These cells are thought not to contribute to sight directly, but have a role in the entrainment of the circadian rhythm and pupillary reflex. A third class of mammalian photoreceptor cell was discovered during the 1990s: the intrinsically photosensitive retinal ganglion cells. Rods primarily mediate scotopic vision (dim conditions) whereas cones primarily mediate to photopic vision (bright conditions), but the processes in each that supports phototransduction is similar. The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form an image of the environment, sight. There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes.
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