The light-sensitive cells of the rods and cones are located. Rods and cones of the retina: structure

The main photosensitive elements (receptors) are two types of cells: one in the form of a stalk - sticks 110-123 million. (height 30 µm, thickness 2 µm), others shorter and thicker - cones 6-7 million. (height 10 µm, thickness 6-7 µm). They are distributed unevenly in the retina. The central fovea of the retina (fovea centralis) contains only cones (up to 140 thousand per 1 mm). Toward the periphery of the retina, their number decreases, and the number of rods increases.

Each photoreceptor - rod or cone - consists of a light-sensitive outer segment containing a visual pigment and an inner segment that contains the nucleus and mitochondria that provide energy processes in the photoreceptor cell.

The outer segment is a photosensitive area where light energy is converted into a receptor potential. Electron microscopic studies have revealed that the outer segment is filled with membrane discs formed by the plasma membrane. In sticks, each outer segment contains 600-1000 disks, which are flattened membrane sacs stacked like a column of coins. There are fewer membrane discs in cones. This partly explains more high sensitivity sticks to the light(the wand can excite everything one quantum of light, a It takes over 100 photons to activate a cone.

Each disc is a double membrane consisting of a double layer phospholipid molecules between which there are protein molecules. Retinal, which is part of the visual pigment rhodopsin, is associated with protein molecules.

The outer and inner segments of the photoreceptor cell are separated by membranes through which the beam passes from 16-18 thin fibrils. The inner segment passes into a process, with the help of which the photoreceptor cell transmits excitation through the synapse to the bipolar nerve cell in contact with it.

The outer segments of the receptors face the pigment epithelium so that light first passes through 2 layers nerve cells and internal segments of the receptors, and then reaches the pigment layer.

cones operate in high light conditions provide day and color vision, and sticks- are responsible for twilight vision.

Visible to us the spectrum of electromagnetic radiation is enclosed between the short-wave (wavelengthfrom 400nm) radiation, which we call violet and long-wave radiation (wavelengthup to 700 nm ) called red. The sticks contain a special pigment rhodopsin, (consists of vitamin A aldehyde or retinal and protein) or visual purple, the maximum of the spectrum, the absorption of which is in the region of 500 nanometers. It resynthesizes in the dark and fades in the light. With a lack of vitamin A, twilight vision is disturbed - "night blindness".

In the outer segments of the three types of cones ( blue-, green- and red-sensitive) contains three types of visual pigments, the maximum absorption spectra of which are in blue (420 nm), green(531 nm) and red(558 nm) parts of the spectrum. red cone pigment was named - "iodopsin". The structure of iodopsin is close to that of rhodopsin.

Consider the sequence of changes:

Molecular Physiology of Photoreception: Intracellular recordings from animal cones and rods have shown that in the dark, a dark current flows along the photoreceptor, leaving the inner segment and entering the outer segment. Illumination leads to the blockade of this current. The receptor potential modulates transmitter release ( glutamate) at the photoreceptor synapse. It has been shown that in the dark the photoreceptor continuously releases a neurotransmitter that acts depolarizing way on the membranes of postsynaptic processes of horizontal and bipolar cells.

Rods and cones have a unique electrical activity among all receptors, their receptor potentials under the action of light - hyperpolarizing, action potentials under their influence do not arise.

(When light is absorbed by a molecule of visual pigment - rhodopsin, an instantaneous isomerization its chromophore group: 11-cis-retinal is converted to trans-retinal. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes colorless and passes into the state methodopsin II As a result, the visual pigment molecule acquires the ability to interact with another membrane proteinG uanosine triphosphate (GTP) -binding protein - transducin (T) .

In complex with metarhodopsin, transducin enters the active state and exchanges ganosite diphosphate (GDP) associated with it in the dark for (GTP). Transducin+ GTP activate another membrane-bound protein molecule, the phosphodiesterase (PDE) enzyme. Activated PDE destroys several thousand cGMP molecules .

As a result, the concentration of cGMP in the cytoplasm of the outer segment of the receptor decreases. This leads to the closure of ion channels in the plasma membrane of the outer segment, which were opened In the dark and through which inside the cell included Na+ and Ca. Ion channels close due to the concentration of cGMP, which kept the channels open, drops. It has now been found that the pores in the receptor open due to cGMP to cyclic guanosine monophosphate .

The mechanism of restoration of the initial dark state of the photoreceptor associated with an increase in the concentration of cGMP. (in the dark phase with the participation of alcohol dehydrogenase + NADP)

Thus, the absorption of light by photopigment molecules leads to a decrease in the permeability for Na, which is accompanied by hyperpolarization, i.e. the emergence of receptor potential. The hyperpolarization receptor potential that has arisen on the membrane of the outer segment then spreads along the cell to its presynaptic ending and leads to a decrease in the rate of mediator release - glutamate . In addition to glutamate, retinal neurons can synthesize other neurotransmitters, such as acetylcholine, dopamine, glycine GABA.

Photoreceptors are interconnected by electrical (gap) contacts. This connection is selective: sticks are connected with sticks, and so on.

These responses from photoreceptors converge on horizontal cells, which lead to depolarization in neighboring cones, a negative Feedback which enhances light contrast.

At the level of receptors, inhibition occurs and the cone signal ceases to reflect the number of absorbed photons, but carries information about the color, distribution, and intensity of light incident on the retina in the vicinity of the receptor.

There are 3 types of retinal neurons - bipolar, horizontal and amacrine cells. Bipolar cells directly bind photoreceptors to ganglion cells, i.e. carry out the transmission of information through the retina in the vertical direction. Horizontal and amacrine cells transmit information horizontally.

Bipolar cells occupy in the retina strategic position, since all the signals that arise in the receptors coming to the ganglion cells must pass through them.

It has been experimentally proven that bipolar cells have receptive fields in which allocate center and periphery (John Dowling- et al. Harvard Medical School).

Receptive field - a set of receptors that send signals to a given neuron through one or more synapses.

Receptive fields size: d=10 µm or 0.01 mm - outside the central fossa.

In the very holed=2.5 µm (due to this, we are able to distinguish between 2 points at visible distance between them is only 0.5 arc minutes-2.5 microns - if you compare, this is a coin of 5 kopecks at a distance of about 150 meters)

Starting from the level of bipolar cells, the neurons of the visual system differentiate into two groups that react in opposite ways to lighting and darkening:

1 - cells, excited by illumination and inhibited by darkness "on" - neurons and

Cells Excited by darkness and inhibited by illumination - " off"- neurons. An on-center cell discharges at a markedly increased frequency.

If you listen to the discharges of such a cell through a loudspeaker, then at first you will hear spontaneous impulses, separate random clicks, and then after turning on the light, a volley of impulses occurs, reminiscent of a machine-gun burst. On the contrary, in cells with an off-reaction (when the light is turned off - a volley of impulses) This division is preserved at all levels of the visual system, up to and including the cortex.

Within the retina itself, information is transmitted impulseless way (distribution and transsynaptic transmission of gradual potentials).

In horizontal, bipolar and amocrine cells, signal processing occurs through slow changes in membrane potentials (tonic response). PD is not generated.

Rod, cone, and horizontal cell responses are hyperpolarizing, while bipolar cell responses can be either hyperpolarizing or depolarizing. Amacrine cells create depolarizing potentials.

To understand why this is so, one must imagine the influence of a small bright spot. The receptors are active in the dark, and light, causing hyperpolarization, reduces their activity. If a excitatory synapse, the bipolar will activate in the dark, a become inactivated in the light; if the synapse is inhibitory, the bipolar is inhibited in the dark, and in the light, turning off the receptor, removes this inhibition, i.e. the bipolar cell is activated. That. whether the receptor-bipolar synapse is excitatory or inhibitory depends on the mediator secreted by the receptor.

Horizontal cells are involved in the transmission of signals from bipolar cells to ganglion cells, which transmit information from photoreceptors to bipolar cells and then to ganglion cells.

Horizontal cells respond to light by hyperpolarization with pronounced spatial summation.

Horizontal cells do not generate nerve impulses, but the membrane has non-linear properties that ensure impulse-free signal transmission without attenuation.

Cells are divided into two types: B and C. B-type cells, or luminosity, always respond with hyperpolarization, regardless of the wavelength of light. C-type cells, or chromatic cells, are divided into two- and three-phase. Chromatic cells respond with either hyper or depolarization depending on the length of the stimulating light.

Biphasic cells are either red-green (depolarized with red light, hyperpolarized with green) or green-blue (depolarized with green light, hyperpolarized with blue). Triphasic cells are depolarized by green light, and blue and red light cause membrane hyperpolarization. Amacrine cells regulate synaptic transmission in the next step from bipolar to ganglion cells.

The dendrites of amacrine cells branch out in the inner layer, where they are in contact with the processes of the bipolars and the dendrites of the ganglion cells. Centrifugal fibers coming from the brain terminate on amacrine cells.

Amacrine cells generate gradual and pulse potentials (phasic nature of the response). These cells respond with a rapid depolarization to turning the light on and off and show a weak

spatial antagonism between center and periphery.

The retina is the main part of the eye visual analyzer. Here is the perception of electromagnetic light waves, their transformation into nerve impulses and transmission to the optic nerve. Day (color) and night vision are provided by special retinal receptors. Together they form the so-called photosensory layer. Based on their shape, these receptors are called cones and rods.

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General concepts

Microscopic structure of the eye

Histologically, 10 cell layers are isolated on the retina. The outer photosensitive layer consists of photoreceptors (rods and cones), which are special formations of neuroepithelial cells. They contain visual pigments capable of absorbing light waves of a certain wavelength. Rods and cones are unevenly distributed on the retina. Most of the cones are located in the center, while the rods are on the periphery. But this is not their only difference:

1. Sticks provide night vision. This means that they are responsible for the perception of light in low light conditions. Accordingly, with the help of sticks, a person can see objects only in black and white.
2. Cones provide visual acuity throughout the day. With their help, a person sees the world in a color image.

Rods are sensitive only to short waves, the length of which does not exceed 500 nm (blue part of the spectrum). But they are active even when scattered light when the photon flux density is reduced. Cones are more sensitive and can perceive all color signals. But for their excitation, light of much greater intensity is required. In the dark, visual work is carried out by sticks. As a result, at dusk and at night, a person can see the silhouettes of objects, but does not feel their colors.

Dysfunction of retinal photoreceptors can lead to various pathologies vision:

violation of color perception (color blindness);
inflammatory diseases of the retina;
stratification of the retinal membrane;
impaired twilight vision (night blindness);
photophobia.

cones

People with good eyesight have about seven million cones in each eye. Their length is 0.05 mm, width - 0.004 mm. Their sensitivity to the flow of rays is low. But they qualitatively perceive the whole gamut of colors, including shades.

They are also responsible for the ability to recognize moving objects, as they respond better to the dynamics of lighting.

The structure of cones

Schematic structure of cones and rods

The cone has three main segments and a constriction:

1. Outer segment. It is he who contains the light-sensitive pigment iodopsin, which is located in the so-called semi-disks - folds of the plasma membrane. This area of the photoreceptor cell is constantly updated.
2. The constriction formed by the plasma membrane serves to transfer energy from internal segment outside. It is the so-called cilia that carry out this connection.
3. The inner segment is the area of active metabolism. Here are mitochondria - the energy base of cells. In this segment, there is an intensive release of energy necessary for the implementation of the visual process.
4. The synaptic ending is an area of synapses - contacts between cells that transmit nerve impulses to the optic nerve.

Three-component hypothesis of color perception

It is known that cones contain a special pigment - iodopsin, which allows them to perceive the entire color spectrum. According to the three-component hypothesis of color vision, there are three types of cones. Each of them contains its own type of iodopsin and is able to perceive only its part of the spectrum.

1. L-type contains erythrolab pigment and captures long waves, namely the red-yellow part of the spectrum.
2. M-type contains chlorolab pigment and is able to perceive medium waves emitted by the green-yellow region of the spectrum.
3. S-type contains the pigment cyanolab and reacts to short waves, perceiving the blue part of the spectrum.

Many scientists dealing with the problems of modern histology note the inferiority of the three-component hypothesis of color perception, since no confirmation has yet been found of the existence of three types of cones. In addition, no pigment has yet been discovered, which was previously given the name cyanolab.

Two-component hypothesis of color perception

According to this hypothesis, all retinal cones contain both erytolab and chlorolab. Therefore, they can perceive both long and middle part spectrum. And its short part, in this case, perceives the pigment rhodopsin contained in the sticks.

In favor of this theory is the fact that people who are not able to perceive the short waves of the spectrum (that is, its blue part) simultaneously suffer from visual impairment in low light conditions. Otherwise, this pathology is called " night blindness and is caused by dysfunction of the retinal rods.

sticks

The ratio of the number of rods (gray) and cones (green) on the retina

The sticks look like small elongated cylinders, about 0.06 mm long. An adult healthy person has approximately 120 million of these receptors on the retina in each eye. They fill the entire retina, concentrating mainly on the periphery. The macula lutea (the area of the retina where vision is most acute) contains practically no rods.

The pigment that makes rods highly sensitive to light is called rhodopsin or visual purple. . In bright light, the pigment fades and loses this ability. At this point, it is only susceptible to short light waves, which make up the blue region of the spectrum. In the dark, its color and qualities are gradually restored.

The structure of the sticks

Rods have a structure similar to that of cones. They consist of four main parts:

1. The outer segment with membrane discs contains the pigment rhodopsin.
2. The connecting segment or cilium makes contact between the outer and inner sections.
3. The inner segment contains mitochondria. Here is the process of generating energy.
4. The basal segment contains nerve endings and carries out the transmission of impulses.

The exceptional sensitivity of these receptors to the effects of photons allows them to convert light stimulation into nervous excitement and send it to the brain. This is how the process of perception of light waves is carried out. human eye- photoreception.

Man is the only living being capable of perceiving the world in all its richness of colors and shades. Eye protection against harmful effects and prevention of visual impairment will help preserve this unique ability for many years.

Hello, dear readers! We all have heard that eye health should be protected from a young age, because lost vision cannot always be returned. Have you ever thought about how the eye works? If we know this, then it will be easier for us to understand what processes provide visual perception of the world around us.

The human eye has a complex structure. Perhaps the most mysterious and complex element is the retina. This is a thin layer of nervous tissue and vessels. But it is on him that essential function processing the information received by the eye into nerve impulses, allowing the brain to create a color three-dimensional picture.

Today we will talk about the receptors of the nervous tissue of the retina - namely, the rods. What is the light sensitivity of the retinal rod receptors and what allows us to see in the dark?

Rods and cones

Both of these elements are funny names- photoreceptors that give an image fixed by the lens and parts of the cornea.

There are a lot of those and others in the human eye. Cones (they look like tiny jugs) - about 7 million, and rods ("cylinders") even more - up to 120 million! Of course, their dimensions are negligible and amount to fractions of millimeters (μm). The length of one stick is 60 microns. The cones are even smaller - 50 microns.

The sticks got their name due to their shape: they resemble microscopic cylinders.

They consist of:

membrane disks;
nervous tissue;
mitochondria.

And they are provided with cilia. A special pigment - the protein rhodopsin - allows cells to "feel" light.

Rhodopsin (this is a protein plus a yellow pigment) reacts to a beam of light in the following way: under the action of light pulses, it decomposes, thus causing irritation of the optic nerve. I must say, the susceptibility of the "cylinders" is amazing: they capture information even from 2 photons!

Differences between photoreceptors in the eye

The differences start with the location. "Jugs" "crowd" closer to the center. They are "responsible" for central vision. In the center of the retina, in the so-called "yellow spot", there are especially many of them.

The density of the accumulation of "cylinders", on the contrary, is higher towards the periphery of the eye.

In addition, the following features can be noted:

cones contain less photopigment than rods;
the total number of "cylinders" is 2 dozen times greater;
sticks are able to perceive any light - diffused and direct; and the cones are exceptionally straight;
with the help of cells located on the periphery, we perceive black and white colors(they are achromatic);
with the help of those gathering in the center - all colors and shades (they are chromatic).

Each of us is able, thanks to the "jugs" to see up to a thousand shades. And the artist's eye is even more sensitive: it sees even up to a million shades of colors!

An interesting fact: in order to carry out the transmission of impulses, several rods require only one neuron. Cones are "more demanding": each needs its own neuron.

"Cylinders" are highly sensitive, "jugs" need stronger light pulses so that they can perceive and transmit them.

In fact, thanks to them we can see in the dark. In conditions of reduced illumination (late in the evening, at night), cones cannot "work". But the sticks begin to act in full force. And since they are located on the periphery, in the dark we better catch movements not directly in front of us, but on the sides.

Oh, and one more thing: sticks react faster.

Take note: when going somewhere in the dark, do not try to stare at the area directly in front of your eyes. You won’t see anything anyway, because the “jugs” located in the center of the retina are now powerless. But if you “turn on” peripheral vision, you will be able to navigate much better. It is the "cylinders" that "work".

Despite the significant difference in the performance of the tasks set by nature, photoreceptors cannot be considered separately from each other. Only together they give a single holistic picture.

By absorbing light quanta, the cells convert the energy into a nerve impulse. It goes to the brain. The result - we see the world!

Why do cats see us better in the dark?

Now, having studied in general terms the structure and functions of photoreceptors, we can answer the question of why our mustachioed pets are much better at navigating in the dark than we are.

The casket opens simply: the structure of the eye of this mammal is similar to a human one. But if a person has about 4 rods per 1 cone, then a cat has 25! It is not surprising that a domestic predator perfectly distinguishes the outlines of objects in almost complete darkness.

Rods and cones are our helpers

"Cylinders" and "jugs" are an amazing invention of nature. If they function correctly, a person sees well in the light and can navigate in the dark.

If they cease to perform their functions in full, there are:

light glare before the eyes;
deterioration of visibility in the dark;
are already in the field of view.

Over time, visual acuity changes for the worse. Color blindness, hemeralopia (decreased night vision), retinal detachment - these are the consequences of a violation of the photoreceptors.

But let's not end our conversation on that sad note. modern medicine learned to cope with most of the diseases that previously caused blindness. The patient is required only an annual preventive examination.

Did you find any benefit in our article? If you have a little less questions related to the structure and work of the organs of vision, we can consider our task completed. And one more thing: please share the received information with your friends, and you can send us your comments and remarks. We are waiting for responses. Your feedback is always welcome!

Thanks to vision, a person cognizes the surrounding reality and orients himself in space. Of course, without the rest of the senses it is difficult to compile a complete picture of the world, but the eyes perceive almost 90% of general information that enters the brain from outside.

By using visual function a person is able to see the phenomena happening next to him, can analyze different events, find differences between one object and another, and also notice an impending threat.

The organs of vision are arranged in such a way that they distinguish not only the objects themselves, but also the color variety of living and inanimate nature. Responsibility for this lies with special microscopic cells - sticks and cones present in the retina of the eye. It is they who are initial link in the chain for transmitting information about the seen object to the occipital part of the brain.

AT structural structure retina cones and rods are assigned a well-defined area. These visual receptors, penetrating the nervous tissue that forms retina, contribute to the rapid conversion of the resulting light flux into a combination of pulses.

An image is formed in the retina, designed with the direct participation of the eye area of the cornea and the lens. At the next stage, the image is processed, after which the nerve impulses, moving along visual pathway deliver information to the right part of the brain. The complex and fully formed device of the eyes makes it possible to instantly process any information.

The main share of photographic receptors is concentrated in the so-called macula. This is the area of the retina located in its central zone. Because of the corresponding color, the macula is also called the yellow spot of the eye.

Cones are visual receptors that respond to light waves. Their functioning is directly related to a special pigment - iodospin. This multicomponent pigment consists of chlorolab (responsible for the perception of the green-yellow spectrum) and erythrolab (sensitive to the red-yellow spectrum). To date, these are two thoroughly studied pigments.

A person with perfect vision has almost seven million cones in the retina. They are microscopic in size and are inferior to sticks in geometrical parameters. The length of a single cone is about fifty micrometers, and the diameter is about four. It should be noted that the sensitivity of cones to light rays is about a hundred times lower than that of rods. However, thanks to them, the eye can qualitatively perceive sharp movements of objects.

The cones form four separate zones. The outer region is represented by semi-disks. The waist acts as a connecting department. Inner area contains a set of mitochondria. Finally, the fourth zone is the area of neural contacts.

The outer region is completely formed by semi-discs formed from the plasma membrane. These are membranous folds of microscopic dimensions, completely covered with sensitive pigments. Regular phagocytosis of these formations, as well as their constant renewal in the receptor body, allow the renewal of the outer region of the cone. Pigment production occurs in this area. Up to a hundred half-disk can be updated per day plasma membranes. For full recovery the entire set of half discs will take approximately two weeks.
The connecting region, protruding the membrane, creates a bridge between the outer and inner portions of the cones. Communication is established with the participation of a pair of cilia and the internal contents of the cells. Cilia and cytoplasm can move from one area to another.
The inner region is the zone of active metabolism. The mitochondria that fill this zone transport the energy substrate for visual function. This part contains the nucleus.
synaptic region. Here there is an energy contact of bipolar cells.

Visual acuity is under the influence of monosynaptic bipolar cells that connect cones and ganglion cells.

There are three types of cones depending on the susceptibility to spectral waves:

S-type. Demonstrate sensitivity to short wavelengths of blue-violet color.
M-type. Cones that capture from the mid-wave spectrum. This is a yellow-green color scheme.
L-type. Sensitive to long wavelength red-yellow colors.

The shape of the sticks is similar to a cylinder, having a uniform diameter along the entire length. The length of these eye receptors is almost thirty times greater than their diameter, so the shape of the rods is visually elongated. The rods of the retina are composed of four elements: membrane discs, cilia, mitochondria and nervous tissue.

The sticks have maximum light sensitivity, which guarantees their response to the smallest light flash. The receptor apparatus of the rods will be activated even when exposed to a single photon of energy. This unique ability of the rods helps a person to navigate at dusk and provides maximum clarity of objects in the dark.

Unfortunately, in their composition, the sticks have only one pigment element, called rhodopsin. It is also referred to as visual purple. The fact that there is only one pigment makes it impossible for these visual receptors to distinguish between shades and colors. Rhodopsin does not have the ability to instantly respond to an external light stimulus, as cone pigments can.

Being a complex protein compound containing a set of visual pigments, rhodopsin belongs to the group of chromoproteins. It owes its name to its bright red color. The purplish hue of the retinal rods has been discovered as a result of numerous laboratory research. Visual purple has two components - a yellow pigment and a colorless protein.

Under the action of light rays, rhodopsin begins to rapidly decompose. The products of its decay affect the formation of visual excitability. Having recovered, rhodopsin maintains twilight vision. From bright lighting the protein decomposes, and its photosensitivity shifts to the blue region of vision. Full recovery stick squirrel healthy person may take approximately half an hour. During this period of time, night vision reaches its maximum level, and a person begins to look at the outlines of objects.

Symptoms of damage to the rods and cones of the eyes

Pathologies marked by damage to these visual receptors are accompanied by the following symptoms:

Visual acuity is lost.
There are sudden flashes and glare before the eyes.
Decreased ability to see in the dark.
A person cannot distinguish between different colors.
Narrows the field of visual perception. AT rare cases tubular vision is formed.

Diseases that are associated with a violation of the photoreceptor functions of rods and cones:

Daltonism m. Hereditary congenital pathology expressed in the inability to distinguish colors.
Hemeralopia. The pathology of the rods causes a decrease in visual acuity in the dark.
Retinal detachment eyes.
Macular degeneration. Violation of the nutrition of the vessels of the eye, leads to a decrease in central vision.

The light-sensitive part of the eye is a mosaic of light-responsive cells (photoreceptors) located on the retina. The retina of the eye contains two types of light-sensitive receptors, occupying an area with a solution of about 170 ° relative to the visual axis: 120 ... 130 million rods (long and thin night vision receptors), 6.5 ... 7.0 million cones (short and thick day vision receptors) . Before reaching the retina, light must first pass through a layer of nervous tissue and a layer blood vessels. Such an arrangement photosensitive elements from point of view common sense is not optimal. Any designer of a television camera would take care to mount the connecting wires so as not to interfere with the light falling on the photocells. The retina is built on a different principle and the reasons for this reversal of the retina are not fully understood.

Rods and cones are tightly adjacent to each other with elongated sides. Their dimensions are very small: the length of the rods is 0.06 mm, the diameter is 0.002 mm, the length and diameter of the cones are 0.035 and 0.006 mm, respectively. Density of rods and cones different areas retina ranges from 20,000 to 200,000 per 1 mm 2 . In this case, cones predominate in the center of the retina, rods - on the periphery. In the center of the retina is the so-called yellow spot oval shape(length 2 mm, width 0.8 mm). There are almost only cones in this place. The "yellow spot" is the area of the retina that provides the clearest sharp vision.

Rods and cones differ in the light-sensitive substances they contain. The substance of the sticks is rhodopsin (visual purple). The maximum light absorption of rhodopsin corresponds to a wavelength of approximately 510 nm (green light), i.e., rods have a maximum sensitivity to radiation with λ = 510 nm . The photosensitive substance in cones (iodopsin) comes in three types, each of which has a maximum absorption in various zones spectrum.

Under the influence of light, the molecules of photosensitive substances dissociate (decompose) into positively and negatively charged particles. When the concentration of ions and, consequently, their total electric charge reach a certain value, under the action of a charge in the nerve fiber, a current pulse arises, which is sent to the brain.

The light decay reactions of rhodopsin and iodopsin are reversible, i.e., after they have been decomposed into ions under the action of light and the charge of the ions has excited a current pulse in the nerve, these substances are restored again in their original light-sensitive form. Energy for recovery is provided by products that enter the eye through an extensive network of tiny blood vessels. Thus, a continuous cycle of destruction and subsequent restoration of photosensitive substances is established in the eye.

If the level of the amount of light acting on the eye does not change with time, then a mobile equilibrium is established between the concentrations of substances in the states of decay and the original light-sensitive form. The value of this concentration depends on the amount of light acting on the eye at a given or previous moment, i.e. light sensitivity eyes change with various levels active light.

It is known that if you enter from a bright light into a very dimly lit room, at first the eye does not distinguish anything. Gradually, the ability of the eye to distinguish objects is restored. After a long stay in the dark (about 1 hour), the sensitivity of the eye becomes maximum, since the concentration of photosensitive substances reaches its upper limit. If, however, after a long stay in the dark, you go out into the light, then at the first moment the eye will be in a state of blindness: the restoration of photosensitive substances lags behind their decay. Gradually, the eye adapts to the level of illumination and begins to work normally.

Recall that the property of the eye to adapt to the level of the amount of acting light, which is expressed by a change in its light sensitivity, is called adaptation.

Sticks - night vision. Rods can react to the smallest amount of light. They are responsible for our ability to see moonlight, the light of the starry sky, and even in cases where this starry sky is hidden by clouds. On fig. 2.2, the dotted curve shows the dependence of the sensitivity of the rods on the wavelength. The rods provide only achromatic or color neutral perception in the form of white, gray and black. Moreover, each wand has no direct connection to the brain. They form groups. Such a device explains the high sensitivity of rod vision, but prevents it from distinguishing the smallest details with its help. These facts explain the general colorlessness and fuzziness of night vision and the validity of the proverb: “At night all cats are

ry".

Rice. 2.2. Relative spectral sensitivity of rods and cones

Cones - day vision. The reaction of cones is more complex than that of rods. Instead of simply distinguishing between light and dark, and perceiving a number of different gray flowers The cones are responsible for the perception of chromatic colors. In other words, with cone vision, we can see different colors. The spectral distribution of the sensitivity of cone vision by wavelength is shown in fig. 2.2 with a solid line. This curve is called the visibility curve, as well as the curve of the spectral sensitivity of the eye. Rod vision, compared to cone vision, is much more sensitive to radiation in the short-wavelength part of the visible spectrum, and the sensitivity to radiation in the long-wavelength (red) part of the spectrum is approximately the same as that of cones. However, the cones continue to respond to small increases in the intensity of the incident light (forming the image on the retina) even when the density of its flux for some time becomes so great that the rods no longer respond to them - they are saturated. In other words, all sticks in this case give the maximum possible number nerve signals. Thus, our daytime vision is provided almost entirely by cones. The shift in sensitivity to light along the wavelength axis from cone (day) vision to rod (or night) vision is called the Purkinje effect (more correctly Purkinet). This "Purkinje shift", named after the Czech scientist Purkinje, who first discovered it in 1823, determines the fact that an object that is red in daylight is perceived by us as black in night or twilight lighting, while an object perceived during the day it looks blue, at night it appears light gray.

Having two types of light-sensitive receivers (rods and cones) in humans is a great advantage. Not all animals are so lucky. Chickens, for example, have only cones and therefore must go to bed at sunset. Owls have only sticks; they have to squint their eyes all day.

Rods and cones - twilight vision. Both rods and cones are involved in dim vision. Twilight is the range of illumination that extends from the illumination produced by radiation from the sky when the sun has sunk more than a few degrees below the horizon to the illumination produced by rising high in the sky. clear sky moon in half phase. Twilight vision also includes vision in a dimly lit (for example, candles) room. Since under such conditions the relative contribution of rod and cone vision to total visual perception is constantly changing, color judgments are extremely unreliable. However, there are a number of products that need to be color-rated using this kind of mixed vision, since they are intended for consumption by us in dim light. An example is the phosphorescent paint used in road signs for dark conditions.

Brain work

Information from the receptors is transmitted to the brain along the optic nerve, which contains about 800,000 fibers. In addition to this direct transmission of excitation from the retina to the brain centers, there is a complex feedback to control, for example, the movements of the eyeballs.

Somewhere in the retina, a complex processing of information takes place - the logarithm of the current density and the transformation of the logarithm into the frequency of impulses. Further, information about the brightness, encoded by the pulse frequency, is transmitted through the optic nerve fiber to the brain. However, not just a current passes through the nerve, but difficult process excitation, some combination of electrical and chemical phenomena. Unlike electric current emphasized by the fact that the speed of signal propagation along the nerve is very low. It lies in the range from 20 to 70 m/s.

The information coming from the three types of cones is converted into impulses and encoded in the retina before transmission to the brain. This encoded information is sent as a brightness signal from all three types of cones, as well as a difference signal for every two colors (Fig. 2.3). The second brightness channel is also connected here, probably originating from an independent rod system.

The first difference color signal is short-circuit signal. It is formed by red and green cones. The second signal is signal J-S, which is obtained in a similar way, except that the information about yellow obtained by adding the input signals

cash from K+Z cones.

Fig.2.3. Visual system model

The brain has been likened more than once to a giant center that collects and processes a large amount of information. Trying to figure out the millions of compounds of this incredibly complex device were in to a large extent successful. We know, for example, that the optic nerve of one eye connects to the optic nerve of the other (cross optic nerves) so that nerve fibers right half of one retina go next to the fibers from the right half of the other retina and, after passing through the relay station (the geniculate body) in the midbrain, they end up in almost the same place in the occipital lobe of the brain, in its back part. Excitations of the retinas are projected in this lobe, and part of them, corresponding to the center of the eye ( yellow spot), in to a large extent increased in comparison with excitations of other parts of the retina. The relay station has the ability for side connections, and itself occipital part has many connections to all other parts of the brain.