Intrinsically photosensitive Retinal Ganglion Cells (ipRGCs), also called
photosensitive Retinal Ganglion Cells (pRGC), or
melanopsin-containing retinal ganglion cells, are a type of
neuron (nerve
cell) in the
retina of the
mammalian eye. While responses to light in mice lacking
rods and
cone cells were first noted in 1923, they were forgotten, then rediscovered in the early 1990s. The source of these responses was shown to be a special type of
retinal ganglion cell, which, unlike other retinal ganglion cells, is intrinsically
photosensitive. This means that they are a third class of retinal
photoreceptors, excited by light even when all influences from classical photoreceptors (
rods and
cones) are blocked (either by applying
pharmacological agents or by dissociating the ganglion cell from the retina). Photosensitive ganglion cells contain the
photopigment melanopsin. The
giant retinal ganglion cells of the primate retina are examples of photosensitive ganglion cells.