Dark Adaptation

Dark adaptation

Rhodopsin, a biological pigment in the photoreceptors of the retina, immediately photobleaches in response to light. Visual phototransduction starts with the isomerizing of the pigment chromophore from 11-cis to all-trans retinal. Then this pigment dissociates into free opsin and all-trans retinal. Dark adaptation of both rods and cones requires the regeneration of the visual pigment from opsin and 11-cis retinal. Therefore, the time required for dark adaptation and pigment regeneration is largely determined by the local concentration of 11-cis retinal and the rate at which it is delivered to the opsin in the bleached rods. The decrease in calcium ion influx after channel closing causes phosphorylation of metarhodopsin II and speeds up the cis-retinal to trans-retinal inactivation. The phosphorylation of activated rhodopsin is mediated by recoverin. The regeneration of the photopigments occurs during dark adaptation albeit at markedly different rates. Rods are more sensitive to light and so take longer to fully adapt to the change in light. Rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about two hours. Cones take approximately 9–10 minutes to adapt to the dark. Sensitivity to light is modulated by changes in intracellular calciumions and cyclic guanosine monophosphate.