The visual sensory system consists of the sensory organ - the eye, the pathways and the cortical sensory area. The eye is a part of the forebrain extended to the periphery. The retina and optic nerve develop from brain tissue. The visual reception apparatus consists of receptors in the retina and optical system eyes. The optical system of the eye includes: the cornea, the chambers of the eye - anterior and posterior, filled with intraocular fluid, the pupil, the lens, and the vitreous body. Their main properties are refraction (refraction) and complete transparency. The refraction of the eye is 60-70 d (d - diopter is the refractive power of a lens with a focal length of 1 m).
Depending on the length of the longitudinal axis of the eye, as well as on
refraction of refractive structures (mainly the lens), the image of visible objects may appear on the retina, in front of or behind it. As the length of the longitudinal axis of the eye decreases, the focal length increases and the image appears behind the retina. To make the image clear, a person is forced to remove the visible object from his eyes. This farsightedness, or hypermetropia,- weak refraction; it is corrected with glasses with biconvex lenses (+).
When the longitudinal axis of the eye lengthens, parallel rays
converge at one point not on the retina, but in front of it. A circle of light scattering appears on the retina. You need to bring the object closer so that its image is focused on the retina. This myopia, or myopia,- strong refraction, which is corrected by glasses with biconcave lenses (-).
If the curvature of the cornea is not the same, then one focus of the image is missing on the retina. This astigmatism, the consequence of which is inaccuracy in determining the distances between parallel lines or concentric
in circles. Astigmatism is corrected with glasses with cylindrical lenses.
In a normal eye, the image of objects on the retina is
real, diminished and inverted. Normal vision of objects is ensured by the cortical part of the visual analyzer. Visible objects have clear contours, since the pupil allows only the central beam of rays to enter the eye.
The function of the pupil is to adapt the eye to light and darkness. The amount of light transmitted by the pupil is controlled by the circular and radial muscles of the iris. The first is innervated by the parasympathetic nerve and constricts the pupil, the second is innervated by the sympathetic nerve and dilates the pupil. Emotions of pain
fear causes a sympathetic reaction of pupil dilation, and an increase in light flux causes a parasympathetic reaction of pupil constriction.
Analysis of light sensations
The retina contains 130 million photoreceptors - rods, which perceive light and determine the visual field, and more than 7 million cones, which perceive color and are responsible for visual acuity. The rods lie in the periphery, and the cones are concentrated in the central fovea of the retina - the macula. There are no photoreceptors on the optic nerve nipple, which is why it is called blind spot. The front part of the retina is also “blind”. The outer layer of the retina contains the pigment fuscin; it absorbs light and makes the image clearer. The perception of light is due to photochemical processes in photoreceptors.
Rod photopigment - rhodosin quickly decomposes in the light and is restored in the dark in the presence of vitamin A. Its sensitivity threshold is very high: the pulse arises from just a few quanta of light. With a lack of vitamin in the body A“night blindness” develops (Hemeralonia).
Yodonsin Cones break down much more slowly than rods. The cones contain 3 photopigments that determine the perception of three colors: blue, red, green. Rods are elements of twilight light, cones are elements of daylight. Broadcast visual information occurs selectively. First, the contours of the object are highlighted, then a holistic perception is formed - in the neurons of the retina. The primary encoding of visual information occurs in the lateral geniculate bodies. Thanks to its decoding, high visual acuity, binocular vision and spatial perception are achieved.
Visual acuity(visus) - the ability to distinguish the smallest distance between two points, depending on the angle of view (the angle between the points going from the two extreme points of the object to the eye). A normal eye can distinguish objects
an angle of 1°. The smaller the angle, the better the vision. The fovea of the retina provides greater visual acuity (central vision). Low visual acuity (hundredths of a percent) - amblyopia. To determine visual acuity, special tables are used, which depict letters and figures of various sizes.
Field of view- this is the space visible with a fixed gaze. This function is provided by rods and characterizes peripheral vision. Loss of part of the visual field in diseases of the retina - scotoma.
Looking at objects with both eyes is called binocular vision. At the same time, we see not two, but one object .
Binocular vision allows you to see objects in more relief and determine the distance from the visible object. This is explained by: bringing the ocular axes together (convergence) when viewing close objects and moving the axes apart (divergence) when viewing distant objects. At the same time, space
is perceived due to eye movements with the crossing of their visual axes on the object.
Humans have color vision and are able to distinguish large number flowers. Modern theory color vision
- polychromatic. The cones contain 3 photoligments, which determine the perception of three primary colors - orange, green, blue-violet. The white color excites all these photocells; their joint excitation gives
sensation of white color.
Colorblindness- This is a congenital color anomaly when red, green, or, less commonly, violet blindness is observed. This anomaly is better detected in poor lighting; in bright lighting, a colorblind person can clearly distinguish all these
colors.
Accommodation- the ability of the eye to clearly see objects at different distances. The accommodation system includes the lens, ciliary muscles and ligaments. The ciliary muscle consists of longitudinal and circular fibers. When viewed from far away
located objects, the annular fibers contract, the ciliary ligament stretches the lens, giving it a flatter shape and thus reducing its refraction. When viewing closely located objects, the longitudinal fibers contract, the ligament sags and the lens, due to its elasticity, takes on a more convex shape; his refraction increases.
Spasm of accommodation consists of prolonged contraction of the ciliary muscle due to visual fatigue. The person becomes myopic. Relevant - for schoolchildren, students, etc.
Paralysis of accommodation may be observed due to a prolonged spasm of accommodation. A person becomes farsighted .
Presbyonia- senile vision - occurs due to the loss of elasticity of the lens. As a result, it becomes flatter and its refraction decreases.
Formation and outflow of intraocular fluid. Intraocular fluid is also called watery. it is constantly formed as an ultrafiltrate of blood from the vessels of the ciliary muscle and flows into the posterior chamber of the eye, then through the pupil into the anterior chamber and from there through the iridocorneal angle -
into the venous sinus of the sclera. When the formation or outflow of aqueous humor is disrupted, glaucoma develops, characterized by a divisive, persistent increase in intraocular pressure (IOP), atrophy of the optic nerves and outcome in
blindness. Normal intraocular pressure is up to 27 mm Hg.
Adaptation. The sensitivity of the eye depends on the light. When moving from darkness to light, temporary blindness occurs. Due to a decrease in the sensitivity of photoreceptors, after some time the eye gets used to the light (light adaptation). When moving from light to darkness, blinding also occurs. After some time, the sensitivity of the photoreceptors increases and vision is restored (dark adaptation).
The visual system, visual analyzer, is a set of light-sensitive organs and parts of the brain that provide the perception and analysis of visual stimuli and the formation of visual sensations and images. During evolution, the visual system improves as the visual organs and nervous system develop. In animals with developed organs of vision, photoreceptors are the input elements of a multilayered nerve formation - the retina. The axons of the terminal neurons of the retina unite into the optic nerve and are sent to the central (brain) parts of the visual system.
In insects, the visual centers are located in the optic lobes of the brain. In fish, amphibians and reptiles, the main visual center is the roof of the midbrain. In mammals, visual signals from the retina enter the cerebral cortex in two ways: through the external geniculate body and through the superior colliculus (analogous to the roof of the midbrain of lower vertebrates).
The main visual zones are concentrated in the occipital part of the cortex, as well as in the temporal, parietal, etc. Most of the visual zones of the cortex are organized retinoscopically, i.e. represents projections, or peculiar “maps” of the retina. The primate cortex, for example, contains at least 15 such maps. In lower vertebrates, a significant part of all visual information processing falls on the retina, where there are specialized elements (detectors) that respond only to biologically important visual objects. For example, frogs have detectors of small dark spots for catching insects. In higher vertebrates, retinal neurons are less specialized: diverse and detailed analysis is carried out in brain centers. In animals with moving eyes, the visual system works in close and inextricable connection with the oculomotor system.
The visual system includes the organs of vision, the pathways of the visual analyzer and the cortical representation of visual information.
Organs of vision
Organs of vision (organum visus) are organs of multicellular animals that provide the perception of light stimuli. The organ of vision is sensitive to energy in the form of electromagnetic radiation with a wavelength in the range of 400-700 nm. The light-sensitive receptors of vertebrates are sensitive to ultraviolet color, but these short waves are filtered out by the fluid media of the eye and do not reach the retina.
Sensitive cells are surrounded by pigment, the purpose of which is to transmit light in a certain direction and absorb excess light rays. In lower animals, such cells are scattered throughout the body (primitive “eyes”), and later a fossa is formed, lined with sensitive cells (retina), to which the nerve approaches. In invertebrates, light-refracting media (lens) appear in front of the fovea to concentrate light rays incident on the retina. In vertebrates, in which the eyes reach the greatest development, muscles that move the eye and protective devices (eyelids, lacrimal apparatus) appear.
A characteristic feature of vertebrates is the fact that the light-sensitive membrane of the eye (retina), containing specific cells, does not develop directly from the ectoderm, but by protrusion from the anterior medullary vesicle. At the first stage of development of the visual analyzer (in fish), at its peripheral end (retina), light-sensitive cells have the form of rods, and in the brain there are only visual centers lying in the midbrain. Such an organ of vision is capable only of light perception and discrimination of objects. In terrestrial animals, the retina is supplemented with new light-sensitive cells - cones, and new visual centers appear in the diencephalon, and in mammals - in the cortex. Thanks to this, the eye gains the ability to see color. All this is connected with the first signal system.
In humans, the higher centers in the cerebral cortex achieve special development, thanks to which he develops abstract thinking associated with visual images and written speech, which are an integral part of the second signaling system, characteristic only of humans.
Embryogenesis of the eye in general occurs as follows. The lateral protrusions of the wall of the anterior medullary vesicle (the part that gives rise to the diencephalon), stretching to the sides, form two optic vesicles, communicating through a hollow, narrowed pedicle with the medullary cavity. The optic nerve is formed from the stalk, and the retina is formed from the peripheral part of the optic vesicle. In connection with the development of the lens, the anterior part of the optic vesicle invaginates towards the stalk, as a result of which the vesicle turns into a double-walled “optic cup”. Both leaves merge into each other at the edge of the glass, forming the rudiment of the pupil. The outer (invaginated) leaf of the glass becomes the pigment layer of the retina, and the inner one becomes photosensitive (the retina itself). In the front part of the optic cup, a lens is formed, which is placed in its cavity, and behind the lens is the vitreous body.
The development of the outer membranes of the eye - the choroid, sclera and cornea - occurs from the mesoderm surrounding the optic cup along with the lens. The sclera with the cornea arises from the outer, denser layer of mesoderm, and from the inner layer, rich in blood vessels, the sclera itself emerges. choroid(chorioidea) with the ciliary body and iris. In the anterior part of the embryonic eye, both layers separate from each other, giving rise to the anterior chamber. The outer layer of mesoderm in this place, having become transparent, forms the cornea. The ectoderm covering the front of the cornea gives rise to the epithelium of the conjunctiva, which extends to the back of the eyelids.
In newborns, due to the shallow orbit and relatively large eyeball, the eye is bulging. This is especially noticeable due to the underdevelopment of the cheekbones and nasal bones. Up to 2 years, the eyeball increases by 40%, by 5 years – by 70% of the original volume, and by 12-14 years it reaches the size of an adult’s eyeball. The cornea is thicker than that of an adult. The formation of the curvature and thickness of the cornea ends in the 7th year of life. The lens in children has great elasticity. With age, elasticity disappears due to the formation of a dense core. Sometimes clouding and hardening of the lens occurs. With age, it is possible to change not only the retraction of the lens, but also the long size of the eye, which leads to farsightedness or nearsightedness.
Eyeball
The eyeball (bulbus oculi) is a spherical body embedded in the eye socket. In the eyeball, there is an anterior pole, corresponding to the most convex point of the cornea, and a posterior pole, located lateral to the exit of the optic nerve. The straight line connecting both poles is called the optical, or external, ocular axis (axis opticus). The portion between the posterior surface of the cornea and the retina is called the internal ocular axis. The latter intersects at an acute angle with the so-called visual line (linea visus), which runs from the object in question through the nodal point to the place of best vision in the central fossa of the retina. The lines connecting both poles along the circumference of the eyeball form meridians, and the plane perpendicular to the optical axis is the eye equator, dividing the eyeball into anterior and posterior halves. The horizontal diameter of the equator is slightly shorter than the external ocular axis (24 mm). The internal ocular axis in a normal eye is 21.3 mm, in myopic eyes (myons) it is longer, and in farsighted eyes (hypermetrons) it is shorter. As a result, the focus of the converging rays in myopic people is in front of the retina, in hypermetronics - behind it. The eyeball is composed of an inner core and the surrounding membranes: outer fibrous, middle vascular and inner reticular.
Fibrous membrane
The fibrous membrane (tunica fibrosa bulbi) is the outer shell of the eyeball and performs a protective function. In the posterior, larger section it forms the tunica albuginea, or sclera, and in the anterior section it forms a transparent cornea. Both sections of the fibrous membrane are separated from each other by a shallow circular groove (sulcus sclerae).
Sclera (sclera) - the back part of the fibrous membrane of the eyeball, contains many elastic and collagen fibers and little basic connective tissue substance; they form a dense plate, the outer layer of which lacks pigment cells. The tunica albuginea on the medial part of the posterior pole of the eye has a lattice structure. Through its openings the processes of neurons penetrate, forming the optic nerve. In the region of the posterior pole and equator of the eyeball, the thickness of the tunica albuginea is 0.3 - 0.4 mm, and near the cornea - 0.6 mm. In the tunica albuginea, arteries are sometimes clearly visible against its white background. The veins are located in the deep layers of the tunica albuginea and are not visible through the palpebral fissure. The venous sinus of the sclera (sinus venosus sclerae) is especially well developed, which is projected onto the surface of the eye along a circular groove. Through the venous channel, fluid is reabsorbed from the anterior chamber of the eye. On the inside, near the venous sinus, the iris is attached to the fibrous membrane, which forms the pectineal ligament (lig. pectinatum anguli iridocornealis), connecting the outer edge of the iris to the sclera.
The cornea (cornea) is the anterior part of the fibrous membrane of the eyeball, located on the anterior cone of the eye, and is a transparent plate convex outward, having 5 layers of epithelium and connective tissue fibers. The latter are enclosed in a colloidal substance of mucopolysaccharide nature. The cornea in the central part is slightly thinner (0.8 mm) than in the periphery (1.1 mm). It contains many sensory nerve endings and is devoid of blood vessels, its nutrition is carried out by the diffusion of nutrients from the fluid of the anterior chamber of the eye and the vessels of the tunica albuginea adjacent to the edge of the cornea.
Choroid
The choroid of the eyeball (tunica fascilisa bulbi) is the middle layer of the eyeball. It contains a plexus of blood vessels and pigment cells. This membrane is divided into 3 parts: the iris, the ciliary body, and the choroid itself. The median location of the choroid between the fibrous and retina helps its pigment layer to retain excess rays falling on the retina and distribute blood vessels in all layers of the eyeball.
The iris (iris) is the anterior part of the choroid of the eyeball, has the appearance of a circular, vertically standing plate with a round hole - the pupil (pupilla). The pupil does not lie exactly in its middle, but is slightly shifted towards the nose. The iris plays the role of a diaphragm, regulating the amount of light entering the eyes, due to which the pupil narrows in strong light and dilates in weak light.
The outer edge of the iris is connected to the ciliary body and sclera; its inner edge, surrounding the pupil, is free. The iris has an anterior surface facing the cornea and a posterior surface adjacent to the lens. The anterior surface, visible through the clear cornea, has different colors in different people and determines the color of the eyes. The color depends on the amount of pigment in the surface layers of the iris. If there is a lot of pigment, then the eyes are brown (brown) up to black; if the pigment layer is poorly developed or even absent, then mixed greenish-gray and blue tones are obtained. The latter mainly arise from the translucency of the black retinal pigment on the back of the iris.
The iris, performing the function of a diaphragm, has amazing mobility, which is ensured by the fine adaptability and correlation of its components. The base of the iris (stroma iridis) consists of connective tissue having a lattice architecture into which vessels are inserted, running radially from the periphery to the pupil. These vessels, which are the only carriers of elastic elements, together with the connective tissue form the elastic skeleton of the iris, allowing it to easily change in size.
Movements of the iris are carried out muscular system, lying deep in the stroma. This system consists of smooth muscle fibers, which are partly located in a ring around the pupil, forming the muscle that constricts the pupil (m. sphincter pupillae), and partly diverge radially from the pupillary opening and form the muscle that dilates the pupil (m. dilatator pupillae). Both muscles are interconnected: the sphincter stretches the dilator, and the dilator straightens the sphincter. The impermeability of the diaphragm to light is achieved by the presence of a double-layer pigment epithelium on its posterior surface. On the anterior surface, washed by fluid, it is covered with the endothelium of the anterior chamber.
The ciliary body (cogrus ciliare) is located on the inner surface at the junction of the sclera and the cornea. On a cross section, it has the shape of a triangle, and when viewed from the posterior pole, it has the shape of a circular ridge, on the inner surface of which there are radially oriented processes (processus ciliares) numbering about 70.
The ciliary body and iris are attached to the sclera by pectineal ligaments, which have a spongy structure. These cavities are filled with fluid that flows from the anterior chamber and then into the circular venous sinus (helmet canal). Ring-shaped ligaments extend from the ciliary processes and are woven into the lens capsule.
The process of accommodation, i.e. adaptation of the eye to near or far vision is possible due to the weakening or tension of the annular ligaments. They are under the control of the muscles of the ciliary body, consisting of meridional and circular fibers. When the circular muscles contract, the ciliary processes move closer to the center of the ciliary circle and the annular ligaments are weakened. Due to internal elasticity, the lens straightens and its curvature increases, thereby reducing the focal length.
Simultaneously with the contraction of the circular muscle fibers, the meridional muscle fibers also contract, which tighten the posterior part of the choroid and the ciliary body as much as the focal length of the light beam decreases. When relaxed due to elasticity, the ciliary body takes its original position and, stretching the annular ligaments, strains the lens capsule, flattening it. In this case, the posterior pole of the eye also takes its original position.
IN old age part of the muscle fibers of the ciliary body is replaced by connective tissue. The elasticity and resilience of the lens also decreases, leading to visual impairment.
The choroid proper (chorioidea) is the posterior part of the choroid, covering 2/3 of the eyeball. The membrane consists of elastic fibers, blood and lymphatic vessels, pigment cells that create a dark brown background. It is loosely fused with the inner surface of the tunica albuginea and easily moves during accommodation. In animals, calcium salts accumulate in this part of the choroid, which form an eye mirror that reflects light rays, which creates conditions for the eyes to glow in the dark.
Retina
The retina (retina) is the innermost layer of the eyeball, extending to the jagged edge (area serrata), which lies at the junction of the ciliary body with the choroid proper. Along this line, the retina is divided into anterior and posterior parts. The retina has 11 layers, which can be combined into 2 sheets: pigment - outer and medulla - inner. The medulla contains light-sensitive cells - rods and cones; their outer photosensitive segments are directed towards the pigment layer, i.e. outwards. The next layer is bipolar cells that form contacts with rods, cones and ganglion cells, the axons of which form the optic nerve. In addition, there are horizontal cells located between the rods and bipolar cells and amacrine cells to integrate the function of ganglion cells.
There are about 125 million rods and 6.5 million cones in the human retina. The macula contains only cones, and rods are located on the periphery of the retina. Retinal pigment cells isolate each light-sensitive cell from each other and from stray rays, creating the conditions for imaginative vision. In bright light, rods and cones are immersed in the pigment layer. The corpse's retina is matte white, without characteristic anatomical features. When examined with an ophthalmoscope, the retina (fundus of the eye) of a living person has a bright red background due to transillumination of the blood in the choroid. Against this background, bright red blood vessels of the fiber are visible.
Cones are photoreceptors in the vertebrate retina that provide daytime (photopic) and color vision. The thickened outer receptor process, directed towards the pigment layer of the retina, gives the cell a flask shape (hence the name). Unlike rods, each cone in the fovea is usually connected via a bipolar neuron to a separate ganglion cell. As a result, cones carry out detailed image analysis and have a high response speed, but low light sensitivity (more sensitive to the action of long waves). In cones, as in rods, there are outer and inner segments, a connecting fiber, the nuclear-containing part of the cell and an internal fiber that carries out synaptic communication with bipolar and horizontal neurons. The outer segment of the cone (a derivative of the cilium), consisting of numerous membrane disks, contains visual pigments - rhodopsins, which react to light of various spectral compositions. The cones of the human retina contain 3 types of pigments, each of them containing one type of pigment, which provides selective perception of one color or another: blue, green, red. The internal segment includes an accumulation of numerous mitochondria (ellipsoid), the contractile element is an accumulation of contractile fibrils (myoid) and glycogen granules (paraboloid). In most vertebrates, an oil droplet is located between the outer and inner segments, selectively absorbing light before it reaches them. visual pigment.
Rods are photoreceptors of the retina that provide twilight (scotopic) vision. The outer receptor process gives the cell a rod shape (hence the name). Several rods are connected by a synaptic connection with one bipolar cell, and several bipolars, in turn, are connected with one ganglion cell, the axon of which enters the optic nerve. The outer segment of the rod, consisting of numerous membranous discs, contains the visual pigment rhodopsin. In most diurnal animals and humans, rods predominate over cones in the periphery of the retina.
At the posterior pole of the eye there is an oval spot - the optic disc (discus n. optici) measuring 1.6 - 1.8 mm with a depression in the center (excavatio disci). The branches of the optic nerve, devoid of myelin sheath, and veins converge radially to this spot; Arteries diverge into the visual part of the retina. These vessels supply blood only to the retina. By the vascular pattern of the retina one can judge the state of the blood vessels of the whole body and some of its diseases (iridology).
4 mm laterally at the level of the optic nerve head lies a spot (macula) with a central fovea (fovea centralis), colored red-yellow-brown. The focus of light rays is concentrated in the spot; it is the place of best perception of light rays. The spot contains light-sensitive cells - cones. Rods and cones lie near the pigment layer. Light rays thus penetrate through all layers of the transparent retina. When exposed to light, rhodopsin in rods and cones breaks down into retinene and protein (scotopsin). As a result of the breakdown, energy is generated, which is captured by the bipolar cells of the retina. Rhodopsin is constantly resynthesized from scotopsin and vitamin A.
Visual pigment is a structural and functional unit of the photosensitive membrane of the photoreceptors of the retina - rods and cones. The visual pigment molecule consists of a chromophore that absorbs light and opsin, a complex of protein and phospholipids. The chromophore is represented by vitamin A 1 aldehyde (retinal) or A 2 (dehydroretinal).
Opsins (rod and cone) and retinals, combining in pairs, form visual pigments that differ in absorption spectrum: rhodopsin (rod pigment), iodopsin (cone pigment, absorption maximum 562 nm), porphyropsin (rod pigment, absorption maximum 522 nm). Differences in the absorption maxima of pigments in animals of different species are also associated with differences in the structure of opsins, which interact differently with the chromophore. In general, these differences are adaptive in nature, for example, species in which the maximum absorption is shifted to the blue part of the spectrum live at greater depths of the ocean, where light with a wavelength of 470 to 480 nm penetrates better.
Rhodopsin, visual purple, is a rod pigment in the retina of animals and humans; a complex protein that includes the chromophore group of the carotenoid retinal (vitamin A 1 aldehyde) and opsin, a complex of glycoprotein and lipids. The maximum of the absorption spectrum is about 500 nm. In the visual act, under the influence of light, rhodopsin undergoes cis-trans isomerization, accompanied by a change in the chromophore and its separation from the protein, a change in ion transport in the photoreceptor and the appearance of an electrical signal, which is then transmitted to the neural structures of the retina. The synthesis of retinal is carried out with the participation of enzymes through vitamin A. Visual pigments close to rhodopsin (iodopsin, porphyropsin, cyanopsin) differ from it either in a chromophore or in opsin and have slightly different absorption spectra.
Cameras of the eye
Chambers of the eye - the space located between the anterior surface of the iris and the posterior side of the cornea is called the anterior chamber of the eyeball (camera anterior bulbi). The anterior and posterior walls of the chamber come together along its circumference at the angle formed by the transition of the cornea into the sclera, on the one hand, and the ciliary edge of the iris, on the other. The angle (angulus iridocornealis) is rounded by a network of crossbars, which together make up the pectineal ligament. Between the crossbars of the ligament there are slit-like spaces (fountain spaces). The angle has an important physiological significance for the circulation of fluid in the chamber, which, through the fountain spaces, is emptied into the Schlemm’s canal located nearby in the thickness of the sclera.
Behind the iris there is a narrower posterior chamber of the eye (camera posterior bulbi), which is limited in front by the posterior surface of the iris, in the back by the lens, and on the periphery by the ciliary body. Through the pupillary opening, the posterior chamber communicates with the anterior chamber. The liquid serves as a nutrient for the lens and cornea, and also participates in the formation of the lenses of the eye.
Lens
The lens is the light-refracting medium of the eyeball. It is completely transparent and has the appearance of lentils or biconvex glass. The central points of the anterior and posterior surfaces are called the poles of the lens, and the peripheral edge, where both surfaces meet each other, is called the equator. The axis of the lens, connecting both poles, is 3.7 mm when looking at distance and 4.4 mm during accommodation, when the lens is made convex. The equatorial diameter is 9 mm. The lens, with its equatorial plane, stands at a right angle to the optical axis, its anterior surface adjacent to the iris, and its posterior surface to the vitreous body.
The lens is enclosed in a thin, also completely transparent, structureless bag (capsula lentis) and is held in its position by a special ligament (zonula ciliaris), which is made up of many fibers running from the lens bag to the ciliary body. Between the fibers there are spaces filled with liquid that communicate with the chambers of the eye.
Vitreous body
The vitreous body (cogrus vitreum) is a transparent jelly-like mass located in the cavity between the retina and the posterior surface of the lens. The vitreous body is formed by a transparent colloidal substance consisting of thin rare connective tissue fibers, proteins and hyaluronic acid. Due to the depression from the lens, a fossa (fossa hyaloidea) is formed on the anterior surface of the vitreous body, the edges of which are connected to the lens bag through a special ligament.
Eyelids
Eyelids (palpebrae) - connective tissue formations covered thin layer skin, limiting with its anterior and posterior edges (limbus palpebralis anteriores et posteriores) the palpebral fissure (rima palpebrum). The mobility of the upper eyelid (palpebra superior) is greater than that of the lower eyelid (palpebra inferior). The lowering of the upper eyelid is carried out due to part of the muscle surrounding the orbit (m. orbicularis oculi). As a result of the contraction of this muscle, the curvature of the arch of the upper eyelid decreases, as a result of which it moves downward. The eyelid is raised by a special muscle (m. levator palpebrae superioris).
The inner surface of the eyelid is lined with a connective membrane - the conjunctiva. In the medial and lateral corners of the palpebral fissure there are ligaments of the eyelids. The medial angle is rounded, it contains a lacrimal lake (lacus lacrimalis), in which there is an elevation - the lacrimal caruncle (caruncula lacrimalis). At the edge of the connective tissue base of the eyelids are placed fat glands(gll. tarsales), called meibomian glands, the secretion of which lubricates the edges of the eyelids and eyelashes.
Eyelashes (cilia) are short, hard hairs that grow from the edge of the eyelid, serving as a lattice to protect the eye from small particles getting into it. The conjunctiva (tunica conjunctiva) starts from the edge of the eyelids, covers their inner surface, and then wraps around the eyeball, forming a conjunctival sac that opens from the front into the palpebral fissure. It is firmly fused with the cartilage of the eyelids and loosely connected to the eyeball. In the places where the connective tissue membrane transitions from the eyelids to the eyeball, folds are formed, as well as the upper and lower vaults, which do not interfere with the movement of the eyeball and eyelids. Morphologically, the fold represents a rudiment of the third eyelid (nictitating membrane).
Lacrimal apparatus
The lacrimal apparatus (apparatus lacrimalis) is a system of organs designed to secrete tears and drain them along the lacrimal ducts. The lacrimal apparatus includes the lacrimal gland, lacrimal canaliculus, lacrimal sac and nasolacrimal duct.
The lacrimal gland (gl. lacrimalis) secretes a clear liquid containing water, the enzyme lysozyme and a small amount of protein substances. The upper most part of the gland is located in the fossa of the lateral angle of the orbit, bottom part- under top part. Both lobes of the gland have an alveolar-tubular structure and 10 - 12 common ducts (ductuli excretorii), which open into the lateral part of the conjunctival sac. The lacrimal fluid along the capillary gap formed by the conjunctiva of the eyelid, conjunctiva and cornea of the eyeball, washes it and merges along the edges of the upper and lower eyelids to the medial corner of the eye, penetrating into the lacrimal canaliculi.
The lacrimal canaliculus (canaliculus lacrimalis) is represented by upper and lower tubules with a diameter of 500 µm. They are located vertically in their initial part (3 mm), and then take a horizontal position (5 mm) and flow into the lacrimal sac with a common trunk (22 mm). The canaliculus is lined flat epithelium. The lumen of the tubules is not the same: narrow spots are located in the corner at the place where the vertical part passes into the horizontal part and at the place where it flows into the lacrimal sac.
The lacrimal sac (saccus lacrimalis) is located in the fossa of the medial wall of the orbit. The medial ligament of the eyelid runs in front of the sac. From its wall begin the bundles of muscle surrounding the orbit. Upper part The sac begins blindly and forms a fornix (fornix sacci lacrimalis), the lower part passes into the nasolacrimal duct. The nasolacrimal duct (ductus nasolacrimalis) is a continuation of the lacrimal sac. This is a straight flattened tube with a diameter of 2 mm, a length including a bag of 5 mm, which opens into the anterior part of the nasal passage. The sac and duct are composed of fibrous tissue; their lumen is lined with flat epithelium.
1 PHYSIOLOGICAL CHARACTERISTICS OF THE VISUAL SENSOR SYSTEM
1.1 Basic vision indicators
1.2 Psychophysical characteristics of light
1.3 Peripheral visual system
2 SOMATOVISCERAL INTERACTIONS
2.1 Psychophysics of cutaneous mechanoreception
2.2 Cutaneous mechanoreceptors
2.3 Psychophysics of thermoreception
2.4 Thermoreceptors
2.5 Visceral sensitivity
2.6 Proprioception
2.7 Functional and anatomical overview of the central somatosensory system
2.8 Transmission of somatovisceral information in the spinal cord
2.9 Somatosensory functions of the brainstem
2.10 Thalamus
2.11 Somatosensory projection areas in the cortex
2.12 Control of afferent input in the somatosensory system
LIST OF REFERENCES USED
The visual system (visual analyzer) is a set of protective, optical, receptor and nervous structures that perceive and analyze light stimuli. In the physical sense, light is electromagnetic radiation with different wavelengths - from short (red region of the spectrum) to long (blue region of the spectrum).
The ability to see objects is related to the reflection of light from their surface. Color depends on which part of the spectrum the object absorbs or reflects. The main characteristics of a light stimulus are its frequency and intensity. Frequency (the reciprocal of wavelength) determines the color of light, intensity - brightness. The range of intensities perceived by the human eye is enormous - about 10 16 . Through the visual system, a person receives more than 80% of information about the outside world.
1.1 Basic vision indicators
Vision is characterized by the following indicators:
1) the range of perceived frequencies or wavelengths of light;
2) the range of light wave intensities from the perception threshold to the pain threshold;
3) spatial resolution - visual acuity;
4) temporal resolution - summation time and critical flicker frequency;
5) sensitivity threshold and adaptation;
6) the ability to perceive colors;
7) stereoscopy - depth perception.
Psychophysical equivalents of light frequency and intensity are presented in Tables 1.1 and 1.2.
Table 1.1. Psychophysical equivalents of light frequency
Table 1.2. Psychophysical equivalents of light intensity
To characterize the perception of light, three qualities are important: hue, saturation and brightness. Tone corresponds to color and changes with the wavelength of light. Saturation refers to the amount of monochromatic light that, when added to white light, produces a sensation corresponding to the wavelength of the added monochromatic light containing only one frequency (or wavelength). The brightness of light is related to its intensity. The range of light intensities from the threshold of perception to values that cause pain is enormous - 160 dB. The brightness of an object perceived by a person depends not only on the intensity, but also on the surrounding background. If the figure (visual stimulus) and the background are equally illuminated, that is, there is no contrast between them, the brightness of the figures increases with increasing physical intensity of illumination. If the contrast between figure and ground increases, the brightness of the perceived figure decreases with increasing illuminance.
Spatial resolution - visual acuity - the minimum angular distance between two objects (points) visible to the eye. Sharpness is determined using special tables of letters and rings and is measured by the value I/a, where a is the angle corresponding to the minimum distance between two adjacent break points in the ring. Visual acuity depends on the general illumination of surrounding objects. In daylight it is maximum; at dusk and in darkness, visual acuity decreases.
The temporal characteristics of vision are described by two main indicators - summation time and critical flicker frequency.
The visual system has a certain inertia: after the stimulus is turned on, time is necessary for the appearance of a visual reaction (this includes the time required for the development chemical processes in receptors). The visual impression does not disappear immediately, but only some time after the effect of light or image on the eye ceases, since it also takes time for the retina to restore visual pigment. There is an equivalence between the intensity and duration of exposure of the eye to light. The shorter the visual stimulus, the greater the intensity it must have to produce a visual sensation. Thus, the total amount of light energy matters for the occurrence of a visual sensation. This relationship between duration and intensity is preserved only for short stimulus durations - up to 20 ms. For longer signals (from 20 ms to 250 ms), complete compensation of the threshold intensity (brightness) due to duration is no longer observed. Any relationship between the ability to detect light and its duration disappears after the stimulus duration reaches 250 ms, and at longer durations intensity becomes decisive. The dependence of the threshold light intensity on the duration of its exposure is called time summation. This indicator is used to assess the function of the visual system.
The visual system retains traces of light stimulation for 150-250 ms after its activation. This indicates that the eye perceives intermittent light as continuous light at certain intervals between flashes. The flash frequency at which a series of consecutive flashes is perceived as continuous light is called the critical flicker frequency. This indicator is inextricably linked with temporal summation: the summation process ensures the smooth merging of successive images into a continuous stream of visual impressions. The higher the intensity of the light flashes, the higher the critical flicker frequency. The critical frequency of flickering pi of average light intensity is 16-20 per 1 s.
Light sensitivity threshold- this is the lowest intensity of light that a person can see. It is 10 -10 - 10 -11 erg/s. In real conditions, the threshold value is significantly influenced by the adaptation process - changes in the sensitivity of the visual system depending on the initial illumination. At low light intensity in the environment, tempo adaptation of the visual system develops. As dark adaptation develops, visual sensitivity increases. The duration of complete dark adaptation is 30 minutes. With an increase in environmental illumination, light adaptation occurs, which is completed in 15-60 s. Differences in dark and light adaptation are associated with the rate of chemical processes of decay and synthesis of retinal pigments.
Perception of light depends on the wavelength of light entering the eye. However, this statement is true only for monochromatic rays, that is, rays with one wavelength. White light contains all wavelengths of light. There are three primary colors: red - 700 nm, green - 546 nm and blue - 435 nm. By mixing primary colors you can get any color. Explain color vision based on the assumption that there are three photoreceptors in the retina various types, sensitive to different wavelengths of light corresponding to the main frequencies of the spectrum (blue, green, red).
Impaired color perception is called color blindness, or color blindness, named after Dalton, who first described this vision defect based on his own experience. Color blindness affects mainly men (about 10%) due to the absence of a certain gene on the X chromosome. There are three types of light vision impairment: protanopia- lack of sensitivity to red color, deuteranopia- lack of sensitivity to green color and tritanopia- lack of sensitivity to blue light. Complete color blindness - monochromacy- is extremely rare.
Binocular vision- the participation of both eyes in the formation of the visual image - is created by combining two monocular images of objects, enhancing the impression of spatial depth. Since the eyes are located at different “points” of the head on the right and left, then in the images recorded with different eyes, there are small geometric differences (disparity), which are greater the closer the object in question is. The disparity of two images is the basis of stereoscopy, that is, depth perception. When a person's head is in a normal position, deviations from exactly corresponding image projections in the right and left eyes occur, the so-called receptive field disparity. It decreases as the distance between the eyes and the object increases. Therefore, at large distances between the stimulus and the eye, the depth of the image is not perceived.
From the outside, the eye is visible as a spherical formation, covered by the upper and lower eyelids and consisting of the sclera, conjunctiva, cornea, and iris. Sclera represents connective tissue white color surrounding the eyeball. Conjunctiva- transparent tissue, equipped with blood vessels, which connects to the cornea at the anterior pole of the eye. Cornea is a transparent protective outer formation, the curvature of the surface of which determines the characteristics of light refraction. Thus, if the cornea has irregular curvature, a distortion of visual images occurs, called astigmatism. Behind the cornea is iris, the color of which depends on the pigmentation of its constituent cells and their distribution. Between the cornea and the iris is the anterior chamber of the eye, filled with fluid - "aqueous humor". At the center of the iris is pupil round in shape, allowing light to enter the eye after passing through the cornea.
It is a set of structures that perceive light energy and form visual sensations. According to modern ideas, 80-90% of all information about the world around us is thanks to. With the help of a visual analyzer, the size of objects, their degree of illumination, color, shape, direction and speed of movement, and the distance at which they are removed from the eye and from each other are perceived. All this allows you to evaluate space, navigate the world around you, perform various types purposeful activity.
Description of the schema fields:
Diagram of the structure of the visual analyzer: 1 - retina, 2 - uncrossed fibers of the optic nerve, 3 - crossed fibers of the optic nerve, 4 - optic tract, 5 - lateral geniculate body, 6 - lateral root, 7 - optic lobes
As it leaves the eye, the optic nerve divides into two halves. The inner half crosses with the same half of the other eye and, together with the outer half of the opposite side, goes to the metathalamus, where the next neuron is located, ending on the cells of the visual zone in the occipital lobe. Some of the fibers of the optic tract are directed to the quadrigeminal cells, from which the tectospinal path of reflex orienting movements associated with vision begins. In addition, in the quadrigeminal region there are connections with the parasympathetic nucleus of Yakubovich, from which the fibers begin oculomotor nerve, providing constriction of the pupil and accommodation of the eye.
Vision for a person is one of the ways of orientation in space. With its help, we receive information about the change of day and night, distinguish between the objects around us, the movement of living and inanimate bodies, and various graphic and light signals. Vision is very important for labor activity person and ensuring his safety.
The peripheral part of the visual sensory system is the eye, which is located in the recess of the skull - eye socket, and is protected by its walls from external influences.
The eye consists of the eyeball and auxiliary structures: lacrimal glands, external eye muscles, eyelids, eyebrows, conjunctiva. The lacrimal gland secretes a fluid that protects the eye from drying out. Uniform distribution of tear fluid over the surface of the eye occurs due to blinking of the eyelids.
Eyeball limited by three shells - outer, middle and inner (Fig. 5.5). The outer layer of the eye is sclera, or tunica albuginea. This is a dense, opaque white fabric, about 1 mm thick, in the front part it becomes transparent cornea.
Rice. 5.5.
- 1 - tunica albuginea; 2 - cornea; 3 - lens; 4 - ciliary body;
- 5 - iris; 6 - choroid; 7 - retina;
- 8 - blind spot; 9 - vitreous body; 10- posterior chamber of the eye;
- 11 - anterior chamber of the eye; 12 - optic nerve (according to A.G. Khripkova, 1978)
Located under the sclera choroid eyes, the thickness of which does not exceed 0.2-0.4 mm. It contains a large number of blood vessels. In the anterior part of the eyeball, the choroid passes into ciliary body And iris (iris). Together these structures make up the tunica media.
There is a hole in the center of the iris - pupil, its diameter can change, causing more or less light to enter the eyeball. The lumen of the pupil is regulated by a muscle located in the iris.
The iris contains a special coloring substance - melanin. Depending on the amount of this pigment, the color of the iris can range from gray and blue to brown and almost black. The color of the iris determines the color of the eyes. If the pigment is absent (such people are called albinos), then light rays can penetrate into the eye not only through the pupil, but also through the tissue of the iris. Albinos have reddish eyes and reduced vision.
The ciliary body contains a muscle connected to the lens and regulating its curvature.
Lens- a transparent, elastic formation shaped like a biconvex lens. It is covered with a transparent bag; along its entire edge, thin, elastic fibers stretch to the ciliary body, which keep the lens in a stretched state.
In the anterior and posterior chambers of the eye there is a clear liquid that supplies nutrients cornea and lens. The cavity of the eye behind the lens is filled with a transparent jelly-like mass - the vitreous body.
Optical system of the eye represented by the cornea, chambers of the eye, lens and vitreous body. Each of these structures has its own optical power indicator.
The eye is an extremely complex optical system, which can be compared to a camera, in which all parts of the eye are the lens, and the retina is the film. Rays of light are focused on the retina, producing a smaller and inverted image. Focusing occurs due to changes in the curvature of the lens (accommodation): when viewing a nearby object, it becomes convex, and when viewing a distant object, it becomes flatter.
The inner surface of the eye is lined with a thin (0.2-0.3 mm) membrane of very complex structure - retina, on which there are light-sensitive cells, or receptors - rods and cones. Cones are concentrated mainly in the central region of the retina - the macula. As you move away from the center, the number of cones decreases and the number of rods increases. On the periphery of the retina there are only rods. Cones are the receptors for color vision, rods - black and white.
The place of best vision is yellow spot, especially its central fovea. This vision is called central vision. The remaining parts of the retina are involved in lateral, or peripheral, vision. Central vision allows you to examine small details of objects, and peripheral vision allows you to navigate in space.
Excitation of rods and cones causes the appearance of nerve impulses in the fibers of the optic nerve. Cones are less excitable, so if weak light enters the fovea, where only the cones are located, we see it very poorly or not at all. Weak light is clearly visible when it hits the lateral surfaces of the retina. Consequently, in bright light it is mainly the cones that function, and in low light it is the rods.
The visual sensation does not occur immediately with the onset of stimulation, but after a certain latent period (0.1 s). It does not disappear with the cessation of light, but remains for some time necessary to remove irritating decay products of light-reactive substances from the retina and restore them.
Receptors in the retina transmit signals along the fibers of the optic nerve only once, at the moment of the appearance of a new object. Next, signals are added about upcoming changes in the image of the object and about its disappearance. Continuous small oscillatory eye movements lasting only 25 ms allow a person to see stationary objects. For example, frogs do not have oscillatory eye movements, so they only see objects that are moving. From this it is clear how great the role of eye movements is in providing vision.
The conductive section of the visual sensory system is represented by optic nerve, nuclei of the superior colliculus of the midbrain, nuclei of the diencephalon.
The central section of the visual analyzer is located in the occipital lobe, and the primary cortex lies in the vicinity of the calcarine sulcus, in the cortex of the lingular and sphenoid gyri (Fig. 5.6). Second
The primary cortex is located around the primary cortex.
(according to E.I. Nikolaeva, 2001)
Normal vision is achieved through two eyes - binocular vision. A person sees differently with his left and right eyes - on the retina of each eye there are different images. But because the image appears on identical points of the retina, a person perceives the object as a single whole. If the rays from the object under consideration fall on non-identical (inappropriate) points of the retina, then the image of the object will appear to be bifurcated. Vision with both eyes is necessary for high-quality perception and representation of the object in question. The perception of the movement of an object depends on the movement of its image on the retina. The perception of moving objects with simultaneous movement of the eyes and head and the determination of the speed of movement of objects are determined not only by visual, but also by centripetal impulses from the proprioceptors of the eye and neck muscles.
Age-related features of the visual sensory system. The development of the visual analyzer begins in the 3rd week of the embryonic period.
Development of the peripheral region. Differentiation of the cellular elements of the retina occurs at 6-10 weeks of intrauterine development. By 3 months of embryonic life, the retina includes all types of nerve elements. In a newborn, only rods function in the retina, providing black and white vision. The cones responsible for color vision are not yet mature and their number is small. And although newborns have functions of color perception, the full inclusion of cones in their work occurs only at the end of the 3rd year of life. The final morphological maturation of the retina ends by 10-12 years.
Development of additional elements of the organ of vision (pre-receptor structures). A newborn's diameter eyeball is 16 mm, and its weight is 3.0 g. The growth of the eyeball continues after birth. It grows most intensively in the first 5 years of life, less intensively - up to 9-12 years. In adults, the diameter of the eyeball is about 24 mm and the weight is 8.0 g. In newborns, the shape of the eyeball is more spherical than in adults, and the anteroposterior axis of the eye is shortened. As a result, in 80-94% of cases they have farsighted refraction. Increased extensibility and elasticity of the sclera in children contributes to slight deformation of the eyeball, which is important in the formation of eye refraction. So, if a child plays, draws or reads with his head bowed low, due to the pressure of the fluid on the front wall, the eyeball lengthens and myopia develops. The cornea is more convex than in adults. In the first years of life iris contains few pigments and has a bluish-grayish tint, and the final formation of its color is completed only by 10-12 years. In newborns due to underdeveloped muscles of the iris pupils narrow. The diameter of the pupils increases with age. At the age of 6-8 years, the pupils are wide due to the predominance of the tone of the sympathetic nerves innervating the muscles of the iris, which increases the risk sunburn retina. At 8-10 years old, the pupil becomes narrow again, and by 12-13 years old, the speed and intensity of the pupillary reaction to light is the same as in an adult. In newborns and children preschool age lens more convex and more elastic than that of an adult, and its refractive power is higher. This makes it possible to clearly see an object when it is closer to the eye than in an adult. In turn, the habit of viewing objects at a short distance can lead to the development of strabismus. Lacrimal glands and regulatory centers develop in the period from 2 to 4 months of life, and therefore tears during crying appear at the beginning of the 2nd, and sometimes at 3-4 months after birth.
Maturation of the conduction department visual analyzer is manifested by: myelination of pathways, beginning at the 8-9th month of intrauterine life and ending by 3-4 years, and differentiation of subcortical centers.
Cortical department the visual analyzer has the main signs of adults already in a 6-7 month old fetus, however nerve cells This part of the analyzer, like other parts of the visual analyzer, is immature. The final maturation of the visual cortex occurs by 7 years of age. Functionally, this leads to the emergence of the ability to form associative and temporary connections in the final analysis of visual sensations. Functional maturation of the visual areas of the cerebral cortex, according to some data, occurs already before the birth of a child, according to others - somewhat later. So, in the first months after birth, the child confuses the top and bottom of an object. If you show him a burning candle, then, trying to grab the flame, he will stretch out his hand not to the upper, but to the lower end.
Development of the functional capabilities of the visual sensory system. ABOUT light-perceiving functions in children can be judged by pupillary reflex, closing of the eyelids with the abduction of the eyeballs upward and other quantitative indicators of light perception, which are determined using adaptometers only from 4-5 years of age. Photosensitive function develops very early. Visual reflex to light (constriction of the pupils) - from 6 months of intrauterine development. A protective blink reflex to sudden light stimulation is present from the first days of life. Closing of the eyelids when an object approaches the eyes appears in the 2-4th month of life. With age, the degree of constriction of the pupils in the light and their dilation in the dark increases (Table 5.1). Constriction of the pupils when fixating an object with the gaze occurs from the 4th week of life. Visual concentration in the form of fixation of gaze on an object with simultaneous inhibition of movements appears in the 2nd week of life and lasts 1-2 minutes. The duration of this reaction increases with age. Following the development of fixation, a method develops.
ability to follow a moving object with the gaze and convergence of visual axes. Up to 10 weeks During life, eye movements are uncoordinated. Coordination of eye movements develops with the development of fixation, tracking and convergence. Convergence occurs at 2-3 weeks and becomes stable at 2-2.5 months of life. Thus, the child has a sense of light essentially from the moment of birth, but clear visual perception in the form of visual samples is not available to him, since although the retina is developed at the time of birth, the fovea has not completed its development, the final differentiation of the cones ends by the end of the year , and the subcortical and cortical centers in newborns are morphologically and functionally immature. These features determine the absence of objective vision and perception of space up to 3 months of life. Only from this time on does the child’s behavior begin to be determined by visual afferentation: before feeding, he visually finds his mother’s breast, examines his hands, and grasps toys located at a distance. The development of object vision is also associated with the perfection of visual acuity, eye motility, and the formation of complex inter-analyzer connections when combining visual sensations with tactile and proprioceptive ones. Differences in the shapes of objects appear in the 5th month.
Table 5.1
Age-related changes in diameter and reaction of pupil constriction to light
Change quantitative indicators Light perception in the form of the threshold of light sensitivity of the dark-adapted eye in children compared to adults is presented in Table. 5.2. Measurements have shown that the sensitivity to light of a dark-adapted eye increases sharply until the age of 20, and then gradually decreases. Due to the great elasticity of the lens, the eyes of children are more capable of accommodation than those of adults. With age, the lens gradually loses elasticity and its refractive properties deteriorate, the volume of accommodation decreases (i.e., the increase in the refractive power of the lens decreases when it is convex), and the point of closest vision is removed (Table 5.3).
Table 5.2
Light sensitivity of the dark-adapted human eye
of various ages
Table 5.3
Changes in the volume of accommodation with age
Color perception in children it appears from the moment of birth, but it appears to be different for different colors. According to the results of an electroretinogram (ERG), the functioning of cones to orange light in children was established from the 6th hour of life after birth. There is evidence that in last weeks embryonic development The cone apparatus is capable of responding to red and green colors. It is believed that from birth to 6 months of age, the order of sensation of color discrimination is as follows: yellow, white, pink, red, brown, black, blue, green, violet. From 6 months, children begin to distinguish all colors. But they are called correctly only from the age of three. Color recognition at an earlier age depends on the brightness, and not on the spectral characteristics of the color. IN school age The discriminating color sensitivity of the eye increases. The sense of color reaches its maximum development by the age of 30 and then gradually decreases. Training is important for the formation of this ability.
Visual acuity it increases with age and in 80-94% of children and adolescents it is greater than in adults (Table 5.4).
Table 5.4
Visual acuity in children of different ages
Improves with age and stereoscopic vision. It begins to form from the 5th month of life. This is facilitated by improving the coordination of eye movements, fixing the gaze on an object, improving visual acuity, and the interaction of the visual analyzer with others (especially with the tactile one). By the 6-9th month, an idea of the depth and distance of objects appears. Stereoscopic vision reaches its optimal level by the age of 17-22, and from the age of 6, girls have higher stereoscopic visual acuity than boys.
Field of view formed by 5 months. Until this time, children cannot evoke a defensive blink reflex when an object is introduced from the periphery. With age, the field of vision grows, especially intensively from 6 to 7.5 years. By age, its size is approximately 80% of the size of the visual field of an adult. Sexual characteristics are observed in the development of the visual field. The expansion of the field of vision continues until 20-30 years. The visual field determines the amount of educational information perceived by the child, i.e. the bandwidth of the visual analyzer, and, consequently, educational capabilities. In the process of ontogenesis, the throughput of the visual analyzer also changes and reaches different age periods the following values (Table 5.5).
Table 5.5
Throughput of the visual analyzer, bit/s
Sensory and motor functions vision develops simultaneously. In the first days after birth, eye movements are asynchronous; if one eye is immobile, the movement of the other can be observed. The ability to fix an object with one’s gaze, or, figuratively speaking, a “fine-tuning mechanism,” is formed between the ages of 5 days and 3-5 months. A reaction to the shape of an object is already observed in a 5-month-old child. In preschoolers, the first reaction is caused by the shape of an object, then by its size, and lastly by color.
At 7-8 years old eye gauge children are significantly better than preschoolers, but worse than adults; has no gender differences. Later, boys' linear eye becomes better than girls'.
The younger the child, the lower the functional mobility (lability) of the receptor and cortical parts of the visual analyzer.
Visual impairment. High plasticity is important in the process of training and raising children with sensory organ defects. nervous system, which allows you to compensate for lost functions at the expense of the remaining ones. It is known that deaf-blind children have increased sensitivity to tactile, gustatory and olfactory analyzers. With the help of their sense of smell, they can navigate the area well and recognize relatives and friends. The more pronounced the degree of damage to the child’s sense organs, the more difficult the educational work with him becomes. The overwhelming majority of all information from the outside world (approximately 90%) enters our brain through visual and auditory channels, therefore for normal physical and mental development For children and adolescents, the organs of vision and hearing are of particular importance.
Among vision defects, the most common are various forms of refractive error of the optical system of the eye or disturbance of the normal length of the eyeball (Fig. 5.7). As a result, rays coming from an object are refracted away from the retina. With weak refraction of the eye due to dysfunction of the lens - its flattening or shortening of the eyeball,
the image of the object appears behind the retina. People with such visual impairments have difficulty seeing close objects; This defect is called farsightedness.
Rice. 5.7. Refraction diagram in farsightedness (A), normal (b) and myopic (V) eye (according to A.G. Khripkova, 1978)
When the physical refraction of the eye increases, for example due to increased curvature of the lens, or elongation of the eyeball, the image of an object is focused in front of the retina, which disrupts perception removed items. This vision defect is called myopia.
When myopia develops, a student has difficulty seeing what is written on the blackboard and asks to be moved to the first desk. When reading, he brings the book closer to his eyes, bows his head strongly while writing, and in a movie or theater he strives to take a seat closer to the screen or stage. When looking at an object, the child squints his eyes. To make the image on the retina clearer, it brings the object in question too close to the eyes, which causes significant stress on the muscular system of the eye. Often the muscles cannot cope with such work, and one eye deviates towards the temple - strabismus occurs. Myopia can develop with diseases such as rickets, tuberculosis, and rheumatism.
Partial impairment of color vision is called color blindness (named after the English chemist Dalton, in whom this defect was first discovered). Colorblind people usually do not distinguish between red and green colors(they seem to them to be different shades of gray). About 4-5% of all men suffer from color blindness. In women it occurs less frequently (up to 0.5%). To detect color blindness, special color tables are used.
Prevention of visual impairment is based on creating optimal conditions for the functioning of the visual organ. Visual fatigue leads to sharp decline performance of children, which affects their general condition. Timely change of activities and changes in the environment in which training sessions are conducted help improve performance.
Of great importance correct mode work and rest, school furniture that meets the physiological characteristics of students, sufficient lighting of the workplace, etc. While reading, every 40-50 minutes you need to take a 10-15 minute break to give your eyes a rest; To relieve tension in the accommodation apparatus, children are advised to look into the distance.
In addition, an important role in the protection of vision and its function belongs to the protective apparatus of the eye (eyelids, eyelashes), which require careful care, compliance with hygiene requirements and timely treatment. Improper use of cosmetics can lead to conjunctivitis, blepharitis and other eye diseases.
Particular attention should be paid to organizing work with computers, as well as watching television programs. If you suspect visual impairment, consult an ophthalmologist.
Up to 5 years of age, hyperopia (farsightedness) predominates in children. With this defect, glasses with collective biconvex glasses (giving a converging direction to the rays passing through them) help, which improve visual acuity and reduce excessive stress of accommodation.
Subsequently, due to the load during training, the frequency of hypermetropia decreases, and the frequency of emmetropia (normal refraction) and myopia (myopia) increases. By the end of school compared to primary grades the prevalence of myopia increases 5 times.
The formation and progression of myopia is facilitated by light deficiency. Visual acuity and stability of clear vision in students decrease significantly by the end of lessons, and this decrease is sharper the lower the illumination level. With an increase in the level of illumination in children and adolescents, the speed of distinguishing visual stimuli increases, the speed of reading increases, and the quality of work improves. When the illumination of workplaces is 400 lux, 74% of work is completed without errors, when illumination is 100 lux and 50 lux - 47 and 37%, respectively.
With good lighting, hearing acuity increases in normally hearing children and adolescents, which also favors performance and has a positive effect on the quality of work. Thus, if dictations were conducted at an illumination level of 150 lux, the number of missing or misspelled words was 47% less than in similar dictations conducted at an illumination level of 35 lux.
The development of myopia is influenced by the educational load, directly related to the need to examine objects at close range, and its duration during the day.
You should also know that students who spend little or no time in the air around midday, when the intensity of ultraviolet radiation is maximum, have disturbances in phosphorus-calcium metabolism. This leads to a decrease in tone eye muscles, which, with high visual load and insufficient lighting, contributes to the development of myopia and its progression.
Children are considered myopic if their myopic refraction is 3.25 diopters or higher, and their corrected visual acuity is 0.5-0.9. Classes are recommended for such students physical culture only according to a special program. They are also contraindicated from performing heavy physical work, prolonged stay in a bent position with a bowed head.
For myopia, glasses with diverging biconcave lenses are prescribed, which convert parallel rays into divergent ones. Myopia in most cases is congenital, but it can increase during school age from junior to senior grades. In severe cases, myopia is accompanied by changes in the retina, which leads to decreased vision and even retinal detachment. Therefore, children suffering from myopia must strictly follow the instructions of the ophthalmologist. Timely wearing of glasses by schoolchildren is mandatory.
