Apparent delay between light-induced receptor current and receptor potential in the Limulus ventral nerve photoreceptor.

We measured receptor potential (ReP) and receptor current (ReC) under voltage-clamp conditions alternatingly evoked by successive identical 10 ms light flashes in the ventral nerve photoreceptor of Limulus. At high stimulus intensities the apparent latency of the ReP is 4-15 ms shorter than that of the ReC. At lower light intensities the difference is even larger. High amplification reveals that the light response starts with two phases corresponding to two current components. The initial first current component rises linearly, is much smaller than the second one and therefore is not detected with standard amplification. This first component, which is more pronounced at low light stimulus intensities, is caused either by a displacement or by a weak ionic current.