Phototransduction

Phototransduction is the process through which light is converted into electrical signals in the rod cells, cone cells, and photosensitive ganglion cells of the retina of the eye. This biological conversion of a light photon into an electrical signal in the retina is known as the visual cycle. Phototransduction occurs via G-protein coupled receptors called opsins which contain the chromophore 11-cis retinal.

Phototransduction is a complicated process. To understand it, one must have an understanding of the structure of the photoreceptor cells involved in vision: the rods and cones. These cells contain a chromophore bound to a cell membrane protein, opsin. Rods deal with low light level and do not mediate color vision. Cones, on the other hand, can code the color of an image through comparison of the outputs of the three different types of cones. Each cone type responds best to certain wavelengths, or colors, of light because each type has a slightly different opsin. The three types of cones are L-cones, M-cones and S-cones that respond optimally to long wavelengths (reddish color), medium wavelengths (greenish color), and short wavelengths (bluish color) respectively.