The Effects of Intracellular Ca²⁺ on the Light Response and on Light Adaptation in Limulus Ventral Photoreceptors

Abstract

The photoreceptors of the horseshoe crab, Limulus, respond to light with a positive-going change in membrane voltage, (i.e. a depolarizing receptor potential). A dark-adapted photoreceptor responds to very dim illumination by producing a series of unitary events, called “quantum bumps”, each about 100 msec. in duration and 10 my in amplitude. Each quantum bump is thought to arise from the absorption of a single photon. In this low intensity range, the frequency of quantum bumps has been measured directly and was found to be nearly proportional to the number of incident photons. (Fuortes and Yeandle, 1964). At higher levels of illumination, individual bumps can no longer be resolved. The light response is characterized by an initial large depolarization, called the “transient”, followed by a smaller “plateau” phase which is maintained for the duration of the stimulus. The amplitude of this plateau phase increases roughly as the logarithm of the stimulus intensity over a very wide range of intensities.