The order of passage of photons through the refractive media of the eye. Eye and vision. Refractive media of the eye
The eye is an organ of vision (Fig. 1), a very complex sensory organ that perceives the action of light. The human eye is irritated by rays of a certain part of the spectrum. It is affected by electromagnetic waves with a length of approximately 400 to 800 nm, which, when afferent impulses enter the visual analyzer of the brain, causes visual sensations. The functions of the eye are very diverse. Through the eye, the shape of objects, their size, distance from the eye, the direction in which they move, their stillness, the degree of illumination, color, and coloration are determined.
Since the most important part of the eye - the retina with the optic nerve - develops directly from the brain tissue, the eye is part of the brain, advanced to the periphery.
refractive media.
The eye consists of two systems:
- 1) optical system of light-refracting media and
- 2) the receptor system of the retina.
The refractive media of the eye include: the cornea, the aqueous humor of the anterior chamber of the eye, the lens and the vitreous body. Each of these media has its own refractive index. The refractive index of the cornea - 1.37; aqueous humor and vitreous body - 1.33; the outer layer of the lens - 1.38; lens nuclei - 1.40. clear vision exists only if the refractive media of the eye are transparent.
The shorter the focal length, the greater the refractive power of the optical system, which is expressed in diopters. Diopter is the refractive power of a lens with a focal length of 1 m. The refractive power of the optical system of the eye is (in diopters): cornea - 43; lens when looking into the distance - 19; at the maximum approach of the object to the eye - 33. the refractive power of the entire optical system of the eye is equal to the distance - 58; and at the maximum approximation of the subject - 70.
The eye is an organ of vision, a very complex sensory organ that perceives the action of light. The human eye is irritated by rays of a certain part of the spectrum. It is affected by a length of approximately 400 to 800 nm, which, when afferent impulses enter the visual analyzer of the brain, causes visual sensations. The functions of the eye are very diverse. Through the eye, the shape of objects, their size, distance from the eye, the direction in which they move, their stillness, the degree of illumination, color, and coloration are determined.
Since the most important part of the eye - the retina with the optic nerve - develops directly from the brain, the eye is part of the brain, advanced to the periphery.
Refractive media
The eye consists of two systems: 1) the optical system of light-refracting media and 2) the receptor system of the retina.
The refractive media of the eye include: the cornea, the aqueous humor of the anterior chamber of the eye, the lens and the vitreous body. Each of these media has its own refractive index. The refractive index of the cornea is 1.37, the aqueous humor and vitreous body is 1.33, the outer layer of the lens is 1.38, the lens nucleus is 1.40. Clear vision exists only if the refractive media of the eye are transparent.
The shorter the focal length, the greater the refractive power of the optical system, which is expressed in diopters. The diopter is the refractive power of a lens with a focal length of 1 m. The refractive power of the optical system of the eye is (in diopters): the cornea - 43, the lens when looking into the distance - 19, with the object closest to the eye - 33. The refractive power of the entire optical system of the eye is for a distance - 58, and at the maximum approximation of an object - 70.
Static and dynamic refraction of the eye and its disorders
Refraction is the optical adjustment of the eye when looking into the distance.
normal eye. When looking into the distance, when a beam of parallel rays falls on the eyes, they, without any change in the curvature of the lens, are collected in focus, just on the retina, in the central fossa. Such an eye is normal, or emmetropic. But there are the following deviations from the norm.
Myopia. It is observed when the length of the eye exceeds the normal (more than 22.5-23.0 mm) or when the power of the refractive media of the eye is greater than normal (the curvature of the lens is greater). A parallel beam of rays, falling on the eye, in these cases gathers into focus in front of the fovea, and therefore a beam of divergent rays falls on the fovea, and the image of the object is blurry. Such an eye is called myopic, or myopic. To obtain a clear image on the retina, it is necessary that the focus falls on the retina, which occurs when a beam of divergent rays falls on a myopic eye; therefore, the short-sighted person brings objects closer to the eye or the eyes to the object and sees clearly only near.
To correct (correct) myopia, biconcave glasses are used, which shift the focus to the retina, which compensates for the increase in the refractive power of the lens. Myopia is often hereditary. However, due to the violation of hygienic rules, it increases at school age from the lower grades to the older ones. Myopia in severe cases is accompanied by changes in the retina, which leads to a decrease in vision and even blindness (with retinal detachment). Therefore, it is necessary to timely prescribe glasses for schoolchildren to correct vision, and general strengthening of the body (physical education, nutrition, general hygiene measures) and compliance with the rules of school hygiene.
farsightedness. When the length of the eye is less than the normal or weak refractive power, the beam of parallel rays, after refraction in the eye, gathers into focus behind the central fovea of the retina. In this case, converging rays fall on the retina, and the image of objects is blurry. Such an eye is neither far-sighted nor hyperopic. The nearest point of clear vision is further away from the far-sighted eye than from the normal one. To correct farsightedness, doubly convex glasses are used, which increase the refractive power of the eye.
Congenital and acquired farsightedness should not be confused with senile farsightedness, which is discussed below.
Astigmatism- the impossibility of convergence of all rays to one point, to one focus. It is usually observed with different curvature of the cornea in its various meridians. If the vertical meridian refracts more, astigmatism is called direct, if the horizontal meridian is reversed. Even normal eyes are slightly astigmatic because the surface of the cornea is not strictly spherical. When viewed from the distance of the best vision of a disk with concentric circles applied to it, a slight flattening of the circles is observed. Astigmatism, which impairs vision, is corrected with the help of cylindrical glasses, which are located along the corresponding meridians of the cornea, in which the curvature is broken.
Accommodation of the eye and its age-related features
Accommodation is the ability of the eye to adapt to a clear vision of objects located at different distances from it. This ability is due to the fact that the lens, due to its elasticity, can change the curvature, and consequently, the refractive power. Therefore, the image of a moving object always falls on the retina, which remains motionless. When looking far away at objects that are far from the eye, the ciliary muscle is relaxed, and the ligament of zon, which is attached mainly to the anterior and posterior surfaces of the lens capsule, is stretched at this time. The tension of the ligament of Zinn causes compression of the lens from front to back and its stretching. Thus, when viewing a distant object, the curvature of the lens is the smallest and, consequently, its refractive power is also the smallest. When an object approaches the eye, the ciliary muscle contracts; while the meridional and radial fibers pull forward the choroid to which they are attached. Therefore, the ligament of zinn relaxes, which stops the squeezing and stretching of the lens. With a very strong relaxation of the Zinn ligament, the lens, due to its gravity, drops to 0.3 mm. Due to its elasticity, the lens becomes more convex and, consequently, its refractive power increases.
The contraction of the ciliary muscle is caused by a reflex due to the influx of afferent impulses into the midbrain and excitation of the parasympathetic fibers that make up the oculomotor nerve.
At rest of accommodation, i.e., when looking into the distance, the radius of curvature of the anterior surface of the lens is 10 mm, and at the highest accommodation stress, i.e., with a clear vision of an object as close as possible to the eye, the radius of curvature of the anterior surface of the lens is 5.3 mm . The corresponding changes in the curvature of the posterior surface of the lens are less (from 6.0) to 5.5 mm).
Accommodation of the eye begins when the object is at a distance of about 65 m from the eye. At this time, the ciliary muscle begins to contract. However, at such a distance of the object from the eye, its contraction is very small. A distinct contraction of the ciliary muscle begins at a distance of an object from the eye of 5-10 m. As the object approaches the eye further, accommodation intensifies more and more, and, finally, a clear vision of the object becomes impossible. The smallest distance of an object from the eye at which the object is still clearly visible is called nearest point of clear vision. In a normal eye distant point of clear vision lies at infinity.
Birds and mammals have the same accommodation mechanism of the eye.
With age, due to the loss of elasticity of the lens, the volume (amplitude) of accommodation decreases. After 10 years, the distant point of clear vision hardly shifts, and the nearest one moves further and further away over the years.
It should be taken into account that when practicing at close range, the eye does not get tired when at least one third of the accommodation volume remains in reserve.
Presbyopia, or presbyopia, is due to the removal of the nearest point of clear vision due to the loss of elasticity of the lens and the corresponding decrease in its refractive power with the greatest increase in accommodation. At the age of 10, the nearest point of clear vision is at a distance of less than 7 cm from the eye, at 20 years old - 8.3 cm, at 30 years old - 11 cm, and by the age of 60-70 it approaches 80-100 cm.
Building an image on the retina
Since the eye is an extremely complex optical system, a simplified model of the eye can be used to study its optical properties and build an image ( reduced eye). The visual axis of the reduced eye, as in the normal eye, passes through the centers of the refractive media of the eye into the central fovea of the retina.
In the reduced eye, only the vitreous is the refractive medium; it lacks the main points that lie at the intersection of the main refractive planes on the visual axis. Both nodal points, which in the true eye are at a small distance of 0.3 mm from each other, are replaced by one point. Nodal points are two conjugate points. A ray passing through one of the points necessarily passes through the other and leaves it parallel to the original direction. Thus, in the reduced eye there is only one nodal point through which the rays pass without being refracted. The nodal point of the reduced eye is placed at a distance of 7.5 mm from the top of the cornea in the posterior third of the lens. The distance from the nodal point to the retina is 15 mm. When constructing an image of an object on the retina, all its points are considered as luminous. From each point, a straight line is drawn through the nodal point to the retina.
The image on the retina is real, inverse and reduced. To determine the size of the image on the retina, one fixes with the eye some long word printed in small print, and determines how many letters the eye sees when it is completely immobile. Then, using a ruler, the length of this row of letters is determined in millimeters, after which it follows from the similarity of the triangles ABO and Ov that the image of these letters on the retina is as many times less than the length of these letters as O is less than BO. Since BO is equal to the distance from the book to the top of the cornea plus 7.5 mm, and O is 15 mm, the length of the image on the retina is easily calculated and thus the diameter of the macula is determined. The yellow spot performs the function of central vision.
Despite the fact that the image is reversed on the retina, we see objects in direct form thanks to the daily training of the brain department of the visual analyzer. To determine the position of an object in space, we use the readings of not only the retina; for example, we perceive the upper part of an object when we raise our eyes, while receiving readings from the proprioceptors of the eye muscles, or we also raise our hand to feel this upper part, or we simultaneously use the readings of other analyzers.
Thus, the determination of the position of objects is based on conditioned reflexes, the testimony of several analyzers, and constant exercise and verification of them in everyday practice.
Pupil reaction and its meaning
In the center of the opaque iris there is a round hole - the pupil.
The pupil passes only the central beam of light rays into the eye, which eliminates the phenomenon of spherical and chromatic aberration. Due to this, the image of the object on the retina is in focus and is clear, not blurry.
The second function of the iris is to regulate the amount of rays entering the eye, and thus to regulate the intensity of retinal irritation.
The regulatory function of the iris is carried out by changing the diameter of the lumen of the pupil. The contraction of the circular, ring, muscle fibers of the iris, forming the sphincter, causes pupillary constriction. The contraction of the radial muscle fibers of the iris, forming a dilator, causes the pupil to dilate. The pupillary sphincter is innervated by the parasympathetic fibers of the oculomotor nerve, and the pupillary dilator is innervated by the sympathetic nerve.
Constriction or dilation of the pupil in one eye is accompanied by constriction or dilation of the pupil in the other, which is probably due to the connection of the nuclei of the oculomotor nerves in the midbrain, thus, the constriction and expansion of the pupils of both eyes occurs reflexively and concomitantly.
Pupil constriction occurs: 1) with increased illumination of the retina; 2) when looking at a close object; 3) in a dream. The expansion of the pupil occurs: 1) with a decrease in the illumination of the retina; 2) with irritations of receptors and nuclei of any afferent nerves, with emotions (pain, anger, fear, etc.), mental excitations; 3) with suffocation, anesthesia.
Constriction of the pupil (miosis) with. In bright light, it has a protective value, as it protects the retina from damage when exposed to bright light. On the contrary, dilation of the pupil (mydriasis) in low light causes more rays to enter the eye than a better visibility of the object is achieved. The most favorable for human vision is the pupil width of 3 mm. With a narrower pupil, the illumination of the retina is insufficient, and with a wider pupil, the eye is blinded. In adults, the pupil width ranges on average from 2.5 to 4.5 mm. The area of the pupil in an adult changes 17 times, which ensures the regulation of the illumination of the retina and the sharpness of the image. Over the years, changes in the area of the pupil decrease. Constriction of the pupil when viewing close objects is associated with excitation of the nuclei of the oculomotor nerves. Pupil constriction during sleep is due to an increase in the tone of the parasympathetic system. The expansion of the pupil during stimulation of the receptors and nuclei of the afferent nerves and emotions depends on the excitation of the sympathetic system and the cerebral hemispheres. The expansion of the pupil during suffocation is associated with the irritating nervous system action of carbon dioxide that accumulates in the blood. Constriction and expansion of the pupil can be caused by a conditioned reflex. The expansion of the pupil under mental influences and its conditioned reflex changes indicate the regulation of the size of the pupil by the cerebral hemispheres.
The structure of the retina, rods and cones
The light-excitable apparatus of the eye is a layer of the retina containing about 130 million rods and about 7 million cones in a person. The outer segments of the rods and cones are composed of a birefringent substance that strongly refracts light. In the outer segments of the rods there is a special purple substance - visual purple, or rhodopsin. The cones contain a purple substance - iodopsin, which, unlike rhodopsin, fades in red light.
The outer segments of these receptors consist of 400-800 of the thinnest plates, or disks, located one above the other. Between the outer and inner segments there are membranes through which 16-18 thin fibrils pass. The lower process of the inner segment is connected to a bipolar neuron. The outermost layer of the retina - the pigment layer - contains the pigment fuscin, which absorbs light and does not allow it to scatter, which ensures clarity of visual perception.
The distribution of rods and cones in the retina is uneven. In humans, cones predominate in the middle of the retina, and rods predominate in its lateral parts. The central fovea of the macula contains almost exclusively cones, while the most peripheral parts of the retina contain exclusively rods.
The cones located in the fovea are thin and elongated. In place of the fovea, the retina becomes much thinner.
In the very center of the macula, each cone is connected through a bipolar neuron to a separate ganglion neuron, which creates the greatest visual acuity. In other parts of the fovea adjacent to its periphery, each bipolar neuron is connected to several cones and many rods, and each ganglion neuron is connected to many bipolar ones. Unlike cones, a large number of rods are connected to a common bipolar neuron (up to 200 rods). Receptors associated with one ganglionic neuron form its receptive field. The receptive fields of different ganglionic neurons are interconnected by horizontal (stellate) and amacrine cells, which leads to the connection of one ganglionic neuron with tens of thousands of receptors. Optic nerve fibers are outgrowths of ganglionic neurons. One ganglion neuron is connected to thousands of neurons in the visual area, and one neuron in this area converges numerous afferent fibers.
Light rays act on the outer layer of the retina, in which the rods and cones are located, beforehand they pass through all the layers of the retina, and therefore through the bipolar neurons located medially from the receptors, and through the ganglionic neurons, which are located medially from the bipolar ones. There are synapses between bipolar and ganglion neurons. From retinal receptors, impulses are transmitted to ganglionic neurons two times slower than from them to the visual area, in which spatial and temporal summation of afferent impulses from the eyes occurs.
Day and dusk vision
Rods and cones are two independent visual apparatuses. The organ of twilight vision, which gives only colorless light sensations, is rods. The organ of daytime vision that gives color sensations is cones. It has been established that there is a reciprocal relationship between rods and cones. When the cones are functioning, the rods are inhibited (LA Orbeli, 1934). The sticks give a feeling of light even in low light. Cones are less excitable to light, and therefore, when a beam of weak light enters the central fossa, where cones are located, but there are no rods or there are extremely few of them, we see it very poorly or not at all. But the same weak light is clearly visible when it acts on the lateral surfaces of the retina. Moreover, it has been established that only sticks function under the action of weak light of less than 0.01 lux on a white surface (lux - lux is a unit of illumination created by one international candle on a surface of 1 m 2 with a perpendicular incidence of light from a distance of 1 m). At light intensities exceeding 30 lux on a white surface, almost exclusively cones function. However, the participation of rods when looking at a light source of high brightness cannot be completely denied. At dusk, in low light, the colors do not differ. In this case, the blue part of the spectrum seems to be light - the forest is red, and the green part of the spectrum appears to be the brightest (J. Purkinje's phenomenon).
During the day, the red part of the spectrum appears brighter, and the brightest is its yellow part. Long wavelengths of red color do not affect the rods, but only excite the cones. Rods are more labile than cones.
This theory of dual vision is supported by the results of studying the structure of the retina of diurnal and nocturnal animals. In the retinas of diurnal animals, in which the vision is adapted to high brightness of light, for example, in chickens, pigeons, there are only cones or almost only cones. In the retinas of nocturnal animals whose vision is adapted to low light, such as owls and bats, there are only rods or almost only rods.
Excitation predominates in the retina of nocturnal animals, while inhibition predominates in diurnal ones. In humans, the retina is mixed, since it contains both rods and cones.
retinal excitability
The excitability of the retina to light is extremely high. It depends not only on the functional state of the eye, but also on the functional state of the neurons of the visual analyzer and on other stimuli that simultaneously act on a person. If we simplify reality and take into account only the stimulus acting on the eye, then the smallest stimulus that first causes a visual sensation characterizes the absolute excitability of the eye. It has been established that the human eye is maximally excitable to the rays of the green part of the spectrum. The threshold intensity of the stimulus that causes a visual sensation is measured in thousandths of a lux, acting on the eye from a distance of 1 km with absolute transparency.
Excitability to color stimuli is greater in the center of the retina, where cones predominate, and excitability to light stimuli is greater in the periphery of the retina, where rods predominate.
A visual sensation occurs when the duration of eye irritation is less than 100 microns under the action of 5-15 light quanta.
The excitability of the retina is regulated by efferent gamma fibers emanating from the reticular formation of the midbrain (R. Granit, 1953).
The highest excitability of the eye to 550 nm waves corresponds to the maximum solar radiation. This proves that the phylogeny of the eye is due to the radiation of the sun. It should be noted that the maximum absorption of light by iodopsin is about 575-580 nm. The greatest excitability of the visual analyzer in people from 20 to 25 years. The greatest lability, measured by the largest time threshold, is at 18-30 years of age.
Consistent images
The visual sensation does not appear immediately with the onset of stimulation, but after a certain latent period of stimulation, which is on average 0.1 s.
The visual sensation does not disappear simultaneously with the cessation of stimulation by light, but remains for some time. The sensation that continues after the cessation of the action of the light stimulus on the eye is called in a consistent way. The sequential image continues for the time necessary for the disappearance of the irritating products of the breakdown of light-reactive substances from the retina and their recovery. When a lit cigarette rotates rapidly in the dark, not individual flashes of light are visible, but a fiery circle. Cinema is based on the phenomenon of successive images. The film strip consists of separate frames, but the gaps between them cannot be distinguished by the eye, but continuous movement is observed. There are positive sequential images, which in their lightness and color correspond to the initial irritation, and negative sequential images, which are negative images of the subject. After the removal of the object in question, several very quickly following each other images are observed, which are separated from each other by fractions of a second. These successive images represent a gradual fading of the visual sensation. For some people, successive images are extraordinarily vivid.
Merging of flickering into a sensation of even, uninterrupted light occurs at a certain high frequency of flickering of light. At the same time, frequent light sensations merge into one light sensation thanks to successive images.
The smallest speed of change - individual flashes of light, at which they cause a fused sensation, is called the critical frequency of flicker fusion. This frequency depends on the light intensity and adaptation.
In humans and cats, the critical frequency of flicker fusion is reached at a frequency of light flashes of about 50 per 1 s.
When watching movies, 24 frames per 1 s are skipped, which exceeds the critical flicker fusion frequency for a given screen illumination.
In some people, more often in children, after the disappearance of the object in question, it is very clearly visible with all the details and only gradually disappears from the field of view. This case of unusually clear and long-lasting visual memory is called eidetism. In children, eidetism is associated with a change in the function of the thyroid or parathyroid glands.
Adaptation
The excitability of the visual analyzer depends on the amount of light-reactive substances in the retina. Under the action of light on the eye, due to the decay of light-reactive substances, the excitability of the eye decreases. This phenomenon is referred to as the adaptation of the eye to light, or light adaptation. For example, when leaving a dark room into bright sunlight, at first we do not distinguish anything, but soon we adapt to the light and see everything perfectly. The drop in the excitability of the eye in the light is greater, the brighter the light. Excitability decreases especially quickly in the first 3-5 minutes. In the first minute of exposure to light, it drops to 90-98%.
On the contrary, in connection with the restoration of light-reactive substances, the excitability of the eye to light in the dark increases, which is referred to as dark adaptation, or tempo adaptation. For example, after staying on the ground, at first we see nothing in a poorly lit room, but gradually we begin to clearly distinguish the objects in it.
The excitability of cones can increase in the dark by 20-60 times, and that of rods by 200-400,000 times. In the first 10 minutes of being in darkness, the excitability of the eye to light increases very rapidly, and then gradually and continuously during the entire time of being in darkness.
The dark adaptation of the cones is many thousand times smaller than the dark adaptation of the rods, but it is faster. Adaptation of cones in the dark ends after 4-6 minutes, and rods - after 45 minutes or more. Vision is carried out against the background of spontaneous impulse activity of retinal ganglion neurons, which increases in the dark.
Dark adaptation decreases under the influence of food starvation, lack of vitamin A, lack of oxygen in the air, with fatigue, etc. It increases to 1.5 hours with simultaneous sound stimulation, cold rubbing, increased short-term ventilation of the lungs, etc.
In addition to light adaptation, there is also color adaptation, or a drop in the excitability of the eye under the action of rays that cause color sensations. The more intense the color, the faster the excitability of the eye falls: after a few seconds it decreases by 50% or more. Excitability falls most rapidly and especially sharply under the action of a blue-violet stimulus, most slowly and least of all - under the action of a green stimulus. The red stimulus occupies a middle position. Thus, the green stimulus reduces the least excitability during prolonged action.
Adaptation occurs not only in the receptors, but also in the visual analyzer of the cerebral hemispheres. Adaptation of vision consists in adaptation to changing illumination, accommodation, convergence, changes in the pupillary reflex, retino-motor phenomena, and rearrangement of cone receptive fields.
When a still image is projected into the human retina, it soon ceases to be distinguished. As a result of adaptation, a person could not see motionless objects, but when trying to fix the gaze on a motionless object, oscillatory eye movements are made. Adaptation is hindered by three types of eye movements that move the image from one group of receptors to another. one) Saccadic voluntary and involuntary jumps of the eyeball begin 0.2-0.3 s after the appearance of a visual stimulus. They are the same in both eyes and are produced at the same time. Arbitrary jumps are regulated by the frontal lobes, and involuntary jumps are regulated by the lower parietal and occipital regions. Each movement lasts 10-20 ms, the intervals between involuntary - from 100 ms to several seconds. 2) Tremor- small fluctuations of the eyes from 30 to 200 in 1 s. 3) Drifting- slow eye movements, each lasting 300 ms. All types of movement are the result of joint reflex activity of retinal receptors and gamma motor neurons of the eye muscles. During each movement, the adaptation of the corresponding receptive field stops, the effect of switching on visual stimulation resumes, and therefore a person can see a stationary object. Frogs do not have such eye movements, so they can only see objects that move in their field of vision.
Modern ideas about the transmission of visual information
Modern studies have shown that in the process of evolution the number of elements that transmit information from receptors increases, and the number of parallel afferent neuron circuits increases. This can be seen in the example of the auditory analyzer and is even more pronounced in the visual analyzer.
The optic nerve contains 800 thousand - 1 million nerve fibers. Each fiber is divided in the diencephalon into 5-6 fibers, each of which ends in synapses on individual cells of the lateral geniculate body. Each single fiber traveling from the geniculate body to the cerebral hemispheres can contact approximately 5,000 neurons of the visual analyzer, and each of the neurons of the visual analyzer receives impulses from 4,000 other neurons. Consequently, the visual pathways expand even more towards the cerebral hemispheres than the auditory ones.
Retinal receptors transmit signals only once, at the moment a new object appears, and then only signals about its changes or disappearance are added. An unchanging image of an object, due to adaptation, ceases to excite the receptors of the retina, so static images are not transmitted. There are retinal receptors that transmit only images of objects, and other receptors that respond only to the appearance or disappearance of a light signal or to its movement.
During wakefulness, afferent impulses are always carried out from the retinal receptors along the optic nerves, which excite or inhibit the eyes under different lighting conditions. There are three types of nerve fibers in the optic nerves. The fiber of the first type gives a discharge of potentials when the light is turned on and does not react to turning it off. In the fiber of the second type, illumination of the eyes causes inhibition of the background of afferent impulses and produces a discharge of potentials when illumination ceases. If the lighting is repeated during the turning off of the light, then the discharge of impulses caused by the turning off of the previous lighting is inhibited in this fiber. Most of the fibers of the optic nerves belong to the third type, which reacts with an increase in afferent impulses both when the eyes are illuminated and when the lighting is turned off (R. Granit, 1956).
Electrophysiological studies and their mathematical analysis made it possible to establish that on the way from the retina to the visual analyzer, the visual transmission is enlarged.
Elements of visual perception - lines. First of all, the visual system highlights the contours of objects. Mechanisms for highlighting the contour and configurations are innate. Thanks to induction, the contours of objects are clearly emphasized.
In the retina there is a spatial and temporal summation of visual stimuli in receptive fields, the number of which in good daylight reaches 800 thousand, which corresponds to the number of fibers in the human optic nerve.
The reticular formation regulates the metabolism of retinal receptors. Its stimulation with electric current through needle electrodes changes the frequency of afferent impulses that occur in the receptors of the retina during a flash of light. The action of the reticular formation is carried out through thin efferent gamma fibers coming from it into the retina, as well as into other receptors, for example, into proprioceptors. As a rule, some time after the onset of stimulation of the retina, the afferent impulse increases sharply, and this effect persists for quite a long time after the cessation of stimulation. Consequently, the excitability of the retina is increased by adrenergic, sympathetic neurons of the reticular formation, which are distinguished by a large latent period and aftereffect.
There are two types of receptive fields in the retina: 1) encoding the simplest configurations of the visual image for individual elements and 2) encoding these configurations as a whole, i.e., enlarging visual images. Therefore, statistical coding begins already in the retina. At the exit from the retina, EPSP and IPSP are recorded in ganglionic neurons, a series of single impulses occur, which arrive through the fibers of the optic nerve to the external geniculate bodies, where the visual image is optimally encoded in large blocks, individual image configurations, direction and speed of its movement are transmitted.
During life, a system of those visual images that are reinforced, i.e., have biological significance, is imprinted by a conditioned reflex (V. D. Glezer and I. I. Tsukkerman, 1961). Therefore, retinal receptors transmit individual visual signals. It is not yet known how they are decoded.
About 30 thousand nerve fibers come out of the central fovea of the human retina, which makes it possible to transmit approximately 900 thousand bits in 0.1 s, and no more than 4 bits are processed in the visual region of the cerebral hemispheres in 0.1 s. Consequently, visual information is not limited to the retina and transmission in nerve fibers, but to decoding in the nerve center.
Perception of space
The eyes are driven by six muscles - four straight and two oblique. Attached to the eyeball are the external, internal, superior and inferior rectus muscles, superior oblique (trochlear) and inferior oblique muscles. The oculomotor nerve (3rd pair) innervates the internal, superior and inferior rectus and inferior oblique muscles. The trochlear nerve (4th pair) innervates the superior oblique muscle. The abducens nerve (6th pair) innervates the external rectus muscle.
The center of rotation of the eye is 1.3 mm behind the center of the eye. From a position where the eye is looking straight ahead, it can turn outward by 42°, inward by 45°, upward by 54° and downward by 57°. Eye movements are made in a friendly manner. The visual axes of the eyes always cross on the object. This occurs as a result of contraction of both internal rectus muscles and is called convergence. Since the main nerve that moves the eye is the oculomotor nerve, which simultaneously strains accommodation and constricts the pupil, when viewing close objects, all three processes - convergence, accommodation and pupil constriction - occur almost simultaneously. Convergence begins 0.16-0.2 s after the appearance of the object, and pupillary constriction begins 0.25-0.5 s after the onset of convergence.
The divergence of the visual axes is called divergence. The perception of space is not an innate ability. It is primarily due to afferent impulses coming from the eyes to the cerebral hemispheres (from the proprioceptors of the ciliary, or accommodative, muscle and oculomotor muscles involved in convergence). It is thanks to these impulses that we learn during our life to determine the distance of objects from the eyes, checking the correctness of this determination with the help of other analyzers. Thus, the perception of distance and depth is based on the formation of conditioned reflexes. In the definition of space, the size of the image of an object on the retina matters in the case when we know the size of the object. An essential role in the perception of distance and depth also belongs to the shadows that are visible on objects.
The perception of the size of objects is determined by the size of their image on the retina and the distance from the eye.
The perception of the movement of an object in the case of immobility of the eye depends on the movement of its image on the retina. The perception of moving objects with the simultaneous movement of the eyes and head and the determination of the speed of movement of objects are due not only to impulses that enter the visual analyzer when various parts of the retina are excited, but also to afferent impulses that flow into the kinesthetic analyzer of the cerebral hemispheres from the receptors of the skin and eye and neck muscles. Temporary connections of the visual and kinesthetic analyzers are formed in the cerebral hemispheres.
After the removal of cataracts in both eyes at 12-18 years of age, vision training required a combination of retinal stimulation with kinesthetic sensations for several months. A long-term increase or decrease in vision causes significant changes in the structure of neurons in the visual area: the growth of dendrites, the number of spines, and the structure of synapses.
The correspondence of sensation in the visual and kinesthetic analyzers to reality is verified by life experience.
First of all, it must be pointed out that the reason for the error may be the presence of lumps or threads of conjunctival secretion, air bubbles, and other formations on the surface of the cornea, which, against the background of a red pupil, look like dark spots or stripes of different sizes and shapes and can be taken for turbidity of the environment. These formations are easy to remove by sweeping the eyelid over the surface of the cornea with a finger or by inviting the patient to close and open the eyes several times.Opacities of media appear more or less dark in transmitted light, depending on their ability to reflect light. Formations with a highly reflective surface may appear not only bright, but also shiny.
It must also be borne in mind that when examined in transmitted light, some areas of transparent media may appear more or less dark, as if clouded, but in reality there is no clouding in this place. The reason for this phenomenon may be the circumstance that in the indicated places the rays emanating from the bottom of the eye, due to reflection or refraction, are so deviated to the side that they either do not enter the eye of the observer at all, or only an insignificant part of them reaches it.
A distinctive feature of such dark areas is often that. that when the direction of the gaze changes, as well as when the eye is illuminated with an ophthalmoscope from different positions, an unusual play of shadows is noted in the area of apparent opacities. For the final elimination of turbidity, it is necessary to resort to side lighting, in which in such cases gray inclusions will not be visible against a dark background.
Opacities in the media of the eye can be mobile and immobile. Mobile is such a clouding that continues to move in the eye after the eye, having made a small movement, again assumes a calm position. Mobile opacities can only be found in liquid media - in the moisture of the anterior chamber or in the liquefied vitreous body. Turbidities in the moisture of the anterior chamber are easily recognized, as they are already detected during examination using side illumination.
The location of many opacities in the media of the anterior segment of the eye (cornea, moisture of the anterior chamber, lens), as is known, can be established with side illumination. The study in transmitted light also makes it possible to precisely localize opacities based on parallax phenomena, that is, by observing the change in the position of opacities relative to the pupil or the light reflex of the cornea with different turns of the eye.
Localization of opacities relative to the pupil.
Imagine that in the media of the eye along the line of the visual axis there is a series of opacities:
a - clouding of the cornea,
c - on the anterior capsule of the lens,
c - on the posterior lens capsule,
d - in the vitreous body.
If such an eye looks directly into the mirror of the ophthalmoscope, then all these opacities, located along the visual line one after the other, will merge into one point located in the center of the pupil (Fig. 30 - top).
Opacification in the pas of the anterior surface of the lens at all turns of the eye will retain its neutral position relative to the pupil, since it is in the same plane with it (Fig. 30 - below).
Opacity a, lying on the cornea, will move in the direction of eye movement when turning: when the eye is turned upward, it will approach the upper edge of the pupil and vice versa.
Opacities with and d, located behind the plane of the pupil, p. the substance of the lens or in the vitreous body move in the direction opposite to the movement of the eye: when you speak the eyes up, they approach the lower edge of the pupil, when you turn down, they will be located eccentrically up. Turbidity makes the greater excursion, the farther it is located from the plane of the pupil.
Localization of opacities relative to the light reflex of the cornea. Here, in fact, it is a matter of localization of opacities relative to the center of rotation of the eye, which is located slightly behind the posterior pole of the lens (about 1.5 mm behind the palms of the lens capsule).
It is obvious that when the eyeball is rotated, the opacity located in the center of the eye rotation will not change its position.
Opacities located anterior to the center of rotation of the eye will move in the direction of movement of the anterior segment of the eye, and opacities localized behind the center of rotation will shift in the opposite direction. This is clearly seen in Fig. 31 - at the top, where a row of opacities is located along the line of the optical axis: a opacification on the cornea, c - on the anterior lens capsule, c - behind the lens, in the center of rotation: eye, d - in the vitreous body, behind the center of rotation of the eye. When the subject is looking straight ahead, all opacities will be merged into one point.
When the eye is turned upward, the cloudiness c, located in the center of rotation of the eye, will not change its location, the cloudiness a and b will move upward, and the cloudiness will move downward (Fig. 31 - below).
But, since the point of rotation of the eye is not indicated by anything, it naturally cannot serve as a guideline in the study; instead of it, they are guided by the position of the light reflex, the cornea. This reflex occurs when the eye is illuminated with an ophthalmoscope and looks like a luminous dot on the surface of the cornea.
According to the laws of optics, the reflex reflected by the surface of a convex mirror always lies on the straight line connecting the light source and the center of curvature of the mirror. Therefore, at. In any position of the eye, the light reflex of the cornea will always be on the line connecting the center of the cornea curvature and the center of the ophthalmoscope mirror, i.e., the reflex will cover the center of the cornea curvature, which almost coincides with the center of rotation of the eye. Hence it is obvious that the light reflex of the cornea at any position of the eyeball indicates the location of the center of rotation of the eye. That is why, when localizing opacities relative to the center of rotation of the eye, they monitor the movement of opacities when turning the eye to the light reflex of the cornea.
The localization of cloudiness relative to the corneal reflex makes it possible to draw the following practical conclusions. If the opacification is located in the anterior part of the vitreous body or in the lens, near the posterior capsule, it almost does not move when turning the eye in relation to the corneal reflex. If the clouding is located in the anterior sections of the lens or in the cornea, it will noticeably mix, and the movement occurs in the direction of eye movement; when the opacity moves in the direction opposite to the movement of the eye, it is located in the vitreous body, the farther from the posterior capsule of the lens, the faster its movement.
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Book article.
:
vitreous body
lens
:
· anterior surface of the capsule
· posterior surface of the capsule
· equator -
· lens substance :
· eyelash band,
Front camera
pectinate ligament iris-corneal angle .
rear camera
Cornea also applies to refractive limbo.
:
· anterior epithelium
· thickness 6-9 microns;
·
· posterior border plate :
· posterior epithelium
Cornea has no blood vessels
Vascular membrane of the eye, its parts. accommodation mechanism.
The choroid represents middle layer eyeball. It consists of the choroid plexus, loose fiber rich in elastic fibers, pigment cells and smooth muscles.
The shell has three parts.
First part - the choroid proper nourishes the retina, lining the inside and behind most of the sclera, loosely connecting with it due to the perivascular space, penetrated by loose connective tissue.
In its microstructure, layers or plates are distinguished :
· supravascular plate from loose, fibrous connective tissue rich in elastic fibers, fibroblasts and melanocytes;
· vascular plate in the form of a plexus of arteries and veins in loose fiber containing many melanocytes and few smooth muscle fibers;
· vascular-capillary record and basal complex fibrous layer and basement membrane.
Second part: ciliary body- the middle section of the choroid - is located in the form of a circular roller, corresponding to the place of transition of the cornea to the sclera posteriorly from the iris, with which it fuses with the outer ciliary edge.
In the ciliary body there are :
· behind - eyelash circle- 4 mm wide - in the form of a circular strip;
in front - ciliary processes- folds thickened up to 3 mm, oriented radially and constituting the ciliary crown; ciliary processes consist of vessels and sympathetic nerves;
· ciliary muscle- from meridional (longitudinal), circular and radial smooth muscle fibers attached to the ciliary belt of the lens.
The third part – iris- the most anterior part of the disk-shaped form. The iris consists
1) from the connective tissue stroma with vessels and pigment epithelium containing melanin;
2) two smooth muscles - the sphincter and pupil dilator.
In the center, the iris has a pupil bounded by the pupillary edge of the choroid, and the opposite edge is called the ciliary. It is connected to the ciliary body by the pectinate ligament.
The choroid contains the ciliary arteries : rear and front; short and long. They form two arterial circle : big along the ciliary edge of the iris, small circle- along the pupillary edge. From the venous network of the choroid are formed vorticose veins(4-6), passing through the sclera and flowing into the ophthalmic veins. The anterior ciliary veins collect blood from the ciliary body, iris and sclera, the posterior - from their own part of the choroid.
The accommodative structures of the eye (muscles, ligaments) and its refractive media (the lens) provide image focusing on the retina, adaptation to light intensity, which allows a person to see equally well both near and far. During accommodation, the curvature of the lens changes, and with it, its refractive power. When viewing closely spaced objects, the lens becomes convex, distant objects - flat.
The mechanism of accommodation is due to the contraction of the ciliary muscles connected to the lens capsule by annular (zinn) ligaments.
With the contraction of circular muscle fibers, the ciliary processes approach the center of the ciliary circle, weakening the tension of the annular ligaments of the ciliary belt of the lens. The internal elasticity of the lens substance is released, and it increases its curvature, which leads to a decrease in the focal length when looking at close-lying objects. Simultaneously with the circular fibers, the meridional fibers contract, which tighten the back of the choroid and the ciliary body as much as the focal length decreases.
When the ciliary muscle is relaxed, the ligaments are stretched, and with them the lens capsule, which flattens it. When shifting the gaze, the auxiliary muscles of the eye work, and the illumination is regulated by the pupil due to its sphincter and dilator.
Age variability
Intrauterine period:
early laying at the beginning of the 3rd week at the head end of the embryo in the form of a thickening of the ectoderm;
rapid development: on the 4th week, an auditory fossa is formed in the ectoderm of the future head, which quickly turns into an auditory vesicle, which already on the 6th week is immersed in the primary brain bladder;
complex differentiation, due to which semicircular canals, utriculus, sacculus with receptor zones arise from the auditory vesicle: scallops, spots and sensory epithelial cells developing in them;
· the membranous labyrinth on the 3rd month is basically formed;
the spiral organ only begins to form from the 3rd month: a cover membrane develops from the thickening of the cochlear duct, under which epithelial sensory cells appear, by the 6th month the structure of the spiral organ becomes more complicated and connection occurs VIII pairs of cranial nerves with receptor zones.
In parallel with the sound-perceiving spiral organ, a sound-conducting organ is formed : outer and middle ear. The tympanic cavity, the auditory tube develop from the 1st visceral pocket, and the auditory ossicles from the first and second visceral arches. The auricle is formed from mesenchyme.
newborn period
The inner ear is well developed and approximates in size to an adult.
The tympanic cavity has thin walls. In the lower wall there are areas of connective tissue. The mucosa is thickened, mastoid cells are absent.
The auditory tube is straight, wide, short (17-21 mm). Its cartilaginous part is poorly developed.
The auditory ossicles are about the size of adults.
The auricle is flat with soft cartilage and thin skin.
The external auditory canal is narrow, long, with a sharp bend, its walls are cartilaginous, with the exception of the tympanic ring.
The auricle grows most rapidly up to 2 years, and then after 10 years, and in length faster than in width. The auditory tube grows slowly in the 1st year, faster in the 2nd.
Refractive media of the eyeball.
The internal nucleus of the eye consists of transparent light-refracting media : vitreous body, lens, aqueous humor of the chambers of the eye.
vitreous body is in the vitreous chamber. Its volume in an adult is 4 ml. In composition, it is a gel-like medium with the presence of special proteins in the skeleton: vitrosin and mucin, with which hyaluronic acid is associated, which ensures the viscosity and elasticity of the body. The primary vitreous body develops from the mesoderm, the secondary - from the mesoderm and ectoderm. The formed vitreous body is a permanent environment of the eye, which is not restored when lost. It is covered along the perimeter with a boundary membrane, which is firmly connected with the ciliary epithelium (the base is the base in the form of a ring protruding anteriorly from the jagged edge) and with the back of the lens capsule (hyaloid-lens ligament).
lens located between the iris and the vitreous body, in a recess (vitreous fossa) and is held by the fibers of the ciliary girdle.
Differences in the lens :
· anterior surface of the capsule(epithelium and fibers) with the most protruding point - the pole;
· posterior surface of the capsule(epithelium and fibers) with a more convex posterior pole;
· equator - the transition of the front surface to the rear surface;
· lens substance from lens fibers and their gluing formation; lens nucleus - lens fibers without nuclei : sclerosed, compacted;
· eyelash band, the fibers of which start from the anterior and posterior surfaces of the capsule in the equatorial region.
The axis of the lens is the distance between the poles, the refractive power of the lens is 18 diopters (dopters).
Front camera located between the cornea and the iris, between the iris and the anterior surface of the lens capsule - the posterior chamber. Both are filled with moisture capable of light refraction.
The front camera is limited around the perimeter pectinate ligament, between the fiber bundles of which there are spaces lined with flat cells iris-corneal angle(fountain spaces) - the path of outflow of moisture into the venous sinus of the sclera. Angle damage underlies the development of angular glaucoma.
rear camera moisture exchange is carried out due to slit-like spaces between the fibers of the ciliary girdle, which, in the form of a common circular slit (petite canal), cover the lens along the periphery.
Cornea also applies to refractive environment, although it is located in the outer shell of the eye, making up its anterior part and participating with its bulge in the formation of the anterior pole of the eyeball. It is transparent, has a round shape with a diameter in an adult of 12 mm, a thickness of 1 mm. In the sagittal plane, it is smoothly curved. On the outer surface the cornea is convex, and on the inner surface it is concave. The radius of curvature is up to 7.5-8 mm, which provides light refraction up to 40 diopters. The cornea grows into the circular groove of the sclera, forming a slight thickening with its peripheral edge - limbo.
There are five layers in the cornea :
· anterior epithelium up to 50 microns thick with numerous free nerve endings; differs in high regeneration and permeability for medicines;
· anterior border plate thickness 6-9 microns;
· own substance from fibrous plates, including bundles of collagen fibers, squamous fibroblasts and an amorphous medium of keratin sulfates, glycosaminoglycans and water;
· posterior border plate thickness 5-10 microns; both records : anterior and posterior consist of collagen fibers and amorphous substance;
· posterior epithelium from flat polygonal cells of various shapes.
Cornea has no blood vessels, nutrition receives diffuse due to the fluid of the anterior chamber and the vessels of the circular sulcus of the sclera.
The refractive media of the eyeball make up the transparent core of the eye. This includes the vitreous body, lens, and aqueous humor in the anterior and posterior chambers. The first two formations are filled vitreous chamber of the eyeball, camera vitrea bulbi.
vitreous body, corpus vitreum (see fig. , ), the outside is covered with a thin transparent vitreous membrane, membrana vitrea, and occupies most of the cavity of the eyeball. It consists of a completely transparent gelatinous mass, devoid of blood vessels and nerves, - vitreous stroma, stroma vitreum. It consists of a delicate network of intertwining thin fibers and a protein-rich liquid - vitreous moisture, humor vitreus. The anterior surface of the vitreous body faces the posterior surface of the lens and bears, according to its shape, a cup-shaped vitreous fossa, fossa hyaloidea. Suitable for her vitreous canal, canalis hyaloidus, which is the remainder of the vascular-embryonic tissue. In the channel sometimes lies vitreous body artery, a. hyaloidea.
The rest of the vitreous body is adjacent to the inner surface of the retina and its shape is close to spherical.
lens, lens, has the shape of a biconvex lens (see Fig. , , ). Posterior surface of the lens, facies posterior lentis, more convex, adjacent to the vitreous body, and anterior surface, facies anterior lentis facing the iris.
Distinguish anterior and posterior poles of the lens, polus anterior et posterior lentis, are the most convex central points of its anterior and posterior surfaces.
The line connecting the anterior and posterior poles of the lens is called lens axis, axis lentis, and is equal to an average of 3.6 mm.
The substance of the lens, substantia lentis, is completely transparent and, like the vitreous body, does not contain vessels and nerves.
The bulk of the lens consists of lens fibers, fibrae lentis, which are elongated hexagonal epithelial cells.
The peripheral sections of the lens are covered from the side of its anterior and posterior surfaces lens capsule, capsula lentis. The latter is a homogeneous transparent shell, thicker on the anterior surface of the lens, where it is located under it lens epithelium, epithelium lentis.
The substance of the lens has an unequal density: in the center it is denser and is called nucleus of the lens, nucleus lentis, and on the periphery less dense is lens cortex, cortex lentis.
The lens, located between the vitreous body and the iris, is fixed by its peripheral rounded edge, called lens equator, equator lentis, to the ciliary body through stretched thin girdle fibers, fibrae zonulares. They are intertwined with the inner end into the lens capsule, and the outer ends start from the ciliary body. The combination of these fibers forms a ligament around the lens - cilia ridge, zonula ciliaris(see fig. , ). Between the fibers of the ciliary ligament are lymphatic girdle spaces, zonula zonularia.
Aqueous moisture, humor aquosus, - a clear, colorless liquid that fills the anterior and posterior chambers of the eyeball - slit-like cavities located in front and behind the iris.
Posterior chamber of the eyeball camera posterior bulbi(see fig. , , ), limited behind the anterior surface of the lens, ciliary girdle and ciliary body; in front - the back surface of the iris. The ciliary processes hang freely into the cavity of the posterior chamber. The rear chamber communicates with girdle spaces, spatia zonularia.
Anterior chamber of the eyeball camera anterior bulbi, formed in front by the posterior concave surface of the cornea, behind - by the anterior surface of the iris.
The anterior and posterior chambers of the eyeball communicate with each other through the pupil.
Aqueous moisture is produced by the vessels of the ciliary body and iris. The outflow of aqueous humor is carried out in the following ways: from the posterior chamber, aqueous humor enters the anterior chamber, from where it flows through the spaces of the iridocorneal angle into the system of tortuous vorticose veins. In addition, from these chambers, moisture can flow into the venous sinus of the sclera, from where, as part of the venous blood, it enters the ciliary and conjunctival veins.
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