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Color vision is one of the primary senses of human beings that provide the perception of our environment. When light penetrates to the eyes through the pupils, it falls on the retina where photons of the light agitate the rod and cone cells. Rod and cone cells are the neuron cells that contain photosensitive pigment rhodopsin.
The activation of the rhodopsin present in the rod and cone cells by the photons of the light is called as photoactivation. During the process of photoactivation, 11-cis-retinal, attached to the rhodopsin is converted into the all-trans-retinal (photoisomerization).
Rhodopsin is a G-protein linked cell surface receptor and its activation induced by the photoisomerized all-trans-retinal initiates the visual signal transduction collectively called as phototransduction. However, it is necessary to regenerate the 11-cis-retinal continuously from the all-trans-retinal to allow the continuation of the phototransduction to maintain the vision.
Visual pigments or photopigments
Consumption of the 11-cis-retinal via the visual signal transduction and its regeneration constitute a cycle called a visual cycle. The visual system of all the vertebrates shares five distinct classes of visual pigments and these are rhodopsin (Rh1), long wavelength-sensitive (LWS), medium wavelength-sensitive (MWS or Rh2), short-wavelength-sensitive-SWS1, and SWS2.
However, all these photopigments bind to the 11-cis-retinal in the same way via a Schiff base that is stabilized by the glutamate residue present in the transmembrane region of the photopigment and, therefore, the difference in absorption of the light is determined by the different types of the photopigments.
These types of rhodopsin are responsible for different types of vision, for example, rhodopsin Rh1 is responsible for the scotopic vision (vision under low light condition) while the other three types of rhodopsin are responsible for the photopic vision (vision under the well-light condition that allows color perception).
Different types of photoreceptor and color perception
Different vertebrates use a specific combination of the rod and cone cells. As, for example, the retina of nocturnal species contains rod and cone cells at the ratio of 200:1 while retina of the diurnal species contains rod and cone cells at the ratio of 20:1.
The aforementioned photopigments are present in the different amounts in the rod and cone cells. As, for example, in the rod cells the Rh1 is abundant while in cone cells the MWS, SWS1, and SWS2 are abundant. However, during the course of evolution, environmental factor and nocturnal living habits of the human have led to the reduction of the four photopigments into the three pigments only Rh1, LWS, and SWS1.
In humans, rhodopsin Rh1 along with two variants of LWS; L/LWS and M/LWS and SWS1 form trichromacy. The distinct sensitivities in the photopigments of the trichromacy can match the visible spectrum by combining the three colors namely red, green and blue. However, a recent study shows that there is fourth photopigment melanopsin that also contributes to the color vision along with these aforementioned pigments.
The interplay of photoreceptors and neurons in the color vision
Perception of the vision is initiated when the light of different wavelength falls on distinct types of photopigments present in the photoreceptor cells. During this interaction, electromagnetic information is converted into the electrochemical information that reaches to the neurons to be analyzed and interplayed accordingly.
After photoreceptor activation, a series of signaling cascade begins that leads to the processing of the visual stimuli. Using the three photoreceptors; SWS1, M/LWS, and L/LWS and neuronal processing, human eyes can discriminate the colors in the visual range of the light of the wavelength ranging from 420 nm to the 680 nm.
Signals from the photoreceptors of the cone cells are processed through the midget ganglion cells where they combine with those of the coming from the horizontal cells and thereby turn on or off the bipolar cells. In addition, SWS1 photoreceptor signaling plays an important role in the transmission of the color sensations such as yellow, blue, green and red.
Based on these models, there are four types of color transmission; yellow, blue, green, and red that is represented by L-(S+M), (S+M)-L, M-(S+M), and (S+L)-M respectively while melanopsin signaling contributes to the contrast sensitivity of the vision.
Bioenergetics of photoreceptor activation
Photoisomerization of the 11-cis-retinal to the all-trans-retinal requires breakage of the π-bond of the 11-cis-retinal. The researchers have calculated the amount of energy required to break the π-bond that is 156.4 kJ per mol in rhodopsin or 61.8 kJ per mol in solution. This calculated energy corresponds to the wavelength λ=765.5 nm, a value that limits the visual spectrum on the infrared side.
While the maximum energy limit of the human visual spectrum is about 314.6 kJ per mol that corresponds to the wavelength λ=380 nm. This energy may be used to break the weak C-H or C-C single bond that would lead to the decay of the chromophore or other biomolecules. This would harm the visual cycle and thus causes loss of the vision. However, cornea and lens limits the light absorption of the short wavelength side of the visual spectrum and thus protects the retina from radiation damage.
Deficiencies in the color vision
There are several disorders related to color vision, including monochromacy, red-green color deficiency, and blue-yellow color deficiency. Monochromacy includes two types; rod monochromacy or achromatopsia and blue cone monochromacy or incomplete achromatopsia.
Red-green color deficiency is a genetic disease that arises when non-homologous recombination of identical sequences of the L/LWS and M/LWS receptor genes is increased i.e. duplication of these genes leads to the red-green color deficiency. However, the loss of functional L/LWS photopigment leads to another vision problem called protanopia, while the loss of functional M/LWS leads to the deuteranopia.
Blue-yellow color deficiency arises due to one of the six known mutations within the SWS1 genes. The mutations affect the folding and H-bonding network as well as stability and activation of the photopigment SWS1.
However, tritanopia is another vision disease related to the SWS1, during which people cannot differentiate the middle to short wavelength region of the visible spectrum. It affects only 0.001-0.2% of the human population and the molecular region behind this disease is the mutation in the gene that leads to the substitution of the key amino acids. It is found that tritanopia increases with the aging and affects the sensitivity of the SWS1. As a result, the lens becomes yellowish in older age.
Reference: Progress in Retinal and Eye Research (Advances in understanding the molecular basis of the first steps in color vision)
Article DOI: 10.1016/j.preteyeres.2015.07.004