Photosensitive ganglion cell

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. They are a class of retinal photoreceptors, excited by light even when all influences from the better known 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.

Compared to the rods and cones, the ipRGCs respond more sluggishly and signal the presence of light over the long term. They represent a very small subset (~1%) of the retinal ganglion cells (Berson, 2003). Their functional roles are non-image-forming and fundamentally different from those of pattern vision; they provide a stable representation of ambient light intensity. They have at least three primary functions.

Photoreceptive ganglion cells have been isolated in humans where, in addition to regulating the circadian rhythm, they have been shown to mediate a degree of light recognition in rodless, coneless subjects suffering with disorders of rod and cone photoreceptors. Work by Farhan H. Zaidi and colleagues showed that photoreceptive ganglion cells may have some visual function in humans.

The photopigment of photoreceptive ganglion cells, melanopsin, is excited by light mainly in the blue portion of the visible spectrum (absorption peaks at ~480 nanometers). The phototransduction mechanism in these cells is not fully understood, but seems likely to resemble that in invertebrate rhabdomeric photoreceptors. Photosensitive ganglion cells respond to light by depolarizing and increasing the rate at which they fire nerve impulses. In addition to responding directly to light, these cells may receive excitatory and inhibitory influences from rods and cones by way of synaptic connections in the retina.

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