<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> A photoreceptor cell is a specialized type of cell found in the retina that is capable of visual phototransduction . The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes . To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential . </P> <P> There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and photosensitive retinal ganglion cells . The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight . The rods are narrower than the cones and distributed differently across the retina, but the chemical process in each that supports phototransduction is similar . A third class of mammalian photoreceptor cell was discovered during the 1990s: the photosensitive ganglion cells . These cells do not contribute to sight directly, but are thought to support circadian rhythms and pupillary reflex . </P> <P> There are major functional differences between the rods and cones . Rods are extremely sensitive, and can be triggered by a single photon . At very low light levels, visual experience is based solely on the rod signal . This explains why colors cannot be seen at low light levels: only one type of photoreceptor cell is active . </P>

Where are the two types of photoreceptors found
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