Bump latency distribution and bump adaptation of limulus ventral nerve photoreceptor in varied extracellular calcium concentrations

Abstract

1) Bumps were recorded as voltage signals following dim flashes of light. Bump amplitude and width did not much depend upon external Ca2+-concentration. However, the distribution of bump latencies was strongly shifted to longer latencies and broadened more than two-fold when the external Ca2+-concentration was lowered from 10 mmol/l to 0.25 mmol/l. Raising the external Ca2+-concentration to 40 mmol/l had the opposite effect. A preadapting light flash caused shortening and narrowing of bump latency distribution similar to the effect of raised external Ca2+-concentration. 2) The bump amplitude is not correlated with the length of the latent period of the bump, indicating that the amplification processes determining the bump size are distinctly different from those which determine the latent period. 3) In a 10 s light/10 s dark cycle, a weak preadapting light flash slightly enlarges, a stronger flash diminishes the average bump amplitude (bump adaptation). The Calcium dependence of bump adaptation which was studied for external Ca2+-concentrations ranging from < 1 nmol/l to 100 mmol/l is relatively weak: Lowering the external Ca2+-concentration to < 1 nmol/l reduces, raising it to 40 mmol/l Ca2+ slightly intensifies the diminution of bump size due to light adaptation. 4) The average amplitude of the “light bumps” (recorded during the 10 s light period) is larger than that of the “dark bumps” (recording during the dark period), because the light-evoked bumps are on the average larger than the spontaneously generated bumps. 5) A preadapting light flash increases the rate both of light-evoked, and of spontaneous bumps.