rhodopsin /rōdop″sin/ [Gk, rhodon, rose, opsis, vision] , the purple pigmented compound in the rods of the retina, formed by a protein, opsin, and a derivative of vitamin A, retinal. Rhodopsin gives the outer segments of the rods a purple color and adapts the eye to low-density light. The compound breaks down when struck by light, and this chemical change triggers the conduction of nerve impulses. Brief periods of darkness allow the opsin and the retinal to reconstitute the rhodopsin, which accounts for the short delay a person experiences in adapting to sudden or drastic changes in lighting, as when moving out of bright sunlight into a darkened room or from darkness into bright light. Closing the eyes is a natural reflex that allows reconstitution of rhodopsin. Compare iodopsin. See also visual purple.
