Intracellular Ca modulates sensitivity and time scale in Limulus ventral photoreceptors

Abstract

IN a photoreceptor cell, light induces a change in membrane voltage (the ‘receptor potential’). The sensitivity of the photoreceptor (defined as the amplitude of the receptor response per unit of stimulus energy) depends on its history of illumination. After a cell has been stimulated by light, its sensitivity becomes reduced (that is, the cell becomes ‘light adapted’). Fuortes and Hodgkin¹ reported that previous illuminations not only reduced the sensitivity of photoreceptors in Limulus lateral eye but also decreased the latency of the responses to subsequent stimuli. They found that the reduction in latency (referred to as a shortened ‘time scale’) was greater with brighter adapting lights, and that the shortening of the time scale was much smaller proportionally than was the reduction in sensitivity. Similar changes in time scale also have been found in Limulus ventral photoreceptors^2,3.