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A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation ) into signals that can stimulate biological processes.
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision .
Melanin is a chromophore that exists in the human epidermal layer of skin responsible for protection from harmful UV radiation. When melanocytes are stimulated by solar radiation, melanin is produced. [7] Melanin is one of the major absorbers of light in some biological tissue (although its contribution is smaller than other components).
In humans, rod cells are exclusively responsible for night vision, as cone cells are only able to function at higher illumination levels. [1] Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. [ 1 ]
[9] [27] The identity of the non-rod, non-cone photoreceptor in humans was found to be a ganglion cell in the inner retina as shown previously in rodless, coneless models in some other mammals. The work was done using patients with rare diseases that wiped out classic rod and cone photoreceptor function but preserved ganglion cell function.
Rhodopsin is a protein found in the outer segment discs of rod cells. It mediates scotopic vision , which is monochromatic vision in dim light. [ 7 ] [ 19 ] Rhodopsin most strongly absorbs green-blue light (~500 nm) [ 20 ] [ 21 ] and appears therefore reddish-purple, hence the archaic term "visual purple".
The retina (from Latin rete 'net'; pl. retinae or retinas) is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs.The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception.
Structurally, cone cells have a cone-like shape at one end where an opsin specific to the type of cone absorbs incoming light. They are typically 40–50μm long, and their diameter varies from 0.5 to 4.0μm, being smallest and most tightly packed at the center of the eye at the fovea. The S cone spacing is slightly larger than the others. [10]