Intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells, are a type of neuron in the retina of the mammalian eye. The presence of ipRGCs were first noted in 1923 when rodless, coneless mice still responded to a light stimulus through pupil constriction, suggesting that rods and cones are not the only light sensitive neurons in the retina. It wasn't until the 1980s that advancements in research on these cells began. Recent research has shown that these retinal ganglion cells, which, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light sensitive protein. Therefore they constitute a third class of photoreceptors, in addition to rod and cone 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. 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. 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.

The axons from these ganglia innervate regions of the brain related to object recognition, including the superior colliculus and dorsal lateral geniculate nucleus.

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