Phototransduction in Rods and Cones, and Beyond
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Retinal rods and cones hyperpolarize to light as a result of a light-triggered cyclic guanosine monophosphate (cGMP) signaling pathway. In this phototransduction process, light activates rhodopsin, which, via the G protein transducin, stimulates a phosphodiesterase to increase cGMP hydrolysis, causing the intracellular free cGMP level to fall and cGMP-gated, nonselective cation channels that are open in darkness to close, hence producing the hyperpolarization. This closure of the cGMP-gated channels, which are Ca2+ permeable, leads to a decrease in the free Ca2+ concentration in the outer segment. This Ca2+ decrease in the light activates multiple negativefeedback mechanisms to produce light adaptation.