| For the greater part of 150 years it was assumed that the mammalian retina contained only two types of photoreceptors; rods and cones. However, a deluge of recent evidence has demonstrated the existence of a third type of mammalian photoreceptor that differs greatly from rods and cones. They utilize a different photopigment, are much less sensitive to light, and have far less spatial resolution; characteristics that fit perfectly with these photoreceptor's primary function of signaling changes in ambient light levels to the brain throughout the day. Most surprisingly, these photoreceptors are ganglion cells, and thus, have the unique ability to communicate directly with the brain. These intrinsically photosensitive retinal ganglion cells (ipRGCs) are a rare sub-population of ganglion cells (1-3%) whose primary role is to signal light for unconscious visual reflexes, such as pupillary constriction, and regulating a number of daily behavioral and physiological rhythms, collectively called circadian rhythms. This latter process, which adjusts circadian rhythms to the light/dark cycle of an animal's environment, is known as photoentrainment. The visual behaviors under ipRGC control are remarkably tonic, and require long integration times of ambient light levels. The unique properties of ipRGCs, both functionally and anatomically, make them well suited for regulating such behaviors. |
