The structure of the human visual analyzer. What is a visual analyzer and a scheme for its construction Visual analyzer and its auxiliary eye apparatus

The visual analyzer of a person, and simply speaking, the eyes, has a rather complex structure and simultaneously performs a lot of different functions. It allows a person not only to distinguish between objects. A person sees an image in color, which many other inhabitants of the Earth are deprived of. In addition, a person can determine the distance to an object and the speed of a moving object. Turning the eyes provides a person with a large viewing angle, which is necessary for safety.

The human eye has the shape of an almost regular sphere. He very complicated, has a lot of small details and at the same time from the outside it is a fairly durable organ. The eye is located in the opening of the skull, called the orbit, and lies there on a fatty layer, which, like a pillow, protects it from injury. The visual analyzer is a rather complex part of the body. Let's take a closer look at how the analyzer works.

Visual analyzer: structure and functions

Sclera

The protein membrane of the eye, which is connective tissue is called the sclera. This connective tissue is quite strong. It provides a permanent shape to the eyeball, which is necessary to maintain an unchanged shape of the retina. All other parts are in the sclera visual analyzer. The sclera does not transmit light radiation. Outside, muscles are attached to it. These muscles allow the eyes to move. Part of the sclera located in front eyeball absolutely transparent. This part is the cornea.

Cornea

There are no blood vessels in this part of the sclera. It is entangled in a dense web of nerve endings. They provide the highest sensitivity of the cornea. The shape of the sclera is a slightly convex sphere. This shape ensures the refraction of light rays and their concentration.

Vascular body

Inside the sclera along its entire inner surface lies vascular body . Blood vessels tightly entwine the entire inner surface eyeball, passing the influx nutrients and oxygen to all cells of the visual analyzer. At the location of the cornea, the vascular body is interrupted and forms a dense circle. This circle is formed by intertwining blood vessels and pigment. This part of the visual analyzer is called the iris.

Iris

The pigment is individual for each person. It is the pigment that is responsible for what color the eyes will have. specific person. For some diseases pigmentation is reduced or disappear altogether. Then the person's eyes are red. In the middle of the iris there is a transparent hole, clean from pigment. This hole can change its size. It depends on the intensity of the light. The diaphragm of a camera is built on this principle. This part of the eye is called the pupil.

Pupil

Smooth muscles are connected to the pupil in the form of intertwining fibers. These muscles provide constriction of the pupil or its expansion. The change in the size of the pupil is interconnected with the intensity of the light flux. If the light is bright, the pupil narrows, and in dim light it expands. This ensures that the light flux reaches the retina of the eye. about the same strength. The eyes act in sync. They rotate at the same time, and when light hits one pupil, both narrow. The pupil is completely transparent. Its transparency ensures that light enters the retina and forms a clear, undistorted picture.

The size of the pupil diameter depends not only on the strength of the illumination. At stressful situations, dangers, during sex, - in any situation when adrenaline is released in the body - the pupil also expands.

Retina

The retina covers the inner surface of the eyeball with a thin layer. It converts the photon stream into an image. The retina consists of specific cells - rods and cones. These cells connect to countless nerve endings. Rods and cones on the surface of the retina, the eyes are located mostly evenly. But there are places of accumulations of only cones or only rods. These cells are responsible for transmitting the image in color.

As a result of exposure to photons of light, a nerve impulse is formed. Moreover, impulses from the left eye are transmitted to right hemisphere, and impulses from the right eye - to the left. An image is formed in the brain due to incoming impulses.

Moreover, the picture turns out to be inverted and the brain then processes, corrects this picture, giving it the correct orientation in space. This property of the brain is acquired by a person in the process of growth. It is known that newborn children see the world upside down and only after some time, the picture of their perception of the world becomes upside down.

In order to obtain a geometrically correct, undistorted image in the human visual analyzer, there is a whole light refraction system. It has a very complex structure:

1. Anterior chamber of the eye
2. Posterior chamber of the eye
3. lens
4. vitreous body

The anterior chamber is filled with fluid. It is located between the iris and the cornea. The liquid in it is rich in many nutrients.

The posterior chamber is located between the iris and the lens. It is also filled with liquid. Both chambers are interconnected. The liquid in these chambers constantly circulates. If, due to a disease, the circulation of the fluid stops, the person’s vision deteriorates and such a person maybe even go blind.

lens - biconvex lens. It focuses the rays of light. Attached to the lens are muscles that can change the shape of the lens, making it thinner or more convex. The clarity of the image received by a person depends on this. This principle of image correction is used in cameras and is called focusing.

Thanks to these properties of the lens, we see a clear image of the object, and we can also determine the distance to it. Sometimes clouding of the lens occurs. This disease is called a cataract. Medicine has learned to replace lenses. Modern doctors consider this operation easy.

Inside the eyeball is the vitreous body. It fills all its space and consists of a dense substance that has jelly consistency. The vitreous keeps the eye in a constant shape and thus provides the geometry of the retina in a constant spherical. This allows us to see undistorted images. The vitreous body is transparent. It transmits light rays without delay and participates in their refraction.

The visual analyzer is so important for human life that nature provides for a whole set of different organs designed to provide correct work and keep his eyes healthy.

Auxiliary device

Conjunctiva

The thinnest layer that covers the inner surface of the eyelid and outer surface eyes is called the conjunctiva. This protective film lubricates the surface of the eyeball, helps to cleanse it of dust and maintain the surface of the pupil in a clean and transparent state. The composition of the conjunctiva contains substances that prevent the growth and reproduction of pathogenic microflora.

lacrimal apparatus

In the region of the outer corner of the eye is the lacrimal gland. It produces a special brackish liquid, which pours out through the outer corner of the eye and washes the entire surface of the visual analyzer. From there, the liquid flows down the duct and enters the lower divisions nose.

Muscles of the eye

Muscles hold the eyeball tightly in the socket, and, if necessary, turn the eyes up, down and to the sides. A person does not need to turn his head to view the subject of interest, and the viewing angle of a person is approximately 270 degrees. In addition, the eye muscles change the size and configuration of the lens, which provides a clear, sharp image of the object of interest, regardless of the distance to it. Muscles also control the eyelids.

eyelids

Movable shutters, if necessary, closing the eye. The eyelids are made up of skin. The lower part of the eyelids is lined with conjunctiva. Muscles attached to the eyelids ensure their closing and opening - blinking. The control of the muscles of the eyelids can be instinctive or conscious. Blink - important function to keep the eye healthy. When blinking, the open surface of the eye is lubricated with the secretion of the conjunctiva, which prevents the development of different kind bacteria. Blinking can occur when an object approaches the eye to prevent mechanical damage.

A person can control the process of blinking. He can somewhat delay the interval between blinks, or even blink the eyelids of one eye - wink. At the border of the eyelids, hairs grow - eyelashes.

Eyelashes and eyebrows.

Eyelashes are hairs that grow along the edges of the eyelids. Eyelashes are designed to protect the surface of the eye from dust and tiny particles present in the air. During a strong wind, dust, smoke, a person closes his eyelids and looks through lowered eyelashes. This happens on a subconscious level. In this case, the mechanism for protecting the surface of the eye from foreign bodies entering it is activated.

The eye is in the socket. At the top of the eye socket there is a superciliary arch. This is a protruding part of the skull that protects the eye from damage during falls and bumps. On the surface of the superciliary arch grow coarse hair- eyebrows that prevent dirt from getting into it.

Nature provides a whole range of preventive measures to preserve human vision. Such a complex structure of an individual organ speaks of its vital importance for saving human life. Therefore, with any initial visual impairment, the most right decision going to see an ophthalmologist. Take care of your eyesight.

To interact with the outside world, a person needs to receive and analyze information from external environment. For this, nature endowed him with sense organs. There are six of them: eyes, ears, tongue, nose, skin and Thus, a person forms an idea about everything that surrounds him and about himself as a result of visual, auditory, olfactory, tactile, gustatory and kinesthetic sensations.

It can hardly be argued that any sense organ is more significant than the others. They complement each other, creating a complete picture of the world. But the fact that most of all information - up to 90%! - people perceive with the help of the eyes - this is a fact. To understand how this information enters the brain and how it is analyzed, you need to understand the structure and functions of the visual analyzer.

Features of the visual analyzer

Thanks to visual perception we learn about sizes, shapes, colors, relative position objects of the surrounding world, their movement or immobility. This is a complex and multi-stage process. The structure and functions of the visual analyzer - a system that receives and processes visual information, and thereby provides vision - are very complex. Initially, it can be divided into peripheral (perceiving the initial data), conducting and analyzing parts. Information is received through the receptor apparatus, which includes the eyeball and auxiliary systems, and then it is sent with the help of the optic nerves to the corresponding centers of the brain, where it is processed and visual images are formed. All departments of the visual analyzer will be discussed in the article.

How is the eye. Outer layer of the eyeball

The eyes are a paired organ. Each eyeball is shaped like a slightly flattened ball and consists of several shells: external, middle and internal, surrounding the fluid-filled cavities of the eye.

The outer shell is a dense fibrous capsule that retains the shape of the eye and protects it. internal structures. In addition, six motor muscles of the eyeball are attached to it. The outer shell consists of a transparent front part - the cornea, and a back, opaque - sclera.

The cornea is the refractive medium of the eye, it is convex, looks like a lens and consists, in turn, of several layers. It doesn't have blood vessels, but there are many nerve endings. The white or bluish sclera, the visible part of which is commonly referred to as the white of the eye, is formed from connective tissue. Muscles are attached to it, providing turns of the eyes.

Middle layer of the eyeball

The middle choroid is involved in metabolic processes, providing nutrition to the eye and the removal of metabolic products. The front, most noticeable part of it is the iris. The pigment substance in the iris, or rather, its quantity, determines the individual shade of a person's eyes: from blue, if there is not enough of it, to brown, if enough. If the pigment is absent, as happens with albinism, then the plexus of vessels becomes visible, and the iris becomes red.

The iris is located just behind the cornea and is based on muscles. The pupil - a rounded hole in the center of the iris - thanks to these muscles regulates the penetration of light into the eye, expanding in low light and narrowing in too bright. The continuation of the iris is the function of this part of the visual analyzer is the production of fluid that nourishes those parts of the eye that do not have their own vessels. In addition, the ciliary body has a direct influence on the thickness of the lens through special ligaments.

In the posterior part of the eye, in the middle layer, there is the choroid, or the vascular proper, almost entirely consisting of blood vessels of different diameters.

Retina

internal, most thin layer, is the retina, or retina, formed nerve cells. Here there is a direct perception and primary analysis visual information. The back of the retina is made up of specialized photoreceptors called cones (7 million) and rods (130 million). They are responsible for the perception of objects by the eye.

Cones are responsible for color recognition and provide central vision allowing you to see the smallest details. Rods, being more sensitive, enable a person to see in black and white colors in poor lighting conditions, and are also responsible for peripheral vision. Most of the cones are concentrated in the so-called macula opposite the pupil, slightly above the entrance of the optic nerve. This place corresponds to the maximum visual acuity. The retina, as well as all parts of the visual analyzer, has a complex structure - 10 layers are distinguished in its structure.

The structure of the eye cavity

The ocular nucleus consists of the lens, the vitreous body and chambers filled with fluid. The lens appears to be convex on both sides clear lens. It has neither vessels nor nerve endings and is suspended from the processes of the ciliary body surrounding it, the muscles of which change its curvature. This ability is called accommodation and helps the eye to focus on close or, conversely, distant objects.

Behind the lens, adjacent to it and further to the entire surface of the retina, is located This is a transparent gelatinous substance that fills most of the volume. This gel-like mass contains 98% water. The purpose of this substance is to conduct light rays, compensate for drops intraocular pressure, maintaining the constancy of the shape of the eyeball.

The anterior chamber of the eye is limited by the cornea and iris. It connects through the pupil to a narrower posterior chamber extending from the iris to the lens. Both cavities are filled with intraocular fluid, which freely circulates between them.

Light refraction

The system of the visual analyzer is such that initially the light rays are refracted and focused on the cornea and pass through the anterior chamber to the iris. Through the pupil, the central part of the light flux enters the lens, where it is more accurately focused, and then through the vitreous body onto the retina. An image of an object is projected on the retina in a reduced and, moreover, inverted form, and the energy of light rays is converted by photoreceptors into nerve impulses. Information further through ophthalmic nerve enters the brain. The place on the retina through which optic nerve, devoid of photoreceptors, therefore it is called a blind spot.

The motor apparatus of the organ of vision

The eye, in order to respond in a timely manner to stimuli, must be mobile. For movement visual apparatus three pairs of oculomotor muscles respond: two pairs of straight and one oblique. These muscles are perhaps the fastest-acting in the human body. The oculomotor nerve controls the movement of the eyeball. He links with four out of six eye muscles, ensuring their adequate work and coordinated eye movements. If the oculomotor nerve ceases to function normally for some reason, this is expressed in various symptoms: strabismus, eyelid drooping, doubling of objects, pupil dilation, accommodation disorders, protrusion of the eyes.

Protective eye systems

Continuing such a voluminous topic as the structure and functions of the visual analyzer, one cannot fail to mention those systems that protect it. The eyeball is located in the bone cavity - the eye socket, on a shock-absorbing fatty pad, where it is reliably protected from impact.

In addition to the orbit, the protective apparatus of the organ of vision includes the upper and lower eyelids with eyelashes. They protect the eyes from outside various items. In addition, eyelids help uniform distribution tear fluid on the surface of the eye, removed when blinking from the cornea tiny particles dust. Eyebrows also perform protective functions to some extent, protecting the eyes from sweat flowing from the forehead.

The lacrimal glands are located in the upper outer corner of the orbit. Their secret protects, nourishes and moisturizes the cornea, and also has a disinfecting effect. Excess fluid through tear duct drains into the nasal cavity.

Further processing and final processing of information

The conductive section of the analyzer consists of a pair of optic nerves that exit the eye sockets and enter special canals in the cranial cavity, further forming an incomplete decussation, or chiasma. Images from the temporal (outer) part of the retina remain on the same side, while images from the inner, nasal part are crossed and transmitted to the opposite side of the brain. As a result, it turns out that the right visual fields are processed by the left hemisphere, and the left - by the right. Such an intersection is necessary for the formation of a three-dimensional visual image.

After decussation, the nerves of the conduction section continue in the optic tracts. Visual information enters that part of the cortex hemispheres the brain responsible for its processing. This zone is located in the occipital region. There, the final transformation of the received information into a visual sensation takes place. This is the central part of the visual analyzer.

So, the structure and functions of the visual analyzer are such that disturbances in any of its sections, whether it be the perceiving, conducting or analyzing zones, entail a failure of its work as a whole. This is a very multifaceted, subtle and perfect system.

Violations of the visual analyzer - congenital or acquired - in turn, lead to significant difficulties in the knowledge of reality and limited opportunities.

REPORT ON THE TOPIC:

PHYSIOLOGY OF THE VISUAL ANALYZER.

STUDENTS: Putilina M., Adzhieva A.

Teacher: Bunina T.P.

Physiology of the visual analyzer

The visual analyzer (or visual sensory system) is the most important of the sense organs of humans and most higher vertebrates. It gives more than 90% of the information going to the brain from all receptors. Thanks to the advanced evolutionary development of precisely the visual mechanisms, the brains of carnivores and primates have undergone drastic changes and achieved significant perfection. Visual perception is a multi-link process that begins with the projection of an image onto the retina and excitation of photoreceptors and ends with the adoption of a decision by the higher parts of the visual analyzer located in the cerebral cortex about the presence of a particular visual image in the field of view.

Structures of the visual analyzer:

Eyeball.

Auxiliary apparatus.

The structure of the eyeball:

The nucleus of the eyeball is surrounded by three shells: outer, middle and inner.

External - a very dense fibrous membrane of the eyeball (tunica fibrosa bulbi), to which the external muscles of the eyeball are attached, performs protective function and thanks to turgor determines the shape of the eye. It consists of an anterior transparent part - the cornea, and an opaque posterior part of a whitish color - the sclera.

The middle, or vascular, shell of the eyeball plays important role in metabolic processes, providing nutrition to the eye and excretion of metabolic products. It is rich in blood vessels and pigment (pigment-rich choroid cells prevent light from penetrating through the sclera, eliminating light scattering). It is formed by the iris, the ciliary body and the choroid itself. In the center of the iris there is a round hole - the pupil, through which light rays penetrate the eyeball and reach the retina (the size of the pupil changes as a result of the interaction of smooth muscle fibers- sphincter and dilator, enclosed in the iris and innervated by parasympathetic and sympathetic nerves). The iris contains a different amount of pigment, which determines its color - "eye color".

The inner, or reticular, shell of the eyeball (tunica interna bulbi), - the retina is the receptor part of the visual analyzer, here there is a direct perception of light, biochemical transformations of visual pigments, a change in the electrical properties of neurons and information is transmitted to the central nervous system. The retina consists of 10 layers:

Pigmentary;

photosensory;

Outer boundary membrane;

Outer granular layer;

Outer mesh layer;

Inner granular layer;

Internal mesh;

Ganglion cell layer;

Layer of optic nerve fibers;

Inner limiting membrane

fovea ( yellow spot). The area of ​​the retina in which there are only cones (color-sensitive photoreceptors); in this regard, it has twilight blindness (hemerolopia); this area is characterized by miniature receptive fields (one cone - one bipolar - one ganglion cell), and as a result, maximum visual acuity

From a functional point of view, the shell of the eye and its derivatives are divided into three apparatuses: refractive (refractive) and accommodative (adaptive), forming the optical system of the eye, and sensory (receptor) apparatus.

Light refracting apparatus

The refractive apparatus of the eye is a complex system of lenses that forms a reduced and inverted image of the outside world on the retina, includes the cornea, chamber moisture - the fluids of the anterior and posterior chambers of the eye, the lens, and the vitreous body, behind which lies the retina that perceives light.

Lens (lat. lens) - a transparent body located inside the eyeball opposite the pupil; Being a biological lens, the lens is an important part of the refractive apparatus of the eye.

The lens is a transparent biconvex rounded elastic formation, circularly fixed to the ciliary body. The posterior surface of the lens is adjacent to the vitreous body, in front of it are the iris and the anterior and posterior chambers.

The maximum thickness of the lens of an adult is about 3.6-5 mm (depending on the tension of accommodation), its diameter is about 9-10 mm. The radius of curvature of the anterior surface of the lens at rest of accommodation is 10 mm, and that of the posterior surface is 6 mm; at maximum accommodation stress, the anterior and posterior radii are equal, decreasing to 5.33 mm.

The refractive index of the lens is not uniform in thickness and averages 1.386 or 1.406 (nucleus), also depending on the state of accommodation.

At rest of accommodation, the refractive power of the lens is on average 19.11 diopters, with a maximum accommodation voltage of 33.06 diopters.

In newborns, the lens is almost spherical, has a soft texture and refractive power up to 35.0 diopters. Its further growth occurs mainly due to an increase in diameter.

accommodation apparatus

The accommodative apparatus of the eye ensures that the image is focused on the retina, as well as the adaptation of the eye to the intensity of illumination. It includes the iris with a hole in the center - the pupil - and the ciliary body with the ciliary girdle of the lens.

Focusing of the image is provided by changing the curvature of the lens, which is regulated by the ciliary muscle. With an increase in curvature, the lens becomes more convex and refracts light more strongly, tuning in to the vision of nearby objects. When the muscle relaxes, the lens becomes flatter, and the eye adapts to seeing distant objects. In other animals, in particular cephalopods, accommodation is dominated by a change in the distance between the lens and the retina.

The pupil is a variable-sized opening in the iris. It acts as the diaphragm of the eye, regulating the amount of light falling on the retina. In bright light, the circular muscles of the iris contract, and the radial muscles relax, while the pupil narrows, and the amount of light reaching the retina decreases, which protects it from damage. In low light, on the contrary, the radial muscles contract, and the pupil expands, letting more light into the eye.

ligaments of cinnamon (ciliary bands). The processes of the ciliary body are sent to the lens capsule. When the smooth muscles of the ciliary body are relaxed, they have the maximum tensile effect on the lens capsule, as a result of which it is maximally flattened, and its refractive power is minimal (this occurs at the time of viewing objects that are at a great distance from the eyes); under conditions of a reduced state of the smooth muscles of the ciliary body, the reverse picture takes place (when viewing objects close to the eyes)

the anterior and posterior chambers of the eye, respectively, are filled with aqueous humor.

The receptor apparatus of the visual analyzer. Structure and functions of individual layers of the retina

The retina is the inner shell of the eye, which has a complex multilayer structure. There are two types of photoreceptors different in their functional significance - rods and cones and several types of nerve cells with their numerous processes.

Under the influence of light rays in photoreceptors, photochemical reactions occur, consisting in a change in photosensitive visual pigments. This causes excitation of the photoreceptors, and then a synoptic excitation of the rod- and cone-associated nerve cells. The latter form the actual nervous apparatus of the eye, which transmits visual information to the centers of the brain and participates in its analysis and processing.

AUXILIARY DEVICE

The auxiliary apparatus of the eye includes protective devices and muscles of the eye. Protective devices include eyelids with eyelashes, conjunctiva and lacrimal apparatus.

The eyelids are paired skin-conjunctival folds that cover the front of the eyeball. The anterior surface of the eyelid is covered with thin, easily folded skin, under which lies the muscle of the eyelid and which, on the periphery, passes into the skin of the forehead and face. The posterior surface of the eyelid is lined with the conjunctiva. The eyelids have anterior lid margins that bear eyelashes and posterior lid margins that merge into the conjunctiva.

Between the upper and lower eyelids there is an eyelid gap with medial and lateral angles. At the medial angle of the slit of the eyelids, the front edge of each eyelid has a slight elevation - the lacrimal papilla, at the top of which the lacrimal canaliculus opens with a pinhole. In the thickness of the eyelids, cartilages are laid that are closely fused with the conjunctiva and largely determine the shape of the eyelids. By the medial and lateral ligaments of the eyelids, these cartilages are strengthened to the edge of the orbit. Quite numerous (up to 40) cartilage glands lie in the thickness of the cartilage, the ducts of which open near the free posterior edges of both eyelids. In persons working in dusty workshops, blockage of these glands is often observed, followed by their inflammation.

The muscular apparatus of each eye consists of three pairs of antagonistically acting oculomotor muscles:

top and bottom straight lines,

Inner and outer straight lines,

Upper and lower oblique.

All muscles, with the exception of the inferior oblique, begin, like the muscles that lift the upper eyelid, from the tendon ring located around the optic canal of the orbit. Then the four rectus muscles are directed, gradually diverging, anteriorly, and after perforation of the Tenon's capsule, they fly with their tendons into the sclera. The lines of their attachment are at different distances from the limbus: the inner straight line - 5.5-5.75 mm, the lower one - 6-6.6 mm, the outer one - 6.9-7 mm, the upper one - 7.7-8 mm.

The superior oblique muscle from the visual opening goes to the bone-tendon block located at the upper inner corner of the orbit and, having spread over it, goes posteriorly and outward in the form of a compact tendon; attached to the sclera in the upper outer quadrant of the eyeball at a distance of 16 mm from the limbus.

The inferior oblique muscle starts from the inferior bone wall of the orbit somewhat lateral to the entrance to the nasolacrimal canal, goes posteriorly and outwardly between the inferior wall of the orbit and the inferior rectus muscle; attached to the sclera at a distance of 16 mm from the limbus (inferior outer quadrant of the eyeball).

The internal, superior and inferior rectus muscles, as well as the inferior oblique muscle, are innervated by branches of the oculomotor nerve, the external rectus by the abducens, and the superior oblique by the trochlear.

When a particular muscle of the eye contracts, it moves around an axis that is perpendicular to its plane. The latter runs along the muscle fibers and crosses the point of rotation of the eye. This means that in most oculomotor muscles (with the exception of the external and internal rectus muscles) the axes of rotation have one or another angle of inclination with respect to the original coordinate axes. As a result, when such muscles contract, the eyeball makes a complex movement. So, for example, the superior rectus muscle, in the middle position of the eye, lifts it up, rotates inwards and turns somewhat towards the nose. Vertical eye movements will increase as the angle of divergence between the sagittal and muscular planes decreases, i.e., when the eye is turned outward.

All movements of the eyeballs are divided into combined (associated, conjugated) and convergent (fixation of objects at different distances due to convergence). Combined movements are those that are directed in one direction: up, to the right, to the left, etc. These movements are performed by muscles - synergists. So, for example, when looking to the right, the external rectus muscle contracts in the right eye, and the internal rectus muscle in the left eye. Convergent movements are realized through the action of the internal rectus muscles of each eye. A variation of them are fusion movements. Being very small, they carry out a particularly precise fixation of the eyes, which creates conditions for unhindered merging of two retinal images in the cortical section of the analyzer into one solid image.

Light perception

We perceive light due to the fact that its rays pass through the optical system of the eye. There, the excitation is processed and transmitted to the central departments. visual system. The retina is a complex shell of the eye containing several layers of cells that differ in shape and function.

The first (outer) layer is pigmented, consisting of densely packed epithelial cells containing the black pigment fuscin. It absorbs light rays, contributing to a clearer image of objects. The second layer - receptor, is formed by light-sensitive cells - visual receptors - photoreceptors: cones and rods. They perceive light and convert its energy into nerve impulses.

Each photoreceptor consists of an outer segment sensitive to the action of light, containing a visual pigment, and an inner segment containing a nucleus and mitochondria, which provide energy processes in the photoreceptor cell.

Electron microscopic studies revealed that the outer segment of each stick consists of 400-800 thin plates, or disks, with a diameter of about 6 microns. Each disk is a double membrane consisting of monomolecular layers of lipids located between layers of protein molecules. Retinal, which is part of the visual pigment rhodopsin, is associated with protein molecules.

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

The human eye has about 6-7 million cones and 110-125 million rods. Rods and cones are unevenly distributed in the retina. The central fovea of ​​the retina (fovea centralis) contains only cones (up to 140,000 cones per 1 mm2). Towards the periphery of the retina, the number of cones decreases, and the number of rods increases. The retinal periphery contains almost exclusively rods. Cones function in bright light conditions and perceive colors; rods are receptors that perceive light rays in conditions of twilight vision.

Irritation of different parts of the retina shows that different colors are perceived best when light stimuli act on the fovea, where cones are located almost exclusively. As you move away from the center of the retina, color perception becomes worse. The periphery of the retina, where only the rods are located, does not perceive colors. The light sensitivity of the cone apparatus of the retina is many times less than that of the elements associated with rods. Therefore, at dusk in low light conditions, central cone vision is sharply reduced and peripheral rod vision predominates. Since sticks do not perceive colors, a person does not distinguish colors at dusk.

Blind spot. The site of entry of the optic nerve into the eyeball - the papilla of the optic nerve - does not contain photoreceptors and is therefore insensitive to light; this is the so-called blind spot. The existence of a blind spot can be verified with the help of Marriott's experiment.

Mariotte did the experiment this way: he placed two nobles at a distance of 2 m against each other and asked them to look at a certain point from the side with one eye - then it seemed to everyone that his counterpart had no head.

Oddly enough, but people only in the 17th century learned that there was a “blind spot” on the retina of their eyes, which no one had thought about before.

retinal neurons. Inside the layer of photoreceptor cells in the retina there is a layer of bipolar neurons, to which a layer of ganglionic nerve cells adjoins from the inside.

Axons of ganglion cells form the fibers of the optic nerve. Thus, the excitation that occurs in the photoreceptor under the action of light enters the optic nerve fibers through nerve cells - bipolar and ganglionic.

Perception of the image of objects

A clear image of objects on the retina is provided by a complex unique optical system of the eye, consisting of the cornea, fluids of the anterior and posterior chambers, the lens and the vitreous body. Light rays pass through the listed media optical system eyes and refract in them according to the laws of optics. The lens plays a major role in the refraction of light in the eye.

For a clear perception of objects, it is necessary that their image is always focused in the center of the retina. Functionally, the eye is adapted for viewing distant objects. However, people can clearly distinguish objects located at different distances from the eye, thanks to the ability of the lens to change its curvature, and, accordingly, the refractive power of the eye. The ability of the eye to adapt to a clear vision of objects located at different distances is called accommodation. Violation of the accommodative ability of the lens leads to impaired visual acuity and the occurrence of myopia or hyperopia.

Parasympathetic preganglionic fibers originate from the Westphal-Edinger nucleus (visceral part of the nucleus III couples cranial nerve) and then go as part of the III pair of cranial nerves to the ciliary ganglion, which lies immediately behind the eye. Here, preganglionic fibers form synapses with postganglionic parasympathetic neurons, which in turn send fibers as part of the ciliary nerves to the eyeball.

These nerves excite: (1) the ciliary muscle, which regulates the focusing of the lenses of the eyes; (2) iris sphincter, pupil constriction.

The source of sympathetic innervation of the eye is the neurons of the lateral horns of the first thoracic segment. spinal cord. The sympathetic fibers leaving from here enter the sympathetic chain and rise to the superior cervical ganglion, where they synaptically communicate with ganglionic neurons. Their postganglionic fibers run along the surface of the carotid artery and further along the smaller arteries and reach the eye.

Here, sympathetic fibers innervate the radial fibers of the iris (which dilate the pupil) as well as some of the extraocular muscles of the eye (discussed below in connection with Horner's syndrome).

The accommodation mechanism that focuses the optical system of the eye is important for maintaining high visual acuity. Accommodation is carried out as a result of contraction or relaxation of the ciliary muscle of the eye. Contraction of this muscle increases the refractive power of the lens, and relaxation reduces it.

The accommodation of the lens is regulated by the mechanism of negative feedback, which automatically adjusts the refractive power of the lens to achieve the highest degree of visual acuity. When eyes focused on some distant object must suddenly focus on a near object, the lens usually accommodates for less than 1 second. Although the exact regulatory mechanism that causes this rapid and precise focusing of the eye is not clear, some of its features are known.

First, with a sudden change in the distance to the point of fixation, the refractive power of the lens changes in the direction corresponding to the achievement of a new state of focus, within a fraction of a second. Secondly, various factors help to change the strength of the lens in the right direction.

1. Chromatic aberration. For example, red rays are focused slightly behind blue rays, since blue rays are more strongly refracted by the lens than red ones. The eyes seem to be able to determine which of these two types of beams is better focused, and this "key" conveys information to an accommodating mechanism to increase or decrease the strength of the lens.

2. Convergence. When the eyes are fixed on a nearby object, the eyes converge. The neural mechanisms of convergence simultaneously send a signal that increases the refractive power of the lens of the eye.

3. The clarity of focus in the depth of the fovea is different compared to the clarity of focus at the edges, since the fovea lies somewhat deeper than the rest of the retina. It is believed that this difference also gives a signal in which direction the lens strength should be changed.

4. The degree of accommodation of the lens fluctuates slightly all the time with a frequency of up to 2 times per second. In this case, the visual image becomes clearer when the lens strength fluctuation changes in the right direction, and less clear when the lens strength changes in the wrong direction. This can give a quick signal to choose the right direction of lens strength change to provide the appropriate focus. The areas of the cerebral cortex that regulate accommodation function in close parallel connection with the areas that control fixative eye movements.

In this case, the analysis of visual signals is carried out in the areas of the cortex corresponding to fields 18 and 19 according to Brodmann, and motor signals to the ciliary muscle are transmitted through the pretectal zone of the brain stem, then through the Westphal-Edinger nucleus and, finally, along the parasympathetic nerve fibers to the eyes.

Photochemical reactions in the receptors of the retina

The retinal rods of humans and many animals contain the pigment rhodopsin, or visual purple, the composition, properties and chemical transformations of which have been studied in detail in recent decades. The pigment iodopsin was found in the cones. The cones also contain the pigments chlorolab and erythrolab; the first of them absorbs the rays corresponding to the green, and the second - the red part of the spectrum.

Rhodopsin is a high molecular weight compound ( molecular mass 270000), consisting of retinal - vitamin A aldehyde and an opsin beam. Under the action of a light quantum, a cycle of photophysical and photochemical transformations of this substance occurs: retinal isomerizes, its side chain is straightened, the bond between retinal and protein is broken, and the enzymatic centers of the protein molecule are activated. A conformational change in the pigment molecules activates Ca2+ ions, which reach the sodium channels through diffusion, as a result of which the conductivity for Na+ decreases. As a result of a decrease in sodium conductivity, an increase in electronegativity occurs inside the photoreceptor cell relative to the extracellular space. The retinal is then cleaved from the opsin. Under the influence of an enzyme called retinal reductase, the latter is converted into vitamin A.

When the eyes are darkened, the regeneration of visual purple occurs, i.e. resynthesis of rhodopsin. This process requires that the retina receives the cis-isomer of vitamin A, from which retinal is formed. If vitamin A is absent in the body, the formation of rhodopsin is sharply disrupted, which leads to the development of night blindness.

Photochemical processes in the retina occur very sparingly; under the action of even very bright light, only a small part of the rhodopsin present in the sticks is split.

The structure of iodopsin is close to that of rhodopsin. Iodopsin is also a compound of retinal with the protein opsin, which is produced in cones and is different from rod opsin.

The absorption of light by rhodopsin and iodopsin is different. Iodopsin absorbs yellow light with a wavelength of about 560 nm to the greatest extent.

The retina is a fairly complex neural network with horizontal and vertical connections between photoreceptors and cells. Bipolar retinal cells transmit signals from photoreceptors to the ganglion cell layer and to amacrine cells (vertical connection). Horizontal and amacrine cells are involved in horizontal signaling between adjacent photoreceptors and ganglion cells.

Color perception

The perception of color begins with the absorption of light by cones - the photoreceptors of the retina (detail below). The cone always responds to the signal in the same way, but its activity is transferred to two different types neurons called ON and OFF type bipolar cells, which, in turn, are connected to ON and OFF type ganglion cells, and their axons carry a signal to the brain - first to the lateral geniculate body, and from there further to the visual cortex

Multicolor is perceived due to the fact that cones react to a certain spectrum of light in isolation. There are three types of cones. Cones of the first type react mainly to red, the second - to green and the third - to blue. These colors are called primary. Under the action of waves of different lengths, cones of each type are excited differently.

The longest wavelength corresponds to red, the shortest - violet;

The colors between red and violet are arranged in the well-known sequence red-orange-yellow-green-cyan-blue-violet.

Our eye perceives wavelengths only in the range of 400-700 nm. Photons with wavelengths above 700 nm are infrared radiation and are perceived in the form of heat. Photons with wavelengths below 400 nm are referred to as ultraviolet radiation, because of their high energy, they are able to have a damaging effect on the skin and mucous membranes; Ultraviolet is followed by x-rays and gamma rays.

As a result, each wavelength is perceived as a particular color. For example, when we look at a rainbow, the primary colors (red, green, blue) seem to be the most noticeable to us.

By optical mixing of primary colors, other colors and shades can be obtained. If all three types of cones fire at the same time and in the same way, a sensation of white color occurs.

Color signals are transmitted along slow fibers of ganglion cells

As a result of mixing signals that carry information about color and shape, a person can see what would not be expected based on the analysis of the wavelength of light reflected from an object, which is clearly demonstrated by illusions.

visual paths:

Ganglion cell axons give rise to the optic nerve. The right and left optic nerves merge at the base of the skull, forming a decussation, where nerve fibers, coming from the inner halves of both retinas, intersect and pass to the opposite side. Fibers from the outer halves of each retina join together with a criss-crossed bundle of axons from the contralateral optic nerve to form the optic tract. The optic tract ends in the primary centers of the visual analyzer, which include the lateral geniculate bodies, the superior tubercles of the quadrigemina, and the pretectal region of the brainstem.

The lateral geniculate bodies are the first structure of the central nervous system where excitation impulses switch on the way between the retina and the cerebral cortex. Neurons of the retina and lateral geniculate body analyze visual stimuli, evaluating their color characteristics, spatial contrast, and average illumination in different parts of the visual field. In the lateral geniculate bodies, binocular interaction begins from the retina of the right and left eyes.

Date: 04/20/2016

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• A little about the structure of the visual analyzer
• Functions of the iris and cornea
• What is the refraction of the image on the retina
• Auxiliary apparatus of the eyeball
• Eye muscles and eyelids

The visual analyzer is paired organ vision, represented by the eyeball, muscular system eyes and ancillary apparatus. With the help of the ability to see, a person can distinguish the color, shape, size of an object, its illumination and the distance at which it is located. So human eye able to distinguish the direction of movement of objects or their immobility. 90% of the information a person receives through the ability to see. The organ of vision is the most important of all the sense organs. The visual analyzer includes an eyeball with muscles and an auxiliary apparatus.

A little about the structure of the visual analyzer

The eyeball is located in the orbit on a fatty pad, which serves as a shock absorber. In some diseases, cachexia (weight loss), the fat pad becomes thinner, the eyes sink deeper eye socket and it seems that they "sunk down". The eyeball has three shells:

• protein;
• vascular;
• mesh.

The characteristics of the visual analyzer are quite complex, so you need to disassemble them in order.

The sclera is the outermost layer of the eyeball. The physiology of this shell is arranged in such a way that it consists of a dense connective tissue that does not transmit light rays. Muscles of the eye are attached to the sclera, providing movement of the eye and conjunctiva. The front part of the sclera has a transparent structure and is called the cornea. Concentrated on the cornea great amount nerve endings, providing its high sensitivity, and there are no blood vessels in this area. In shape, it is round and somewhat convex, which allows for the correct refraction of light rays.

The choroid consists of a large number of blood vessels that provide trophism to the eyeball. The structure of the visual analyzer is arranged in such a way that the choroid is interrupted at the point where the sclera passes into the cornea and forms a vertically located disk consisting of plexuses of blood vessels and pigment. This part of the shell is called the iris. The pigment contained in the iris is different for each person, and it provides the color of the eyes. In some diseases, the pigment may decrease or be completely absent (albinism), then the iris becomes red.

In the central part of the iris there is a hole, the diameter of which varies depending on the intensity of illumination. Rays of light penetrate the eyeball to the retina only through the pupil. The iris has smooth muscles - circular and radial fibers. She is responsible for the diameter of the pupil. Circular fibers are responsible for the constriction of the pupil, they are innervated by the peripheral nervous system and the oculomotor nerve.

The radial muscles are classified as sympathetic nervous system. These muscles are controlled from a single brain center. Therefore, the expansion and contraction of the pupils occurs in a balanced way, regardless of whether it affects one eye bright light or both.

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Functions of the iris and cornea

The iris is the diaphragm eye apparatus. It regulates the flow of light rays to the retina. The pupil constricts when fewer light rays hit the retina after refraction.

This happens when the light intensity increases. When the illumination decreases, the pupil expands and enters the fundus large quantity Sveta.

The anatomy of the visual analyzer is designed so that the diameter of the pupils depends not only on lighting, this indicator is also affected by some body hormones. So, for example, when frightened, it stands out a large number of adrenaline, which is also able to act on the contractility of the muscles responsible for the diameter of the pupil.

The iris and cornea are not connected: there is a space called the anterior chamber of the eyeball. The anterior chamber is filled with a fluid that performs a trophic function for the cornea and participates in the refraction of light during the passage of light rays.

The third retina is a specific perceiving apparatus of the eyeball. The retina is made up of branched nerve cells that emerge from the optic nerve.

The retina is located just behind the choroid and lines most of the eyeball. The structure of the retina is very complex. Only capable of perceiving objects rear end retina, which is formed by special cells: cones and rods.

The structure of the retina is very complex. Cones are responsible for the perception of the color of objects, rods - for the intensity of light. Rods and cones are interspersed, but in some areas there is an accumulation of only rods, and in some - only cones. Light hitting the retina causes a reaction within these specific cells.

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What is the refraction of the image on the retina

As a result of this reaction, a nerve impulse is produced, which is transmitted along the nerve endings to the optic nerve, and then to the occipital lobe of the cerebral cortex. It is interesting that the pathways of the visual analyzer have a complete and incomplete intersection with each other. Thus, information from the left eye enters the occipital lobe of the cerebral cortex on the right and vice versa.

An interesting fact is that the image of objects after refraction on the retina is transmitted upside down.

In this form, information enters the cerebral cortex, where it is then processed. Perceiving objects as they are is an acquired skill.

Newborn babies perceive the world upside down. As the brain grows and develops, these functions of the visual analyzer are developed and the child begins to perceive external world in true form.

The refraction system is represented by:

• front camera;
• posterior chamber of the eye;
• lens;
• vitreous body.

The anterior chamber is located between the cornea and the iris. It provides nutrition to the cornea. The posterior chamber is located between the iris and the lens. Both the anterior and posterior chambers are filled with fluid that is able to circulate between the chambers. If this circulation is disturbed, then a disease occurs that leads to impaired vision and can even lead to loss of it.

The lens is a biconvex transparent lens. The function of the lens is to refract light rays. If the transparency of this lens changes in some diseases, then a disease such as a cataract occurs. To date, the only treatment for cataracts is lens replacement. This operation is simple and quite well tolerated by patients.

The vitreous body fills the entire space of the eyeball, providing a constant shape of the eye and its trophism. The vitreous body is represented by a gelatinous transparent liquid. When passing through it, the rays of light are refracted.