Dependence of bump rate and bump size in Limulus ventral nerve photoreceptor on light adaptation and calcium concentration

Abstract

Bumps were recorded in Limulus ventral nerve photoreceptor as deflections in membrane voltage during 10 s illuminations by dim light which were repeated every 20 s. The bump amplitude vs frequency distribution and its dependence on the intensity of a preadapting light flash are described. Light adaptation which diminishes the average bump amplitude alters the character of the bump amplitude distribution from a curve with a convex region to a continuously falling concave curve. Weak light adaptation can increase frequency (and height) of the bumps elicited by constant stimuli. Raising the external Ca2+-concentration from 10 to 40 mmol/l augments the effect of a preadapting light flash in diminishing the bump amplitudes and also increases the bump frequency. The results are consistent with the assumptions