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Concept
Intrinsically photosensitive retinal ganglion cell
Summary
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of (something like) ipRGCs was first suspected in 1927 when rodless, coneless mice still responded to a light stimulus through pupil constriction, This implied that rods and cones are not the only light-sensitive neurons in the retina. Yet research on these cells did not advance until the 1980s. Recent research has shown that these retinal ganglion cells, 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:
They play a major role in synchronizing circadian rhythms to the 24-hour light/dark cycle, providing primarily length-of-day and length-of-night information. They send light information via the retinohypothalamic tract (RHT) directly to the circadian pacemaker of the brain, the suprachiasmatic nucleus of the hypothalamus. The physiological properties of these ganglion cells match known properties of the daily light entrainment (synchronization) mechanism regulating circadian rhythms. In addition, ipRGCs could also influence peripheral tissues such as the hair follicle regeneration through SCN-sympathetic nerve circuit.
Photosensitive ganglion cells innervate other brain targets, such as the center of pupillary control, the olivary pretectal nucleus of the midbrain.
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