrically with displacement, producing a rectification; and the amplitude declined at high and low frequencies. To determine the cellular elements responsible for generation of the microphonic potential, the equivalent circuit of the epithelium was estimated from morphological and electrophysiological data, and responses to step displacement stimuli were recorded. Four elements in particular shape the microphonic potential: the complex impedance of the extracellular current path, the saturating displacement-conductance curve, an adaptation mechanism which shifts that curve, and a voltage-dependent K+ conductance in the basolateral hair cell membrane. A quantitative model incorporating these elements accurately reproduces the observed responses.