Rhodopsin is a common visual pigment that is part of the rod-shaped visual receptors in the retina of vertebrates. This substance has a very high photosensitivity and is a key component of photoreception. Another name for rhodopsin is visual purple.

V currently rhodopsins include pigments not only of the rods, but also of the rhabdomeric visual receptors of arthropods.

General characteristics of the pigment

By chemical nature, rhodopsin is a membrane protein of animal origin containing a chromophore group in its structure. It is she who determines the ability of the pigment to capture light quanta. The rhodopsin protein has a molecular weight of approximately 40 kDa and contains 348 amino acid units.

The light absorption spectrum of rhodopsin consists of three bands:

• α (500 nm);
• β (350 nm);
• γ (280 nm).

Rays γ are absorbed by aromatic amino acids in the composition of the polypeptide chain, and β and α - by the chromophore group.

Rhodopsin is a substance that can break down under the influence of light, which triggers an electrotonic signal transmission pathway along nerve fibers. This property is also characteristic of other photoreceptor pigments.

Rhodopsin structure

According to the chemical structure, rhodopsin is a chromoglycoprotein, which consists of 3 components:

• chromophore group;
• 2 oligosaccharide chains;
• water insoluble protein opsin.

The chromophore group is vitamin A aldehyde (retinal), which is in the 11-cis form. This means that the long part of the retinal chain is bent and twisted into an unstable configuration.

In the spatial organization of the rhodopsin molecule, 3 domains are distinguished:

• intramembrane;
• cytoplasmic;
• intradisk.

The chromophore group is located in the intramembrane domain. Its connection with opsin is carried out through the Schiff base.

Scheme of phototransformation

The mechanism of phototransformation of the rhodopsin pigment under the action of light is based on the reaction of cis-trans isomerization of retinal, i.e., on the conformational transition of the 11-cis-form of the chromophore group to the straightened trans-form. This process is carried out at a tremendous speed (less than 0.2 picoseconds) and activates a number of further transformations of rhodopsin, which occur already without the participation of light (dark phase).

The product formed under the action of a light quantum is called photorhodopsin. Its peculiarity is that trans-retinal is still associated with the opsin polypeptide chain.

From the completion of the first reaction to the end of the dark phase, rhodopsin sequentially undergoes the following series of transformations:

• photorhodopsin;
• bathorhodopsin;
• luminorhodopsin;
• metarhodopsin Ia;
• metarhodopsin Ib;
• metarhodopsin II;
• opsin and all-trans retinal.

These transformations are accompanied by stabilization obtained from the light quantum of energy and conformational rearrangement of the protein part of rhodopsin. As a result, the chromophore group is finally separated from the opsin and immediately removed from the membrane (the trans form has a toxic effect). After that, the process of regeneration of the pigment to its original state is started.

The regeneration of rhodopsin occurs due to the fact that outside the membrane, trans-retinal again acquires a cis-form, and then returns back, where it again forms a covalent bond with opsin. In vertebrates, recovery has the character of enzymatic resynthesis and occurs with the expenditure of energy, while in invertebrates it is carried out due to photoisomerization.

The mechanism of signal transmission from the pigment to the nervous system

The active component in triggering phototransduction is metarhodopsin II. In this state, the pigment is able to interact with the transducin protein, thereby activating it. As a result, tranducin-bound GDP is replaced by GTP. At this stage, a huge number of transducin molecules (500-1000) are simultaneously activated. This process is called the first stage of amplifying the light signal.

Then the activated transducin molecules interact with photodiesterase (PDE). This enzyme, in its active state, is able to very quickly destroy the cGMP compound, which is necessary to keep the ion channels in the receptor membrane open. After transducin-induced activation of PDE molecules, the concentration of cGMP drops to such a level that the channels close and sodium ions no longer enter the cell.

A decrease in the concentration of Na + in the cytoplasm of the outer part of the receptor leads the cytoplasmic membrane to a state of hyperpolarization. As a result, a transmembrane potential arises, which propagates to the presynaptic terminal, reducing the release of the neurotransmitter. This is precisely the semantic result of the process of all transformations in the visual receptor.

Protein modifications characteristic of various species, can differ significantly in structure and molecular weight.

Functions of rhodopsin

Rhodopsin belongs to the superfamily of transmembrane GPCRs (G-protein coupled receptors). When light is absorbed, the conformation of the protein portion of rhodopsin changes and it activates the G protein transducin, which activates the enzyme cGMP phosphodiesterase. As a result of activation of this enzyme, the concentration of cGMP in the cell decreases and cGMP-dependent sodium channels close. Since sodium ions are constantly pumped out of the cell by ATPase, the concentration of sodium ions inside the cell falls, which causes its hyperpolarization. As a result, the photoreceptor releases less of the inhibitory neurotransmitter glutamate, and nerve impulses arise in the bipolar nerve cell, which is "disinhibited".

Absorption spectrum of rhodopsin

The specific absorption spectrum of the visual pigment is determined both by the properties of the chromophore and opsin, and by the nature chemical bond between them (for more on this, see the review:). This spectrum has two maxima - one in the ultraviolet region (278 nm), due to opsin, and the other in the visible region (about 500 nm), - the absorption of the chromophore (see figure). The transformation under the action of light of the visual pigment to the final stable product consists of a series of very fast intermediate steps. By studying the absorption spectra of intermediate products in rhodopsin extracts at low temperatures at which these products are stable, it was possible to describe in detail the entire process of visual pigment bleaching.

In the living eye, along with the decomposition of the visual pigment, the process of its regeneration (resynthesis) is constantly going on. With dark adaptation, this process ends only when all the free opsin has combined with retinal.

Day and night vision

It can be seen from the absorption spectra of rhodopsin that reduced rhodopsin (under weak "twilight" illumination) is responsible for night vision, and during daylight "color vision" (bright illumination) it decomposes, and its maximum sensitivity shifts to the blue region. In sufficient light, the rod works in conjunction with the cone, being the receiver of the blue region of the spectrum. Complete recovery of rhodopsin in humans takes about 30 minutes.

Rhodopsin in skin cells

According to a 2011 study at Brown University, melanocyte skin cells also contain rhodopsin. Rhodopsin reacts to ultraviolet radiation and triggers the production of melanin

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Etymology

The name "rhodopsin" comes from other Greek. ρόδον - rose, etc. - Greek. όπσις - vision .

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An excerpt characterizing Rhodopsin

- Yes, let's go with me to the mound, you can see from us. And it’s still tolerable with us on the battery, ”said the adjutant. - Well, are you going?
“Yes, I am with you,” said Pierre, looking around him and looking for his bereator with his eyes. Here, only for the first time, Pierre saw the wounded, wandering on foot and carried on a stretcher. On the same meadow with fragrant rows of hay, through which he had passed yesterday, across the rows, awkwardly turning his head, lay motionless one soldier with a fallen shako. Why didn't they bring it up? - Pierre began; but, seeing the stern face of the adjutant, who looked back in the same direction, he fell silent.
Pierre did not find his bereytor and, together with the adjutant, rode down the hollow to the Raevsky barrow. Pierre's horse lagged behind the adjutant and shook him evenly.
- You, apparently, are not used to riding, count? the adjutant asked.
“No, nothing, but she jumps a lot,” Pierre said in bewilderment.
- Eh! .. yes, she was wounded, - said the adjutant, - right front, above the knee. Bullet must be. Congratulations, Count,” he said, “le bapteme de feu [baptism by fire].
Passing through the smoke along the sixth corps, behind the artillery, which, pushed forward, fired, deafening with its shots, they arrived at a small forest. The forest was cool, quiet and smelled of autumn. Pierre and the adjutant dismounted from their horses and walked up the mountain.
Is the general here? asked the adjutant, approaching the mound.
“We were just now, let’s go here,” they answered him, pointing to the right.
The adjutant looked back at Pierre, as if not knowing what to do with him now.
"Don't worry," said Pierre. - I'll go to the mound, can I?
- Yes, go, everything is visible from there and not so dangerous. And I'll pick you up.
Pierre went to the battery, and the adjutant rode on. They did not see each other again, and much later Pierre learned that this adjutant's arm had been torn off that day.
The barrow that Pierre entered was that famous one (later known by the Russians under the name of the kurgan battery, or Raevsky battery, and by the French under the name la grande redoute, la fatale redoute, la redoute du center [large redoubt, fatal redoubt, central redoubt ] a place around which tens of thousands of people were laid and which the French considered the most important point of the position.
This redoubt consisted of a mound, on which ditches were dug on three sides. In a place dug in by ditches stood ten firing cannons protruding through the openings of the ramparts.
Cannons stood in line with the mound on both sides, also firing incessantly. A little behind the cannons were infantry troops. Entering this mound, Pierre never thought that this place dug in with small ditches, on which several cannons stood and fired, was the most important place in battle.
Pierre, on the contrary, it seemed that this place (precisely because he was on it) was one of the most insignificant places of the battle.
Entering the mound, Pierre sat down at the end of the ditch surrounding the battery, and with an unconsciously joyful smile looked at what was happening around him. Occasionally, Pierre would get up with the same smile and, trying not to interfere with the soldiers loading and rolling the guns, who constantly ran past him with bags and charges, walked around the battery. The cannons from this battery continuously fired one after another, deafening with their sounds and covering the whole neighborhood with gunpowder smoke.
In contrast to the eerie feeling between the infantry soldiers of the covering, here, on the battery, where a small number of people engaged in business are white limited, separated from others by a ditch, here one felt the same and common to all, as if family animation.
The appearance of the non-military figure of Pierre in a white hat first struck these people unpleasantly. The soldiers, passing by him, looked with surprise and even fear at his figure. The senior artillery officer, a tall, pockmarked man with long legs, as if in order to look at the action of the last gun, approached Pierre and looked at him curiously.
A young, round-faced officer, still a perfect child, obviously just released from the corps, disposing of the two guns entrusted to him very diligently, turned sternly to Pierre.
“Sir, let me ask you out of the way,” he said to him, “it’s not allowed here.
The soldiers shook their heads disapprovingly, looking at Pierre. But when everyone was convinced that this man in a white hat not only did nothing wrong, but either sat quietly on the slope of the rampart, or with a timid smile, courteously avoiding the soldiers, walked along the battery under the shots as calmly as along the boulevard, then little by little, a feeling of unfriendly bewilderment towards him began to turn into affectionate and playful participation, similar to that which soldiers have for their animals: dogs, roosters, goats, and in general animals living with military teams. These soldiers immediately mentally accepted Pierre into their family, appropriated and gave him a nickname. “Our master” they called him and they affectionately laughed about him among themselves.
One core blew up the ground a stone's throw from Pierre. He, cleaning the earth sprinkled with a cannonball from his dress, looked around him with a smile.

Rhodopsin is the main visual pigment of retinal cells in vertebrates (including humans). It belongs to complex chromoprotein proteins and is responsible for "twilight vision". In order to enable the brain to analyze visual information, the retina converts light into nerve signals, determining the sensitivity of vision in the range of illumination - from starry night to sunny noon. The retina is formed by two main types of visual cells - rods (about 120 million cells per human retina) and cones (about 7 million cells). The cones, which are predominantly concentrated in the central region of the retina, function only in bright light and are responsible for color vision and sensitivity to fine details, while the more numerous rods are responsible for vision in low light conditions and turn off in bright light. Thus, at dusk and at night, the eyes are not able to clearly determine the color of an object, since the cone cells do not work. Visual rhodopsin is contained in the light-sensitive membranes of rod cells.

Rhodopsin provides the ability to see when "all cats are gray."

Under the action of light, the photosensitive visual pigment changes, and one of the intermediate products of its transformation is directly responsible for the appearance of visual excitation. After the transfer of excitation in the living eye, the process of pigment regeneration takes place, which then again participates in the process of information transfer. Complete recovery of rhodopsin in humans takes about 30 minutes.

Andrey Struts, head of the Department of Medical Physics at the St. Petersburg State Pediatric Medical Academy, and his colleagues from the University of Arizona managed to clarify the mechanism of action of rhodopsin by studying the protein structure using NMR spectroscopy. Their work is published Nature Structural and Molecular Biology .

“This work is a continuation of a series of publications on rhodopsin, which is one of the G-protein coupled receptors. These receptors regulate many functions in the body, in particular, rhodopsin-like receptors regulate the frequency and strength of heart contractions, immune, digestive and other processes. Rhodopsin itself is a visual pigment and is responsible for the twilight vision of vertebrates. In this paper, we publish the results of studies of the dynamics, molecular interactions, and mechanism of rhodopsin activation. For the first time, we obtained experimental data on the mobility of ligand molecular groups in the binding pocket of rhodopsin and their interaction with surrounding amino acids.

Based on the information obtained, we also proposed for the first time the mechanism of receptor activation,”

Struts told Gazeta.Ru.

Studies of rhodopsin are useful both from the point of view of fundamental science for understanding the principles of the functioning of membrane proteins, and in pharmacology.

“Since proteins belonging to the same class as rhodopsin are the target of 30-40% of currently developed drugs, the results obtained in this work can also be used in medicine and pharmacology to develop new drugs and treatments»,

Struts explained.

Research on rhodopsin was carried out by an international team of scientists at the University of Arizona (Tucson), but Andrey Struts intends to continue this work in Russia.

“My collaboration with the head of the group, professor, began in 2001 (before that, I worked at the Research Institute of Physics of the St. state university and at the University of Pisa, Italy). Since then, the composition of the international group has repeatedly changed, it included specialists from Portugal, Mexico, Brazil, and Germany. Working all these years in the USA, I remained a citizen of Russia and did not lose contact with the Faculty of Physics of St. Petersburg State University, of which I am a graduate and where I defended my PhD thesis. And here I should especially note the comprehensive and comprehensive training that I received at the Faculty of Physics of St. Petersburg State University and specifically at the Department of Molecular Optics and Biophysics, which allowed me to easily integrate into a team that was new to me and successfully deal with new topics, master new equipment for me.

Currently, I have been elected head of the Department of Medical Physics at the St. Petersburg State Pediatric Medical Academy (SPbSPMA) and I am returning to my homeland, but my cooperation with Professor Brown will continue no less actively. Moreover, I hope that my return will allow the University of Arizona to establish cooperation with St. Petersburg State University, St. Petersburg State Medical Academy, Russian State Humanitarian University and other universities in Russia. Such cooperation would be beneficial to both parties and would help promote the development of domestic biophysics, medicine, pharmacology, etc.

Specific scientific plans include the continuation of the study of membrane proteins, which are currently poorly understood, as well as the use of magnetic resonance imaging for the diagnosis of tumors.

In this area, I also have a certain backlog, obtained during my work at the medical center of the University of Arizona, ”explained Strutz.

visual pigments

visual pigments

The structure of rhodopsin

Cones and color vision

color blindness

Properties of photoreceptor channels

Molecular structure of cGMP-gated channels

Signal transduction in photoreceptors

Metabolic cascade of cyclic GMP

Vertebrate receptors that depolarize on exposure to light

Signal amplification in the cGMP cascade

Signals in response to single light quanta

Literature

visual pigments

Visual pigments are concentrated in the membranes of the outer segments. Each stick contains about 10 8 pigment molecules. They are organized into several hundred discrete discs (about 750 in monkey wand) that are not connected to the outer membrane. In cones, the pigment is located in special pigment folds that are a continuation of the outer cell membrane of the photoreceptor. Pigment molecules make up about 80% of all disc proteins. Visual pigments are so densely packed in the membranes of the outer segment that the distance between two visual pigment molecules in a rod does not exceed 10 nm. Such dense packing increases the probability that a photon of light passing through a layer of photoreceptor cells will be captured. The following question arises: how do signals arise when light is absorbed by visual pigments?

Light absorption by visual pigments

The events that occur when light is absorbed by the rod pigment, rhodopsin, were studied using psychophysiological, biochemical, and molecular techniques. The visual pigment molecule consists of two components: a protein, called opsin, and a chromophore, 11-cis-vitamin A-aldehyde, called retinal (Fig. 1). It should be clarified that the chromophore contains a chemical group that gives color to the compound. The quantitative characteristics of the absorption capacity of the pigments were studied using spectrophotometry. When rhodopsin, the visual pigment of the rods, was illuminated with light of different wavelengths, blue-green light with a wavelength of about 500 nm was best absorbed. A similar result was also obtained by illuminating a single rod under a microscope with beams of light with different wavelengths. An interesting relationship has been found between the absorption spectrum of rhodopsin and our perception of twilight light. Quantitative psychophysical studies performed on humans have shown that bluish-green daylight with a wavelength of about 500 nm is optimal for the perception of twilight light in the dark. During the day, when rods are inactive and only cones are used, we are most sensitive to the red color corresponding to the absorption spectrum of cones (we will talk about this later).

When one photon is absorbed by rhodopsin, retinal undergoes photoisomerization and passes from the 11-cis to the trans configuration. This transition occurs very quickly: in about 10-12 seconds. After that, the protein part of the pigment also undergoes a series of transformational changes, with the formation of a number of intermediate products. One of the conformations of the protein moiety, metarhodopsin II, is most important for signal transduction (we will discuss this later in this chapter). Figure 2 shows the sequence of events during decolorization and regeneration of active rhodopsin. Metarhodopsin II is formed after 1 ms. Pigment regeneration after its disintegration occurs slowly, within a few minutes; this requires the transport of retinal from photoreceptors to the pigment epithelium.

The structure of rhodopsin

On the molecular level opsin protein consists of 348 amino acid residues, forming 7 hydrophobic zones, each of which consists of 20-25 amino acids, making up 7 transmembrane helices. The N-terminus of the molecule is located in the extracellular space (i.e., inside the rod disk), and the C-terminus is located in the cytoplasm.

Fig.1. The structure of vertebrate rhodopsin embedded in the photoreceptor membrane. The helix is ​​somewhat unrolled to show the location of the retinal (indicated in black). C - C-terminus, N - N-terminus.

Fig.2. The efflorescence of rhodopsin in the light. In the dark, 11-cis-retinal is tightly bound to the protein opsin. Photon capture leads to the isomerization of all cis retinal in the retinal throne. In this case, the opsin all-tron-retinal complex quickly turns into metarhodopsin II, which dissociates into opsin and all-tron retinal. The regeneration of rhodopsin depends on the interaction of photoreceptors and pigment cells. Metarhodopsin II activates and maintains the second messenger system.

Retinal is connected to opsin through a lysine residue located in the seventh transmembrane segment. Opsin belongs to a family of proteins with 7 transmembrane domains, which also includes metabotropic mediator receptors, such as adrenergic and muscarinic receptors. Like rhodopsin, these receptors signal to second messengers via G-protein activation. Rhodopsin is remarkably stable in the dark. Bayor calculated that spontaneous thermal isomerization of the rhodopsin molecule takes about 3000 years, or 10 23 more than for photoisomerization.

Cones and color vision

The amazing research and experiments performed by Young and Helmholtz in the 19th century drew attention to the very important issue of color vision, and the scientists themselves gave a clear and accurate explanation of this phenomenon. Their conclusion about the existence of three various types color photoreceptors has stood the test of time and has been subsequently validated at the molecular level. Again, we can quote Helmholtz, who compared the perception of light and sound, color and sound tone. One can envy the clarity, power and beauty of his thought, especially when compared to the confusing vitalistic concepts that were widespread in the 19th century:

All differences in color tones depend on the combination in various proportions of the three primary colors ... red, green and violet ... Just as the perception of sunlight and its warmth depends ... on whether the rays of the sun hit the nerves, coming from the receptors of vision or from the receptors of thermal sensitivity. As Young suggested in his hypothesis, the difference in the perception of different colors depends simply on which of the 3 types of photoreceptors is more activated by this light. When all three types are equally excited, the result is white...

Rice. 3. Sensitivity spectra of human photoreceptors and various visual pigments. (A) Sensitivity curves of three color visual pigments showing absorption peaks at wavelengths corresponding to cyan, green and red. (B) Sensitivity spectra of cones to blue, green, and red, and rods (shown in black) in macaques. The responses were recorded using suction electrodes, averaged and normalized. The curves of the rod spectrum were obtained in the study of visual pigments in humans. (C) Comparison of monkey and human cone spectra using a color sensitivity test. The continuous curve shows an experiment to determine the sensitivity to color in humans, when presented with light of different wavelengths. The dotted line shows the results predicted based on the registration of currents in individual cones, after correcting for the absorption of light in the lens and pigments on the way to the outer segment. The agreement between the results of both experiments is surprisingly high.

If we project onto White screen two beams of light of different colors at the same time ... we see only one color, more or less different from both of these colors. We can better understand the outstanding fact that we are able to perceive all the shades in the composition of external light by a mixture of three primary colors, if we compare the eye with a dry ... In the case of sound ... we hear longer waves as low tones, and short waves - both high and piercing, besides this, the ear is able to catch a lot at the same time sound waves, i.e. many notes. However, they β in this case do not merge into one complex chord, just as different colors ... merge into one complex color. The eye cannot tell the difference if we change orange to red or yellow; but if we hear the notes do and mi sounding at the same time, such a sound does not seem to us like a note re. If the ear perceived musical tones as the eye perceives colors, each chord could be represented by a combination of three constant notes, one very low, one very high, and one intermediate, producing all possible musical effects only by changing the relative loudness of these three notes. .. However, we are able to see the smooth transition of colors from one to another through an infinite number of shades and gradations ... The way we perceive each of the colors ... depends mainly on the structure of our nervous system. It must be admitted that at present, neither in humans nor in tetrapods has an anatomical basis been described to confirm the theory of color perception.

These accurate and far-sighted predictions have been confirmed by a series of different observations. Using spectrophotometry, Wald, Brouck, McNicol, and Dartnell et al showed the presence of three types of cones with different pigments in the human retina. Baylor and colleagues also managed to divert currents from the cones of monkeys and humans. The three cone populations were found to have different but overlapping ranges of sensitivity to the blue, green, and red portions of the spectrum. The optimal wavelengths for the excitation of electrical signals exactly coincided with the peaks of light absorption by visual pigments, established using spectrophotometry and psychophysical experiments to measure the sensitivity of the eye to the color spectrum. Ultimately, Natais cloned and sequenced the genes encoding the opsin pigment in three types of cones that are sensitive to red, green, and blue.

visual pigment

The outer segments of rod and cone cells contain many disks consisting of double membranes. The structural and functional unit of the light-sensitive membrane of photoreceptors are visual pigments. In the mechanism of vision, these molecules provide two main physiological functions:

First, they absorb light in a characteristic wavelength region, that is, they determine the spectral range of photoreceptor cells.

Secondly, the visual pigment molecule triggers the photoreceptor process.

The first function is based on the absorption spectrum of molecules, which depends on the nature of the chromophore group and its covalent or non-covalent interaction with the protein part of the molecule. The second is based on the ability of a molecule to change its conformation upon absorption of light: first the chromophore, and then the protein. And also, the visual pigment molecules at one of the phototransformation stages acquire the ability to interact with other proteins involved in the photoreception mechanism. (Byzov A.L., 1992)

The visual pigment is a chromoglycoprotein. This molecule contains one chromophore group, two oligosaccharide chains, and a water-insoluble membrane protein (opsin). The visual pigment is a relatively small molecule: the molecular weight of rod rhodopsin in vertebrates, such as bovine rhodopsin, is about 39 kDa; The polypeptide chain of a protein consists of 348 amino acid residues. The largest molecules of rhodopsin are found in insects - 383 amino acid residues and in cephalopods (octopus) - 455 residues.

The chromophore group of visual pigments in vertebrates is retinal-1 (vitamin A1 aldehyde), retinal-2, or 2,3-dehydroretinal (vitamin A2 aldehyde). The position of the absorption maxima of visual pigments found in rods and cones of vertebrates varies widely. Therefore, for a better understanding, all pigments are classified according to the nature of the chromophore, regardless of origin. Therefore, all retinal - 1 - containing visual pigments are referred to as rhodopsins, retinal-2-containing - to porphyropsins. In invertebrates (arthropods, cephalopods), 3- and 4-dehydroretinal have also been found as chromophores. 3-oxyretinal-containing pigments are called "xanthopsins".

J. Wald proposed a classification of visual pigments based on a combination of two types of retinals - retinal - 1 and - 2, as well as two types of opsins - rod and cone. However, this simple classification turned out to be too limited and Lately almost never used. Although the term "iodopsin" is still used for the pigment of red-sensitive cones with l = 550-570 nm (for example, in birds or humans), and the term "cyanopsin" is used for the cone pigment l = 620 nm in turtles and fish.

The visual pigment of the rod consists of a large molecule of the protein rhodopsin, the pigment itself, one of the chemical forms of vitamin A.

Rhodopsin was the first membrane protein of animal origin for which such a polypeptide chain fold was established. It consists of the protein olein and the aldehyde vitamin A-retinal. With a lack of vitamin A, visual perception is disturbed, and rods are faster than cones. Particularly high density of rhodopsin molecules in the disc membranes from the side facing the incident light. The absorption of light by the pigment is the first stage of transformations leading to the disintegration and discoloration of the visual pigment. And this leads to a change in the ion permeability of the photoreceptor membrane and the appearance of a visual signal.

When light hits (and for a stick, 3-5 light quanta are enough), this molecule breaks up into protein and pigment parts. In this case, ions with positive and negative charges are released, i.e., an electrically charged medium is formed. Through the cell membrane of the rod, this biocurrent is transmitted, through a system of nerve fibers and cells, to the cortex of the occipital lobes of the brain. After some time, the disintegrated parts of the molecule recombine and the visual pigment is ready to absorb light. Knowing the mechanism of light perception, one can understand the importance of vitamin A for vision.

The membranes of the cone disks contain other chemical composition pigments: iodopsin, chlorolab, erythrolab. There are three different types of cones, each type containing predominantly only one pigment. The most studied cone pigment is iodopsin. The different visible colors depend on the ratio of the three kinds of cones stimulated.

The nucleus of the eye

Inside eyeball is the nucleus of the eye. It consists of aqueous humor, lens and vitreous body. All of these components are transparent. And according to physical laws, transparent media refract light, so the transparent media of the eye are also called refractive media.

Between the posterior surface of the cornea and the anterior surface of the iris is a space called the anterior chamber, and between the posterior surface of the iris and the anterior surface of the lens is the posterior chamber. Both chambers are filled with aqueous humor - an intraocular fluid that provides metabolism in the cornea, lens and vitreous body.

The lens is a transparent, elastic, dense formation located directly behind the iris. The lens substance is enclosed in a capsule into which the fibers of the zinn ligament are woven. Thus, the lens, due to the zinn ligaments, is in the middle position, as if suspended on the ligaments. The lens is a biconvex transparent lens. Its property is to refract the course of light rays entering the eye and enlarge the image. The line between the anterior and posterior surfaces of the lens is called the equator. The lens grows throughout life, its optics and physical qualities change all the time. However, despite the growth, the dimensions of the lens remain constant. This happens because the new layers are superimposed, compacting the previous ones and pushing them towards the center. As a result, the nucleus of the lens is formed. In the nucleus, the cells are so compressed that, over time, their metabolism deteriorates, and they lose their transparency. A clouding of the lens is called a cataract.

The lens, being a living tissue, has an amazing property - to change the curvature. This happens so that objects located at different distances from the eye are in focus. For close objects, the ciliary body muscle contracts, the ligament of zinus relaxes, the tension of the lens capsule weakens, and the lens substance expands. Becoming more convex, the lens increases the optical power of the eye. When looking at distant objects, opposite muscle movements occur and the lens narrows.

With age, due to the formation of the nucleus, the elasticity of the lens decreases. It can no longer expand at the right time to look at close objects, this phenomenon is called presbyopia.

The vitreous body is a transparent jelly-like mass behind the lens. It occupies two posterior thirds of the volume of the eye. In some diseases, the vitreous body becomes cloudy, causing a decrease in vision. Together, the cornea, lens, aqueous humor, and vitreous form the optical system of the eye.