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Through genetic manipulations that enabled mouse embryo cells to produce melanopsin, researchers have been able to make these cells photosensitive. Likewise, other researchers have shown that frog eggs also became light-sensitive when manipulated to express the gene for melanopsin. Following a similar protocol, a third research team has even succeeded in making an embryonic human kidney cell light-sensitive . In fact, the cells in all of these experiments began to behave like the subpopulation of retinal ganglion cells that contain melanopsin, thus clearly indicating that this pigment makes cells intrinsically photosensitive.

Link : Melanopsin


Thanks to various tagging methods, scientists now know that a certain sub-population of the ganglion cells in the human retina contain a photosensitive pigment and project their axons directly into the suprachiasmatic nuclei as well as into other brain structures that are concerned with the intensity of ambient light.

These light-sensitive ganglion cells have large receptive fields, because of their long, widely dispersed dendrites. In these cells, accurate reception of information on shape, orientation, and movement is sacrificed to general sensitivity. These cells clearly constitute another light-sensitive system that runs parallel to the visual system but is dedicated to detecting light intensity rather than to forming images.

The number of these light-sensitive ganglion cells in each human retina is relatively small (only about 2000). Their electrophysiological response to light stimuli is quite different from that of the retina’s rods and cones. In response to light, rod and cone cell membranes hyperpolarize rapidly, but the membranes of these photosensitive ganglion cells instead depolarize, and far more slowly. This type of response is similar to that found in the photosensitive cells of invertebrates such as flies and octopi, which supports the idea that phylogenetically, this signalling system is far older than the visual one.

Source: Ralph Nelson,

Because they respond to light stimuli so slowly, these ganglion cells can integrate information over a long period—up to 5 minutes, according to some authors. This is exactly what you would expect of a non-visual system dedicated to signalling the overall intensity of light, rather than to transmitting detailed information about visual images.

These ganglion cells, which receive their inputs from the amacrine and bipolar cells in the retina’s inner plexiform layer, appear to use two neurotransmitters: glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP).

Many experiments have shown that even though they do have indirect connections with the retina’s better-known photoreceptor cells (the rod and cones), these ganglion cells are also intrinsically photosensitive (see sidebar). Their photosensitivity is, at least in part, attributable to the fact that they contain the photosensitive pigment melanopsin in their dendrites, their proximal axons, and their cell membranes.

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