Abstract

There is a particular ganglion cell in the retina that responds to light in the absence of all rod and cone photoreceptor input, the intrinsically photosensitive retinal ganglion cell (ipRGC). The existence of this cell was predicted from observations over decades but not established till the 80s when it was shown that a novel photopigment, melanopsin, was expressed in a small subset of ganglion cells in the inner retina of rodents and primates. Phototransduction in mammalian ipRGCs more closely resembles that of invertebrate rather than vertebrate photoreceptors and is mediated by transient receptor potential channels. Through particular attributes of melanopsin, irradiance information is conveyed centrally via the optic nerve to influence several functions, including photoentrainment of the biological clock, pupillary light reflex, sleep, arousal states and some aspects of vision. Irradiance signals interface directly with the autonomic nervous system to regulate rhythmic gene activity in major organs of the body.