Intrinsic Photosensitivity of Retinal Ganglion Cells

Rod and cone cells are proven photoreceptors of the eye, and responsible for scotopic and photopic vision.  The natural circadian cycle is tuned to environmental influence and can be reset with exposure to light.  Mice and people that lack rods and cones still posses the ability to reset their circadian clock, meaning rods and cones are not the only way to perceive light.  Melanopsin is a protein that, according to the amino acid sequence, is very similar to proteins found in rod and cone cells such as rhodopsin and color opsins.  Melanopsin has been found to be present in retinal ganglion cells, which were previously thought to act only as output cells from rods and cones to the rest of the brain.  Circadian rhythm experiments have concluded that eyeless mice are unable reset their circadian clock, but mice genetically modified to lack rods and cones can reset their circadian clocks.  This means that the mechanism for setting this circadian clock lies within the retina and is still photosensitive in blind animals.  Recent experiments tested if melanopsin could act as the mechanism to set circadian rhythm, modify pupil size, and influence conscious visual perception.