There are two distinct classes of image-forming photoreceptors in the vertebrate retina: rods
and cones. Rods are able to detect single photons of light whereas cones operate
continuously under rapidly changing bright light conditions. Absorption of light by rod-and
cone-specific visual pigments in the outer segments of photoreceptors triggers a
phototransduction cascade that eventually leads to closure of cyclic nucleotide-gated
channels on the plasma membrane and cell hyperpolarization. This light-induced change in …