Hair cells in the bullfrog's sacculus, a vestibular organ sensitive to linear acceleration, show sensory adaptation: the response to a constant stimulus peaks near the stimulus onset, then decays. This has been studied in two different preparations. In an excised in vitro preparation of the saccular sensory epithelium, intracellular responses of hair cells to step deflections of their hair bundles were recorded. The second set of experiments was conducted in vivo using steps of vertical linear acceleration as stimuli. The hair cell response was recorded extracellularly in the form of the saccular microphonic potential. Both the intracellular response to direct hair bundle deflection and the extracellular response to acceleration adapted to a steady-state value within the first 100 ms following the step onset. In both cases, the response decline was largely due to a shift in the operating range of the cells in the direction of the constant stimulus. This shift occurred without significant change in dynamic range or in sensitivity within the operating range. Thus the hair cells appear to respond to static stimuli by resetting the bias point of the operating range in the direction of the stimulus.
The response of primary saccular neurons to acceleration steps also showed pronounced sensory adaptation. Comparison of the afferent activity and saccular microphonic potential suggests that adaptation of afferent responses to acceleration steps may be due largely to the adaptive operating range shift in the hair cell responses.
The adaptation of saccular neurons to acceleration steps may be explained by the following simple model. The acceleration step causes displacement of the saccular otolith and deflection of the underlying hair bundles. The hair cells respond initially to the displacement, then adapt (as observed in vitro), and this information is faithfully translated postsynaptically into afferent spike rate. However, the possibility exists that the in vitro and in vivo adaptive shifts in operating range are not the same process. In vivo, one cannot distinguish an operating range shift within the hair cells from one due to mechanical adaptation of the stimulus to the hair bundles.
