Photoreceptors are the cells in retina that respond to light. Rods are the predominant photoreceptor overall. Cones are sparse in comparison to rods. The rods and cones contain photopigment, which in rods is termed rhodopsin and in cones is termed photopsin. Rhodopsin is a combination of the proteins scotopsin and retinal. The activated rhodopsin then activates a G-protein called transducing.
The activation of phosphodiesterase, results in cGMP being converted to GMP and therefore the cGMP-gated Na+ channels are closed. The cell becomes hyperpolarised and there is a reduction in the release of photoreceptor neurotransmitters. The sensitivity of the photoreceptors in reduced, and this process is called light adaptation.