Retinal Photoreceptors in Humans

In humans, the visual system uses millions of photoreceptors to view, perceive, and analyze the visual world. Moreover, the photoreceptor is the only neuron in humans capable of phototransduction (with an exception being the recently discovered photosensitive ganglion cell). All photoreceptors in humans are found in the outer nuclear layer in the retina at the back of each eye, while the bipolar and ganglion cells that transmit information from photoreceptors to the brain are in front of them. This arrangement requires two specializations: a fovea in each retina (for high visual acuity) and a blind spot in each eye, where axons from the ganglion cells can go back through the retina to the brain. Normalized typical human cone responses (and the rod response) to monochromatic spectral stimuli Normalized typical human cone responses (and the rod response) to monochromatic spectral stimuli

Humans have three types of photoreceptors: rods, cones and photoreceptive ganglion cells. Both are neurons that transduce light into a change in membrane potential through the same signal transduction pathway (see below). However, they differ in the nature of the opsin they contain, and therefore in their function. Rods are used primarily to see at low levels of light, while cones are used to determine color, depth, and intensity. Furthermore, there are three types of cones, which differ in the spectrum of wavelengths of photons over which they absorb (see graph). A single cone or rod cannot tell color; color vision requires interactions of three types of cones (see below).