The Light Energy Dependence of the Limulus Photoreceptor Current in Two Defined States of Adaptation

The light energy dependence of the Limulus ventral nerve photoreceptor current was deter­ mined for two reproducible states o f adaptation. In the double logarithmic plot the energy dependence of the current amplitude and that of the current-time integral, which represents the total charge transport through the membrane during the response, show several sections with different slopes: 1) a linear section (slope about 1) 2) a supralinear section (slope 2 4 ) 3) a sublinear section (slope < 0.5) and 4) a second sublinear section (slope > 0.5, only in current-time integral curve). A significant deviation from slope 1 indicates that the single events (bumps) which constitute the macroscopic receptor current, are not independent o f each other in size and/or number. It may indicate a positive or negative cooperativity. Light adaptation reduces the size of the receptor current and shifts the response vs. energy characteristics towards higher light energies. For lightand dark-adapted cells the transitions between the sections occur at same amplitude or area values, only the light energy values are different. The energy dependence o f the time parameters, which characterize the shape of the light response, shows the same partition in 4 sections as the receptor current. The time parameters are shortened whith increasing stimulus energy and light adaptation. The amount of the shortening is greater in the supralinear section of the amplitude or area vs. energy characteristics than in the other sections (except for latency and tim e-to-peak with greatest shortening in the linear or first sublinear section). Section 4 is correlated with a new increase of response duration and simultane­ ously with the appearance of the second response com ponent. l he results can be interpreted sufficiently in terms of the bump-speck-model and of an auto­ matic gain control mechanism.