Rhodopsin vision in the dark. Several closely related opsins differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have eight other opsins besides rhodopsin, as well as cryptochrome (light-sensitive, but not an opsin). The photopsins are found in the cone cells of the retina and are the basis of color vision. They have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light. The remaining opsin, melanopsin, is found in photosensitive ganglion cells and absorbs blue light most strongly. In rhodopsin, the aldehyde group of retinal is
Retina the retina, which in its entirety comprises ten distinct layers, including an outer layer of pigmented epithelial cells. The only neural cells that are directly sensitive to light are the photoreceptor cells, which are of two types: rods and cones. Rods function mainly in dim light and provide black-and-white vision while cones are responsible for the perception of colour. A third type of photoreceptor, the photosensitive ganglion cells, is important for entrainment and reflexive responses to the brightness of light. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses that are sent