“Prebumps”: Evidence for Double-H its at Functional Subunits in a Rhabdomeric Photoreceptor

In fly photoreceptors the latency of “bumps” due to single photon absorption is in the mean 60-70 (± 20) ms (10 °C). With increasing stimulus intensities first the amplitude of the receptor potential increases without shortening of the latency; with still higher intensities the latency is reduced also. The first reduction occurs if neighboured microvilli are simultaneously hit by one light quantum each or if individual microvilli are hit by two quanta. Such “double hits” lead to a discrete reduction in latency which is manifested as a “prebump”.