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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 .
Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells (rods and cones) in the vertebrate retina.A photon is absorbed by a retinal chromophore (each bound to an opsin), which initiates a signal cascade through several intermediate cells, then through the retinal ganglion cells (RGCs) comprising the optic nerve.
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.
This change acts as a molecular switch to activate a signal transduction mechanism within the cell. Depending on the type of rhodopsin, it either opens an ion channel (for example in bacteria) or activates an associated G protein and triggers a second messenger cascade (for example in animal eyes).
Generally, the basic morphologies are spheres (coccus) and round-ended cylinders or rod shaped (bacillus). But, there are also other morphologies such as helically twisted cylinders (example Spirochetes), cylinders curved in one plane (selenomonads) and unusual morphologies (the square, flat box-shaped cells of the Archaean genus Haloquadratum ...
Steps 3, 4, 5, and 6 occur in rod cell outer segments; Steps 1, 2, and 7 occur in retinal pigment epithelium (RPE) cells. RPE65 isomerohydrolases are homologous with beta-carotene monooxygenases; [ 7 ] the homologous ninaB enzyme in Drosophila has both retinal-forming carotenoid-oxygenase activity and all- trans to 11- cis isomerase activity.
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 rod opsins are called scotopsins when unbound to retinal and rhodopsins or porphyropsins when bound to retinal (1 and 2, respectively). Rod opsins mediate scotopic vision (dim light). [ 8 ] Compared to cone opsins, the spectral sensitivity of rhodopsin is quite stable, not deviating far from 500 nm in any vertebrate.