Retinal photoreceptor cell. Illustration of a photoreceptor cell in the eye's retina responding to light. The banded areas are the location of the photoreceptor rhodopsin that contains the chromophore chemical retinal. In darkness (left), the cation channels are open. When light impacts rhodopsin (red, inset at upper right), it changes shape and interacts with the G-protein transducin (orange) and activates a phosphodiesterase enzyme (yellow). This closes the cation channels, triggering inhibitory neurotransmitter activity (bottom left and bottom centre) that sends signals to the brain. For this artwork with labels, see C023/8839.