Recent studies on biological anatomy of the eye discovered an additional photoreceptor within the mammalian eye. The cells discovered mediate the primary non-image visual activities with the vision system. The functioning of these cells aids in various significant processes including the regulation of the papillary reflex activity in response to light, as well as, the circadian photo entrainment. These cells, called the intrinsically photosensitive retinal ganglion cells respond to more than the absolute light. The ipRGCs have a unique feature of activity, as they differ from the usual photoreceptor cells of cones and rods. The rods and cones mediate on the vision of images by signaling the contrasts in light after adaptation. Interestingly, the ipRGCs also do adapt to light contrast. The cells show sensitivity to flash of light, as is the case with other photoreceptors. The factor of action of the intrinsically photosensitive ganglion cells is from the mechanism of transduction in which the adaption leads to the function of the eye. The transduction mechanism features various process activities as described in this paper. Additionally, the intrinsically photosensitive retinal ganglion cells (ipRGCs) have the presence of an express photo pigment melanopsin. The melanopsin is a retinal ganglion cell photo pigment that aids in the synchronization of the clock in the suprachiasmatic nucleus (SCN) . The ipRGCs since their discovery have evolved, and scholars continue to explore them. Task of these cells is to facilitate vision in time where the visual light is not necessary. These cells also have differing grounds between the normal photoreceptor cells and the ipRGCs. This paper evaluates several points featuring the intrinsically photoreceptive retinal ganglion cells, their mechanism of transduction, as well as, the difference in function and structure between the ipRGCs and other photoreceptor cells.
Vision is among the various sense organs that mammals possess. It consists of the process that constitutes the formation of image of the object in observation via the photoreceptors that that facilitate the image formation pathway. Photoreceptors are those cells interacting with the neural network within the retina to send signals to brain for interpretation. The cells in the retina that facilitate the image formation pathway are the rod and cone . Another pathway for vision does not entail the image-forming pathway that applies the photosensitive cells to enable vision. This vision procedure entails the alternative of an evolutionary ancient photo transduction system. This system of the photo transduction has a direct link to various spots in the brain facilitating vision. This system is what entails the intrinsically photosensitive retinal ganglion cells (ipRGCS). These cells contain the photo pigment called melanopsin. However, the melanopsin is incapable of detecting photons while it is also receiving the synaptic input from the rod and cone photoreceptors through the bipolar cells. Therefore, the intrinsically photosensitive retinal ganglion cells are the retinal sensory for the subconscious visual processing. It facilitates the circadian photo entrainment along with the light reflex of the pupil. They fall under the irradiance detectors, have various roles, and specified procedure of meeting their purpose in the visual system of the mammal . They also have own neural pathways some of which blur the boundary between image-forming and non-image-forming processes of visual.
The Intrinsically photosensitive retinal ganglion cell
The intrinsically photosensitive retinal ganglion cells constitute a rare sub-population of the ganglion cells, about 1 -- 3% . They are the group of cells with the primary role of signaling light for the unconscious visual reflexes. These unconscious vision reflexes include the papillary constrictions and other movements that regulate the daily behavioral and psychological rhythms; collectively referred to as circadian rhythms on a daily basis. Therefore, the intrinsically photosensitive retinal ganglion cells are a third class of the mammalian photoreceptor cell types, differing significantly from the usual retinal vision that involves the rods and cones. Indeed, their extraordinary aspect and contribution to vision of the mammal emanates from the different photo pigment that it applies. These photo pigments are way less sensitive to the light and consequently, have a spatial resolution that is far less. Therefore, the photoreceptors are the ganglion cells with a unique ability to convey signal directly to the brain. The latter procedure that adjusts the circadian rhythms of the eye depending on the environment alongside other factors is what entails the photo entrainment process.
The unique aptitude of the intrinsically photosensitive retinal ganglion cell to facilitate; such visionary system movements is due to the exclusive possession of the photo pigment called melanopsin. The melanopsin is a clone from the frog dermal melanophores, with many orthologs in various mammalian species, including mice, monkey and humans. The analysis of the melanopsin reveals an amino-acid sequence with a seven-transmembrane structure, a feature common to all the G-protein coupled receptors. Surprisingly, it is the observation that the melanopsin has a higher homology to the invertebrate rhabdomeric opsins, r-opsins that it shares with the ciliary opsins, c-opsins, of the vertebrate species. This factor indicates the difference in the procedure of light signaling through a different mechanism; differing from that used in the rods and cones of vertebrates. The use of the rod-less and cone-less mouse in the experimentations proved valuable in characterizing the photo pigment of the intrinsically photosensitive retinal ganglion cells. However, at their discovery, there were doubts on the reality of the cone-less and rod-less cells signaling light . Therefore, to establish the existence of these cells, the scholar, Berson and his colleagues who discovered the intrinsically photosensitive cells used a targeted patch-clamp; taking recordings. They established that the retro labeled ganglion cells responded to light even in the presence of a cocktail of pharmacological blocker that eliminates every the rod and cone signaling in the retina. Additionally, the tests indicated that even after the mechanical isolation of the ganglion cells, the cells still intrinsically managed to detect light, resting all the doubts of the ability of the intrinsically photosensitive ganglion cells being actual photoreceptors. Since the original discovery of the melanopsin ganglion cells, there are other three varieties of the melanopsin cells. However, the first melanopsin cells, (M1 cell) are the one that studies concentrate on most.
The ability of the intrinsically photosensitive retinal ganglion cells (ipRGCs) in responding to light is from the exclusive expression of the photo pigment called melanopsin. The melanopsin is an original clone from the frog dermal melanophores. The hydrophobicity of the cells predicts that it has a sequence of 7-trans-membrane structure. This factor is unique to all the G-protein coupled receptors. However, it is notable that, the ability of the ipRGCs to detect and direct light signal is through the different mechanism used, which differs from that of the vertebrate rods and cones. However, initially there were doubts in comparison of melanopsin and a group of blue-light absorbing flavoproteins called crypto chromes in the ipRGCs . The crypto chromes are the circadian photo pigments in invertebrates. However, with time, there is overwhelming evidence that ipRGCs work due to the presence of the melanopsin cells. It is also observable form experiments and studies; that animal those do not have the melanopsin show significant difficulties in establishing multiple visual reflexes including pupillary constriction and photo entrainment. Additionally, the melanopsin from mice show the existence of intrinsic photosensitive retinal ganglion cells. Nonetheless, with all this evidence, there are controversies regarding the ability of melanopsin to function as a true photo pigment. However, these controversies do not hold, as when the melanopsin gene gets exposure to light, they exhibit normal light sensitivity to the other multiple cells of light detection . Thus, the melanopsin, which is the photo pigment, that facilitates the intrinsic response of the retinal ganglion cells are actual photo pigments. The ganglion cell population consists of 3% of the cells in the retina and has an expansive distribution throughout the entire retina . They have a high density of distribution. The dendritic strands of the ganglion cells are quite large, spanning up to 500 micrometers. The cells, thus, create a widespread overlapping plexus occurring in the retinal inner plexiform layer (IPL). Therefore, from this expansive network of the dendritic spread, the intrinsic photosensitive retinal ganglion cells initiate the spatial convergence process that leads to the wide receptive fields in target structures. The dendrites of these ganglion cells terminate in the outermost sub-layer of the inner plexiform layer.
Its similarities and differences to other retinal cells
The retina has a peripheral location within the eye structure. It is the neural portion of the eye, and to a larger extent, part of the central nervous system. The retina develops, as an out pocketing of the optic vesicle, which then undergoes invagination, becomes the optic cup. The outer wall of the cup gives rise to the eye epithelium while the inner wall gives the retina. The epithelium plays a key role in maintaining the photoreceptors, renewing photo pigments and removing phagocytes from the photoreceptor discs. In this retina, various neurons facilitate vision. These include the photoreceptors, bipolar cells, ganglion…