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The Senses
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Help Quelques notions sur la couleur How Do We See Colors? Color Vision
Role of the Rods and Cones Different types of cone photoreceptor Rods & Cones
PHOTORECEPTORS
HOW LIGHT GETS CONVERTED INTO NERVE IMPULSES

The two kinds of photoreceptors in the retinas of vertebrates—the rods and cones—differ in many ways, both anatomically and functionally. The main difference is the opposite roles that they play in vision. The rods provide what is called scotopic vision: they are very sensitive to low levels of light but cannot distinguish colours. The cones provide photopic vision: they require bright light but let us see the world around us in colour, and more sharply.

In both cases, however, the neural response is the same—the hyperpolarization of the photoreceptor cells—and is initiated by the same phenomenon: the absorption of light energy by photopigments embedded in the discs of the photoreceptors' outer segments. In the rods, the photosensitive pigment is rhodopsin, which has its peak sensitivity at around 500 nanometres (nm) in the visible-light band of the electromagnetic spectrum.

In the cones, the photosensitive pigment is opsin, a transmembrane protein that is very similar to rhodopsin. Opsin comes in three different varieties, distinguished by differences in their amino acid sequences that result in differences in their light-absorption curves, with peaks in the blue, green, and red portions of the visible light spectrum, respectively.

All three varieties of opsin are present in all cones. But there are three types of cones, in each of which a different variety of opsin heavily predominates, making it more sensitive to a different part of the colour spectrum, as shown in the diagram here. "Blue" cones containing mostly blue-sensitive opsin are excited chiefly by a wavelength of around 420 nm, "green" cones by a wavelength around 530 nm, and "red" cones by a wavelength near 560 nm.

But the attachment of color names to cones can be misleading because the cones are not maximally sensitive exactly in the red, green and blue parts of the spectrum. As you can see on the picture above, the blue cones are most sensitive in the violet, and the red cones in the yellow-green part of the spectrum. Consequently, it is more accurate to refer to the three types of cones containing mostly blue-, green-, and red-sensitive pigment as S-cones, M-cones, and L-cones, respectively, where S, M, and L stand for short, medium, and long wavelength.

An object whose colour falls anywhere in the visible spectrum will therefore excite all three types of cones to varying extents. For example, a green object will stimulate green cones for the most part, but also red cones to a lesser extent, and blue cones to a still lesser one. Our perception of colours thus depends on this superimposition of the various absorption spectra of the three types of cones, and subsequently, of course, on the complex neuronal interactions between the retina and the rest of the brain.

Colour blindness, or daltonism, is a vision defect characterized by the inability to differentiate certain colours or hues. Its name comes from that of the English physicist John Dalton (1766-1844), who suffered from this condition himself.

About 8% of all men are colour blind to varying degrees, and slightly less than 1% of all women. The reason for this difference is that the main form of colour blindness is hereditary, and the genetic mutations that cause it occur on the X chromosome. Since the mutated gene is recessive, women, who have two X chromosomes, can carry the gene without being colour blind, if the other X chromosome is unaffected. But men have only one X chromosome, so if the mutated gene is present on it, they are automatically colour blind.

Cases of total colour blindness, known as achromatopsy, in which someone sees the world only in shades of grey, are very rare. Usually, people who are colour blind have trouble in telling red from green, or, much more rarely, blue from yellow.

Classic red/green colour blindness is the result of a lack of red cones in the retina. Forms of colour blindness are usually classified according to the type of cone affected. Thus there are three kinds of colour blindness, corresponding to the three kinds of cones. Blindness to green, due to deficiency of the green pigment, is called deuteranopia, and is the most common form.

Link : UN ETRE VENU D'AILLEURS ? Experience : Are You Color Blind? Link : LE DALTONISME, C'EST... Link : What Is Colorblindness and the Different Types? Link : Le Daltonisme

 

       

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Were dinosaur ancestors creatures of the night? Visual pigments and visual transduction Rhodopsin La iodopsine
Rhodopsin - a G-protein binding receptor in the retina of the eye La photo excitation Animation : Photoisomerization of rhodopsin
Researcher
Dr. Reiner Vogel DERIC BOWNDS

Dark adaptation is a two-step process in which the eyes make the transition from photopic (cone-based) to scotopic (rod-based) vision. Once you have spent a certain amount of time in a well-lit room, your eyes' light-sensitivity threshold is very high. If you then move into a darker room, this threshold falls rapidly for the first 5 or 6 minutes, then seems as if it were going to stabilize asymptotically. But around the 7th minute, the threshold starts to fall even more. About half an hour later, it reaches a second asymptotic level, much lower than the first. This minimum level is the threshold for scotopic vision, whereas the initial level represented the threshold for photopic vision.

Link: Light and Dark Adaptation
HOW LIGHT GETS CONVERTED INTO NERVE IMPULSES
PHOTORECEPTORS

The transduction of light energy into variations in photoreceptors' membrane potential begins with the absorption of photons by light-sensitive pigment proteins in the discs of the photoreceptors' outer segments. These pigment proteins belong to a family known as the opsins. The pigment protein in rods is called rhodopsin, while the pigment protein in cones is called iodopsin. A single rod can contain up to 100 million molecules of rhodopsin in its outer segment discs.

Rhodopsin molecules contain seven transmembrane domains and somewhat resemble metabotropic synaptic receptors in their structure. In fact, rhodopsin can be regarded as a receptor protein whose agonist is already bound to it. This agonist is a molecule called retinene (or retinal) that is derived from Vitamin A. It is bound to the middle of the seventh transmembrane domain of the rhodopsin molecule. More specifically, it is the 11-cis form of retinene, which, when it absorbs light, isomerizes to the all-trans form. This isomerization converts the rhodopsin to its active form, metarhodopsin II. This reconfiguration of the retinene molecule thus produces the same effect as if a neurotransmitter had suddenly bound to a receptor.

Link : Photoisomerization of rhodopsin Link: Kent Wilson, phtotisomerization, the movie

Next, the metarhodopsin II stimulates a particular G protein called transducin. Like all G proteins, the transducin then activates another enzyme, in this case phosphodiesterase (PDE). When the PDE is activated, it converts the cGMP that is present in the rod's cytoplasm in the absence of light into regular GMP. The resulting decrease in cGMP closes the sodium channels in the rod's cell membrane, thus hyperpolarizing the cell. As a result, fewer neurotransmitter (most likely glutamate) molecules are released from the photoreceptor's synaptic endings.

Note that a signal amplification occurs at two points in this biochemical cascade. Every metarhodopsin II molecule activates about 100 G-protein molecules, and every phosphodiesterase molecule hydrolyzes about 1 000 molecules of cGMP into GMP. It is this amplification phenomenon that enables rods to detect the presence of a single photon of light.


The Vitamin A that our bodies produce from the beta carotene in many of the foods we eat (including, most famously, carrots) is needed to synthesize the retinene bound to the centre of the rhodopsin molecule. Indeed, a severe Vitamin A deficiency impairs night vision, because of the smaller amounts of retinene being produced. During the daytime, however, there is generally enough light to allow relatively normal vision despite low levels of visual pigments.

Link : Vitamine A Link : Vitamin A

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