How the eye works. Physiology of vision

Vision plays a very important role in the life of man and most animals, ensuring the perception of information about objects and properties of the environment - light, shape, size, color, etc.

The organ of vision - the eye (50) - is located in the eye socket of the skull. Of eyeball  out the optic nerve connecting it with the brain. The eyeball consists of the inner core and the three shells surrounding it — the outer, middle, and inner. The outer shell - the sclera, or the protein shell, is a rigid, opaque connective tissue capsule, which passes into the transparent cornea from the front, through which light enters the eye. Under it is the choroid, which passes in front of the ciliary body, where the ciliary muscle is located, regulating the curvature of the lens, and the iris, in the center of which there is a hole - the pupil - is able to narrow and expand under the influence of muscles embedded in the thickness of the iris. The choroid is rich in blood vessels and contains a black pigment layer that absorbs light. In the inner lining of the eye - the retina - there are light-sensitive receptors - rods and cones. In them, the energy of light turns into a process of excitation, which is transmitted along the optic nerve to the occipital lobe of the cerebral cortex. Cones are concentrated in the center of the retina, opposite the pupil - in the yellow spot - and provide daytime vision, perceiving the colors, shape and details of objects. At the periphery of the retina there are only sticks, which are irritated by a weak dim light, but do not have the ability to perceive colors.

ANALYZERS SENSITIVES

The place where the optic nerve leaves the retina is receptor-free and is called a blind spot.

The inner core of the eyeball forms (together with the cornea) the optical system of the eye and consists of the lens, vitreous body and aqueous humor of the anterior and posterior chambers of the eye. The transparent and elastic crystalline lens, located behind the pupil, has the shape of a biconvex lens. Together with the cornea and intraocular fluids, it refracts the rays of light entering the eye and focuses them on the retina. With the reduction of the ciliary muscle, the lens changes its curvature, taking shape for the far or near vision. The refracted rays of light from the object in question, falling on the retina, form on it a reduced inverse image of the object. However, we see objects in direct form due to the daily training, the visual analyzer, which is achieved by the formation of conditioned reflexes, the testimony of other analyzers, their interactions, constant testing of visual sensations, and everyday practice.

The auxiliary apparatus of the eye consists of protective devices, the lacrimal and motor apparatus. The protective formations include eyebrows, eyelashes and eyelids, covered on the inside with a mucous membrane that passes to the eyeball. Tears, secreted by the lacrimal gland, wash the eyeball, constantly moisturize the cornea and drain through the lacrimal canal into the nasal cavity. The motor apparatus of each eye consists of six muscles, the contractions of which allow changing the direction of the gaze.

In people with normal vision, a clear image of objects appears on the retina.

Visual impairment is often the result of an abnormal length of the eyeball. Myopia develops with an increased longitudinal axis of the eye. Parallel rays coming from distant objects are collected (focused) in front of the retina, into which the diverging rays fall, and as a result a blurry image is obtained. When myopia is prescribed glasses with diffusing biconcave glass, reducing the refraction of the rays so that the image of objects occurs on the retina.

When the shortened axis of the eyeball is observed   farsightedness.  The image is focused behind the retina. Biconvex glasses are required for correction. Presbyopic vision usually develops after 40 years, when the lens loses its elasticity, hardens and loses its ability to change the curvature, which makes it difficult to see clearly at close range. The eye loses the ability to clearly see objects of different sizes.

Observance of simple rules of hygiene of sight allows to prevent an overstrain and to avoid a vision disorder.

It is necessary that the workplace is well lit, but not too bright light, which should fall to the left. Sources of artificial light should be covered with lamp shades. When reading, writing, working with small objects, the distance from objects to the eyes should be 30-35 cm. It is harmful to read while lying down or in a moving vehicle. Smoking and alcohol have a detrimental effect on vision. To avoid infectious diseases of the eye, you need to protect them from dust, do not rub them with your hands, wipe only with a clean handkerchief or towel.

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1. Where are the photosensitive eye receptors?

• in the retina
• in the lens
• in the iris

The retina is a very thin and very delicate layer of cells - the visual receptors.

2. What are the protective membranes of the eye called?

• lens and pupil
• albuginea and cornea
• choroid

The eyeball is covered with a dense albuminous membrane that protects it from mechanical and chemical damage and penetration of foreign particles and microorganisms from the outside. This shell in front of the eye is transparent. She is the cornea, which transmits the rays of light.

3. In which part of the analyzer does the difference in stimulation begin?

• in the cerebral cortex
• in sensitive nerves
• in receptors

Irritation begins at the receptors.

4. Pigmentation of which part of the eye determines its color?

• retina
• lens
• iris

The iris is the anterior part of the choroid. The pigment in it determines the color of the eye. With a small amount of pigment eyes - gray and blue, with a large - brown or black, in the absence of it - red (white mice, rats, rabbits).

5. Place the projection of the object in the eyeball.

• retina
• lens
• pupil

The retina has a complex structure and contains a photosensitive device - rods and cones. The outer layer is covered with black pigment. It absorbs light, preventing its reflection and scattering, which contributes to the clarity of visual perception.

6. In which part of the ear are sound-sensitive receptors?

• in the auditory ossicles
• in the snail
• in eardrum

The auditory part is called a cochlea, it perceives sound vibrations, and turns them into nervous excitement. According to the processes of the centrifugal neurons entering the auditory nerve, excitation is carried out in the medulla, and then in the auditory part of the cerebral cortex. Here ends the path of the auditory analyzer.

7. Where are the conductive bones?

• in the snail
• in the middle ear
• in the auditory cortex

The main function of the middle ear is to conduct sounds from the eardrum through the conductive bones (auditory) to the oval window.

8. What external stimuli distinguish receptors of the nasal cavity?

• smells
• subject shape
• taste qualities

Odor perception is carried out with the help of special receptors located in the nasal cavity. The processes of the olfactory cells form the olfactory nerve, which carries excitation in the central nervous system. The receptors of the olfactory organ are excited only by gaseous substances.

9. The analyzer is called ...

• receptors
• nerves
• no right answer

Functional systems that provide analysis (difference) of stimuli acting on the body.

10. What is the sensitive part of the visual analyzer?

• optic nerve
• sticks and cones
• pupil

The photosensitive device - sticks and cones. Cones function in bright light and distinguish colors and details of objects. Thanks to the chopsticks a person sees in the twilight.

11. The conductive part of the visual analyzer.

• retina
• pupil
• optic nerve

Axons of neurons form the optic nerve. In the retina, light is converted into nerve impulses that are transmitted along the optic nerve to the brain to the visual cortex of the cerebral hemispheres. In this zone, the final distinction of irritation occurs - the shape of objects, their color, size, light, location and movement.

12. What is the cause of myopia in children?

• elongated eyeball
• optic nerve fatigue
• loss of lens flexibility

The crystalline lens is a transparent avascular biconvex body. Myopia develops from prolonged eyestrain (which leads to a loss of flexibility of the lens), associated with a lack of lighting.

13. Disruption of function leads to night blindness ...

• lens
• cones
• chopsticks

Disruption of the normal activity of the chopsticks in the retina causes a disease known as "night blindness". The patient sees well during the day, but as dusk approaches, his vision deteriorates and he almost stops seeing.

14. Where does the conversion of sound waves into biocurrents occur?

• in snail receptors
• in the auditory area
• in the auditory ossicles

The snail is an organ that perceives sound vibrations and turns them into nervous excitement.

15. What colors and their combinations have the most favorable and beneficial effect on the higher nervous activity of a person?

• red and yellow
• blue and green
• their diversity and brightness

The most favorable and beneficial effect on the higher nervous activity of a person is blue and green.

Information about the world around 90% of people receive through the organ of vision. The role of the retina is visual function. The retina consists of photoreceptors of a special structure - cones and rods.

Rods and cones are photographic receptors with a high degree of sensitivity, they convert light signals from outside into impulses perceived by the central nervous system, the brain.

When illuminated - during daylight - cones experience an increased load. Rods are responsible for twilight vision - if they are not active enough, night blindness appears.

Cones and rods in the retina have a different structure, since their functions are different.

The structure of the human organ of vision

The organ of vision also includes the vascular part and the optic nerve, transmitting signals received from the outside to the brain. The division of the brain that receives and transforms information is also considered one of the divisions of the visual system.

Where are the sticks and cones? Why aren't they listed? These are the receptors of the nervous tissue that make up the retina. Thanks to the cones and chopsticks, the retina receives a picture fixed by a section of the cornea and lens. Impulses transmit an image to the central nervous system, where information processing takes place. This process is carried out in a matter of seconds - almost instantly.

Most of the sensitive photoreceptors are located in the macula, the so-called central region of the retina. The second name of the macula is the yellow spot of the eye. This name was given to the macula because when examining this area, a yellowish tint is clearly visible.

The structure of the outer part of the retina includes pigment, in the inner - light-sensitive elements.

Cones in the eye

Cones were called because they are shaped like flasks, only very small. In an adult, the retina includes 7 million of these receptors.

Each cone consists of 4 layers:

• outer - membrane discs with iodopsin color pigment; It is this pigment that provides high sensitivity in the perception of light waves of various lengths;
• binding layer - the second layer - constriction, which allows to form the shape of a sensitive receptor - consists of mitochondria;
• the inner part is the basal segment, a link;
• synaptic region.

Currently, only 2 photosensitive pigments in the composition of the photoreceptors of this type - chloroab and erythrolab are fully studied. The first is responsible for the perception of the yellow-green spectral region, the second - the yellow-red.

Sticks in the eyes

The rods of the retina are cylindrical, the length exceeds the diameter by 30 times.

The composition of the sticks includes the following elements:

• membrane discs;
• cilia;
• mitochondria;
• nerve tissue.

Maximum photosensitivity is provided by pigment rhodopsin (visual purple). It cannot distinguish between color shades, but it even reacts to the minimal light flashes that it receives from outside. The chop receptor is excited even by a flash, whose energy is only one photon. It is this ability that makes it possible to see at dusk.

Rhodopsin is a protein from the group of visual pigments, belongs to chromoproteins. His second name - visual purple - he received during research. Compared with other pigments, it stands out sharply with a bright red tint.

The composition of rhodopsin two components - a colorless protein and yellow pigment.

The reaction of rhodopsin to the light beam is as follows: when exposed to light, the pigment decomposes, causing excitation of the optic nerve. In the daytime, the sensitivity of the eye shifts to the blue area, at night - the restoration of visual purple takes place within 30 minutes.

During this time, the human eye adapts to twilight and begins to more clearly perceive the surrounding information. This is what explains why in the dark they begin to see more clearly over time. The less light comes in, the more the twilight vision sharpens.

Eye cones and wands - functions

Photoreceptors cannot be considered separately - in the visual apparatus they form a single whole and are responsible for visual functions and color perception. Without coordinated work of the receptors of both types, the central nervous system receives distorted information.

Color vision is provided by the symbiosis of rods and cones. The rods are sensitive in the green part of the spectrum - 498 nm, not more, and then cones with different types of pigment are responsible for perception.

To assess the yellow-red and blue-green range, long-wavelength and medium wave cones with wide photosensitive zones and internal overlapping of these zones are involved. That is, photoreceptors react simultaneously to all colors, but they are excited to their own more intensely.

It is impossible to distinguish colors at night; one color pigment can only react to light flashes.

Diffuse biopolar cells in the retina form synapses (the point of contact between a neuron and a cell that receives a signal, or between two neurons) with several rods at once — this is called synaptic convergence.

Increased perception of light radiation is provided by monosynaptic bipolar cells connecting cones to the ganglion cell. The ganglion cell is a neuron that is located in the eye retina and generates nerve impulses.

Together, the rods and cones connect the amacrylic and horizontal cells, so that the first processing of information takes place even in the retina itself. This provides a quick response to what is happening around him. Amacrylic and horizontal cells are responsible for lateral inhibition - that is, the excitation of one neuron produces   "Soothing"  action on another, which increases the sharpness of perception of information.

Despite the different structure of photoreceptors, they complement the functions of each other. Thanks to their coordinated work, it is possible to get a clear and precise image.

The whole complex superstructure described above exists so that the retina can work, which is itself an amazing structure. It converts light into nerve signals, allows us to see in conditions from a starry night to a sunny day, distinguishes wavelengths, which allows us to see colors, and provides accuracy sufficient to detect human hair or mote from a distance of several meters.

The retina is a part of the brain that separates from it on early stages  development, but still associated with it through a bundle of fibers - the optic nerve. Like many other structures of the central nervous system, the retina is plate-shaped, in this case about a quarter of a millimeter thick. It consists of three layers of the bodies of nerve cells, separated by two layers of synapses, formed by axons and dendrites of these cells.

The layer of cells on the posterior surface of the retina contains light-sensitive receptors - rods and cones. Sticks, much more numerous than cones, are responsible for our vision in low light and turn off in bright light. Cones do not react to weak light, but they are responsible for the ability to see subtle details and for color vision.

The number of rods and cones varies markedly in different parts of the retina. In the very center, where the ability of our vision to distinguish subtle details is maximum, there are only cones. This stripped band is about half a millimeter in diameter called central fossa.Cones are available all over the retina, but most densely packed in the central fossa.

Since the rods and cones are located on the posterior surface of the retina, the incoming light must pass through two other layers in order to stimulate them. We do not know exactly why the retina is arranged in such a strange way - as if turned upside down. One of the possible reasons is that behind the receptors there is a layer of cells containing black pigment melanin (it is also present in the skin). Melanin absorbs the light that has passed through the retina, preventing it from reflecting back and scattering inside the eye; it plays the same role as the black color of the inside of the camera. Cells containing melanin also contribute to the chemical recovery of the photosensitive visual pigment, which is discolored in the light (see Chapter 8). To perform both functions, it is necessary that melanin is located close to the receptors. If the receptors lay ahead, the pigment cells would have to be located between them and the next layer of nerve cells, in a region already filled with axons, dendrites and synapses.

Be that as it may, the layers in front of the receptors are fairly transparent and probably do not seriously harm the clarity of the image. However, on the central millimeter, where our vision is most acute, the consequences of even a small decrease in clarity would be catastrophic, and evolution apparently “tried” to soften them — shifting other layers to the periphery, forming a ring of thickened retina here and exposing the central cones so that they were on the surface itself. The resulting small depression is the central fossa.

Moving from the back layer to the front layer, we find ourselves in the middle layer of the retina, located between the rods and cones, on the one hand, and the ganglion cells, on the other. This layer contains three types of neurons: bipolar, horizontal and amacrine cells. Bipolar cellshave inputs from receptors, as shown in fig. 21, and many of them transmit signals directly to ganglion cells. Horizontal cellsconnect receptors and bipolar cells with relatively long bonds running parallel to the retinal layers; in a similar way amacrine cellsbind bipolar cells with ganglion cells.

Fig. 21. The enlarged fragment of the retina on the right shows the relative position of its three layers. This seems surprising, but before the light reaches the rods and cones, it must pass through the layers of ganglion and bipolar cells.

The layer of neurons on the front of the retina contains ganglion cells,the axons of which pass along the surface of the retina, gather into a bundle at the blind spot and leave the eye, forming the optic nerve. Each eye has about 125 million rods and cones, but only 1 million ganglion cells. In view of this difference, the question arises: how can the detailed visual information be stored?

Examining the connections between retinal cells can help solve this problem. One can imagine two ways of information flow through the retina: a direct path from photoreceptors to bipolar cells and further to ganglion cells, and an indirect path in which horizontal cells can be included between receptors and bipolar cells, and between bipolar cells and ganglion cells, amacrin cells (see fig. 22, illustrating these direct and indirect links). These connections have already been very thoroughly studied by Ramon and Kahal around 1900. The direct path is very specific, or compact,in the sense that one bipolar cell has inputs only from one receptor or from a relatively small number of them, and one ganglion cell from one or relatively few bipolar cells. The indirect path is more diffuse, or “blurred,” due to wider lateral connections. The total area occupied by receptors associated with one ganglion cell in direct and indirect ways, is only about a millimeter. This zone, as you may recall from Chapter 1, is receptive fieldganglion cell - the area of ​​the retina, light stimulation of which can affect the impulses of this ganglion cell.

Fig. 22. A transverse incision of the retina approximately in the middle between the central fossa and the far periphery, where there are more rods than cones. The full height of the picture in nature corresponds to about a quarter of a millimeter.

This general scheme is true for the entire retina, but in the details of the connections there are large differences between the central fossa, where the direction of gaze is projected and where our ability to see subtle details is maximum, and the periphery of the retina, where visual acuity decreases sharply. When moving from the central fossa to the periphery, the network of direct paths from receptors to ganglion cells becomes completely different. In or near the central fossa on the straight path, as a rule, one cone is connected with one bipolar cell, and one bipolar - with one ganglion cell. However, as the gradual transition to the outer regions, more and more receptors are converging on bipolar and bipolar on ganglion cells. This high degree of convergence, which we see in most of the retina, together with very compact paths in and around the center itself, makes it possible to understand why, despite the 125: 1 ratio between the number of receptors and the number of optic nerve fibers, some of the retina (its center ) can still provide acute vision.

The general scheme of retinal paths with their direct and indirect components has been known for many years, and their connection with visual acuity was understood long before it was possible to figure out the role of the indirect path. Her understanding suddenly became possible when they began to study the physiology of ganglion cells.

The organ of vision is very sensitive and one of our important analyzers. With the help of the organ of vision a person perceives the world. The eye provides an idea of ​​the illumination of the object, its color, shape, size, distance at which it is from us, the movement of the object.

In the diverse work activity of people, in the performance of numerous very delicate works, the eye plays a paramount role.

The eye has a very complex structure and consists of several parts.

The eye is located in the eye socket of the skull. From the eyeball out the optic nerve connecting it to the brain. The eyeball consists of the inner core of the three shells surrounding it: the outer, middle inner. The outer shell - the sclera, or the protein shell, is a rigid, opaque connective tissue capsule, which passes into the transparent cornea from the front, through which light enters the eye. Under it is the choroid, which passes into the front of the ciliary body, where the ciliary muscle is located, which regulates the curvature of the crystalline lens, and the iris, in the center of which there is a hole (pupil) that can narrow under the influence of muscles embedded in the thickness of the iris. The choroid is rich in blood vessels and contains a black pigment layer that absorbs light.

In the inner shell - the retina are light-sensitive receptors - rods and cones. In them, the energy of light turns into a process of excitation, which is transmitted along the optic nerve to the occipital lobe of the cerebral cortex. Cones are concentrated in the center of the retina, opposite the pupil - in the yellow spot and provide day vision, perceiving the colors, shape and details of objects. At the periphery of the retina there are only sticks, which are irritated by a weak twilight light, but are insensitive to color.

The place where the optic nerve leaves the retina is receptor-free and is called a blind spot. The inner core of the eyeball forms (together with the cornea) the optical system of the eye and consists of the lens, vitreous body and aqueous humor of the anterior and posterior chambers of the eye. The transparent and elastic crystalline lens, located behind the pupil, has the shape of a biconvex lens. Together with the cornea and intraocular fluids, it refracts the rays of light entering the eye and focuses them on the retina.

When the ciliary muscle is reduced, the lens changes its curvature, taking shape for far and near vision. The adaptation of the eye to obtaining distinct images of objects at different distances is called accommodation. It occurs due to a change in the curvature of the lens. The refracted rays of light from the object in question, falling on the retina, form on it a reduced inverse image of the object.

However, we see objects in a direct form due to the daily training of the visual analyzer, which is achieved by the formation of conditioned reflexes, constant testing of visual sensations, and everyday practice.

The auxiliary apparatus of the eye consists of protective devices, the lacrimal and motor apparatus. The protective formations include eyebrows, eyelashes and eyelids, covered on the inside with a mucous membrane that passes to the eyeball. Tears, secreted by the lacrimal gland, wash the eyeball, constantly moisturize the cornea and drain through the lacrimal canal into the nasal cavity. The motor apparatus of each eye consists of six muscles, the reduction of which allows you to change the direction of the gaze.

Retinal receptors — rods and cones — differ in both structure and function. Day vision is associated with cones, and twilight with rods. In the sticks there is a red substance - rhodopsin. In the light, as a result of a photochemical reaction, it disintegrates, and in the dark it is restored within 30 minutes from its own cleavage products. That is why a person, entering a dark room, does not see anything at first, and after a while begins to discern objects.

Cones contain another photosensitive substance - iodopsin. It disintegrates in the dark and is restored to light in 3-5 minutes. The cleavage of iodopsin in the light gives a color sensation. Of the two receptors of the retina, only cones are sensitive to color, of which there are three types in the retina: some perceive red color, others green, and some blue. Depending on the degree of excitation of cones and a combination of stimuli, various other colors and their shades are perceived.

Blurred vision

In people with normal vision, a clear image of objects appears on the retina, as it focuses on the center of the retina. Disruption of the normal activity of the sticks in the retina causes a disease known as "night blindness", expressed in the fact that with the onset of darkness a person completely loses his sight.

The ability of an eye to examine objects at different brightness of illumination is called adaptation. It is disturbed by a lack of vitamin A and oxygen, as well as by fatigue.

Visual impairment is often the result of an abnormal length of the eyeball. Myopia develops with an increase in the longitudinal axis of the eye. The eyeball is enlarged in size, the image of distant objects, even in the absence of accommodation of the lens, is obtained in front of the retina. Such an eye clearly sees only close objects and is therefore called short-sighted. Glasses with concave glasses, pushing the image on the retina, correct myopia.

When the axis of the eyeball is shortened, hyperopia is observed. The image is focused behind the retina. Biconvex glasses are required for correction. Presbyopic vision usually develops after 40 years, when the lens loses its elasticity, hardens and loses its ability to change the curvature, which makes it difficult to see clearly at close range. The eye loses the ability to clear vision of objects removed from each other. Congenital hyperopia is associated with a reduced size of the eyeball or a weak refractive power of the cornea or lens. In contrast to senile, with congenital hyperopia, accommodation of the lens may be normal.

Observance of simple rules of hygiene of sight allows to prevent an overstrain and to avoid vision disorders.

It is necessary that the workplace is well lit, but not too bright light, which should fall to the left. Sources of artificial light should be covered with lamp shades.

When reading, writing, working with small objects, the distance from objects to the eyes should be 30–35 cm. It is harmful to read while lying down or in a moving vehicle.

To avoid infectious diseases of the eyes, you need to protect them from dust, from various mechanical effects, do not rub your hands, wipe only with a clean handkerchief or towel. Visual impairment may occur due to lack of vitamin A.