Intrinsically photosensitive retinal ganglion cells ipRGCsalso called photosensitive retinal ganglion cells pRGCor melanopsin-containing retinal eyebsll cells mRGCsare a type of neuron in the retina of the mammalian eye. The presence of ipRGCs were first noted here 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 s that advancements in research on these cells began.
Recent research has shown that these retinal ganglion cellsunlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsina light-sensitive protein. Therefore they constitute a third class of photoreceptors, in addition to rod and cone more info. Compared to the rods and cones, the ipRGCs respond more sluggishly and signal the presence of light over the long term. 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.
Zaidi and colleagues showed that photoreceptive ganglion cells may have some visual function in humans. The phototransduction mechanism in these cells is not fully understood, but seems likely to resemble that in invertebrate rhabdomeric photoreceptors.
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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. These photoreceptor cells project both throughout the retina and into the brain. They contain the photopigment melanopsin in varying quantities along the cell membrane, including on the axons up to the optic disc, the soma, and dendrites of the cell. Results of studies in mice suggest that the axons of ipRGCs are unmyelinated.
Unlike other photoreceptor pigments, melanopsin has the ability to act as both the excitable photopigment and as a photoisomerase. Dopamine has functions in the light-adaptation process by up-regulating melanopsin transcription in ipRGCs and thus increasing the photoreceptor's sensitivity.
One postsynaptic target of ipRGCs is the suprachiasmatic nucleus SCN of the hypothalamus, which serves as the circadian clock in an organism. Other post synaptic targets of ipRGCs include: the intergenticulate leaflet IGLa cluster of neurons located in the thalamus, which play a role in circadian entrainment; the olivary pretectal nucleus OPNa cluster of neurons in the midbrain that controls the pupillary light reflex; the ventrolateral preoptic nucleus VLPOlocated in the hypothalamus and is a control center for sleep; as well as to [ the outermost tunic of the eyeball is the ] the amygdala.
Using various photoreceptor knockout mice, researchers have identified the role of ipRGCs in both the transient and sustained signaling of the pupillary light reflex PLR. At brighter light intensities the sustained PLR occurs, which involves both phototransduction of the rod providing input to the ipRGCs and phototransduction of the ipRGCs themselves via melanopsin.
Researchers have suggested that the role of melanopsin in the sustained PLR is due to its lack of adaptation to light stimuli in contrast to rod cells, which exhibit adaptation. Experiments with rodless, coneless humans allowed another possible role for the receptor to be studied. Ina new role was found for the photoreceptive ganglion cell.
Zaidi link colleagues showed that in humans the retinal ganglion cell photoreceptor contributes to conscious sight as well as to non-image-forming functions like circadian the outermost tunic of the eyeball is the, behaviour and pupillary reactions.
Zaidi and colleagues' work with rodless, coneless human subjects hence has also opened the door into image-forming visual roles for the ganglion cell photoreceptor. The discovery that there are parallel pathways for vision was made: one classic rod- and cone-based arising from the outer retina, the other a rudimentary visual brightness detector arising from the inner retina. The latter seems to be activated by light before the former. It has been suggested by the authors of the rodless, coneless human model that the receptor could be instrumental in understanding many diseases, including major causes of blindness worldwide such as glaucomaa disease which affects ganglion cells.
In other mammals, photosensitive ganglia have proven to have a genuine role in conscious vision. Tests conducted by Jennifer Ecker et al. Most work suggests that the peak spectral sensitivity of click receptor is between and nm. Lockley et al. In work by Zaidi, Lockley and co-authors using a rodless, coneless human, it was found that a very click nm stimulus led to some conscious light perception, meaning that some rudimentary vision was realized. InClyde E. Keeler observed that the pupils in the eyes of blind mice he had accidentally bred still responded to light.]
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