home Diseases Electrical phenomena in the retina and optic nerve. Structure and function of the retina

Electrical phenomena in the retina and optic nerve. Structure and function of the retina

Adjacent to the pigment layer from the inside is a layer of photoreceptors: rods and cones. In the retina of each human eye there are 6-7 million cones and 110-123 million rods. They are distributed unevenly in the retina. The central fovea of the retina (fovea centralis) contains only cones (up to 140 thousand per 1 mm2). Towards the periphery of the retina, their number decreases, and the number of rods increases, so that at the far periphery there are only rods. Cones function in high light conditions; they provide daylight. and color vision; the much more light-sensitive rods are responsible for twilight vision.

Color is perceived best when light is applied to the fovea of the retina, where cones are located almost exclusively. This is also where visual acuity is greatest. Color perception and spatial resolution become progressively worse as you move away from the center of the retina. The periphery of the retina, where only the rods are located, does not perceive color. But the light sensitivity of the cone apparatus of the retina is many times less than that of the rod apparatus, therefore at dusk, due to a sharp decrease in “cone” vision and the predominance of “peripheral” vision, we do not distinguish color (“all cats are gray at night”).

Impaired rod function, which occurs when there is a lack of vitamin A in food, causes a twilight vision disorder - the so-called night blindness: a person becomes completely blind at dusk, but during the day vision remains normal. On the contrary, when the cones are damaged, photophobia occurs: a person sees in dim light, but goes blind in bright light. In this case, complete color blindness may develop - achromasia.

The structure of a photoreceptor cell. A photoreceptor cell - rod or cone - consists of a light-sensitive outer segment containing visual pigment, an inner segment, a connecting stalk, a nuclear part with a large nucleus and a presynaptic ending. The rod and cone of the retina face their light-sensitive outer segments towards the pigment epithelium, i.e. in the direction opposite to light. In humans, the outer segment of the photoreceptor (rod or cone) contains about a thousand photoreceptor disks. The outer segment of the rod is much longer than the cone and contains more visual pigment. This partly explains the higher sensitivity of the rod to light: a rod can be excited by just one quantum of light, but more than a hundred quanta are required to activate a cone.

The photoreceptor disk is formed by two membranes connected at the edges. The disc membrane is a typical biological membrane, formed by a double layer of phospholipid molecules, between which there are protein molecules. The disc membrane is rich in polyunsaturated fatty acids, which causes its low viscosity. As a result, the protein molecules in it rotate quickly and move slowly along the disk. This allows proteins to collide frequently and, when interacting, form functionally important complexes for a short time.

The inner segment of the photoreceptor is connected to the outer segment by a modified cilium, which contains nine pairs of microtubules. The inner segment contains a large nucleus and the entire metabolic apparatus of the cell, including mitochondria, which provide the energy needs of the photoreceptor, and a protein synthesis system, which ensures the renewal of the membranes of the outer segment. Here the synthesis and incorporation of visual pigment molecules into the photoreceptor membrane of the disc occurs. In an hour, on average, three new discs are re-formed at the border of the inner and outer segments. Then they slowly move from the base of the outer segment of the rod to its apex. Eventually, the apex of the outer segment, containing up to a hundred now old disks, breaks off and is phagocytosed by the cells of the pigment layer. This is one of the most important mechanisms for protecting photoreceptor cells from molecular defects that accumulate during their light life.

The outer segments of the cones are also constantly renewed, but at a slower rate. It is interesting that there is a daily rhythm of renewal: the tips of the outer segments of the rods mainly break off and are phagocytosed in the morning and daytime, and the tips of the cones in the evening and night.

The presynaptic terminal of the receptor contains a synaptic ribbon, around which there are many synaptic vesicles containing glutamate.

Visual pigments. The rods of the human retina contain the pigment rhodopsin, or visual purple, the maximum absorption spectrum of which is in the region of 500 nanometers (nm). The outer segments of the three types of cones (blue-, green- and red-sensitive) contain three types of visual pigments, the maximum absorption spectra of which are in the blue (420 nm), green (531 nm) and red (558 nm) parts of the spectrum. The red cone pigment is called iodopsin. The visual pigment molecule is relatively small (with a molecular weight of about 40 kilodaltons), consists of a larger protein part (opsin) and a smaller chromophore (retinal, or vitamin A aldehyde).

Retinal can be found in various spatial configurations, i.e., isomeric forms, but only one of them, the 11-cis isomer of retinal, acts as the chromophore group of all known visual pigments. The source of retinal in the body is carotenoids, so their deficiency leads to vitamin A deficiency and, as a consequence, to insufficient rhodopsin resynthesis, which in turn causes impaired twilight vision, or “night blindness.” Molecular physiology of photoreception. Let us consider the sequence of changes in molecules in the outer segment of the rod that are responsible for its excitation. When a quantum of light is absorbed by a molecule of visual pigment (rhodopsin), instant isomerization of its chromophore group occurs in it: 11-cis-retinal is straightened and converted into completely trans-retinal. This reaction lasts about 1 ps. Light acts as a trigger, or trigger, factor that initiates the photoreception mechanism. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes discolored and passes into the state of metarhodopsin II.

As a result of this, the visual pigment molecule acquires the ability to interact with another protein - the near-membrane guanosine triphosphate-binding protein transducin (T). In complex with metarhodopsin II, transducin enters an active state and exchanges guanosine diphosphate (GDP) bound to it in the dark for guanosine triphosphate (GTP). Metharhodopsin II is capable of activating about 500-1000 transducin molecules, which leads to an increase in the light signal.

Each activated transducin molecule associated with a GTP molecule activates one molecule of another near-membrane protein - the phosphodiesterase enzyme (PDE). Activated PDE destroys cyclic guanosine monophosphate (cGMP) molecules at a high speed. Each activated PDE molecule destroys several thousand cGMP molecules - this is another step in signal amplification in the photoreception mechanism. The result of all the described events caused by the absorption of a light quantum is a drop in the concentration of free cGMP in the cytoplasm of the outer segment of the receptor. This in turn leads to the closure of ion channels in the plasma membrane of the outer segment, which were open in the dark and through which Na+ and Ca2+ entered the cell. The ion channel closes due to the fact that, due to a drop in the concentration of free cGMP in the cell, cGMP molecules that were bound to it in the dark and kept it open leave the channel.

A decrease or cessation of entry into the outer segment of Na+ leads to hyperpolarization of the cell membrane, i.e., the appearance of a receptor potential on it. Concentration gradients of Na+ and K+ are maintained on the plasma membrane of the rod by the active work of the sodium-potassium pump, localized in the membrane of the inner segment.

The hyperpolarizing receptor potential that arises on the membrane of the outer segment then spreads along the cell to its presynaptic end and leads to a decrease in the rate of release of the transmitter (glutamate). Thus, the photoreceptor process ends with a decrease in the rate of neurotransmitter release from the presynaptic ending of the photoreceptor.

The mechanism for restoring the original dark state of the photoreceptor, i.e., its ability to respond to the next light stimulus, is no less complex and perfect. To do this, it is necessary to reopen the ion channels in the plasma membrane. The open state of the channel is ensured by its connection with cGMP molecules, which in turn is directly caused by an increase in the concentration of free cGMP in the cytoplasm. This increase in concentration is ensured by the loss of metarhodopsin II's ability to interact with transducin and the activation of the enzyme guanylate cyclase (GC), capable of synthesizing cGMP from GTP. Activation of this enzyme causes a drop in the concentration of free calcium in the cytoplasm due to the closure of the membrane ion channel and the constant operation of the exchanger protein, which releases calcium from the cell. As a result of all this, the concentration of cGMP inside the cell increases and cGMP again binds to the ion channel of the plasma membrane, opening it. Through the open channel, Na+ and Ca2+ begin to enter the cell again, depolarizing the receptor membrane and transferring it to a “dark” state. The release of the transmitter from the presynaptic ending of the depolarized receptor is again accelerated.

Retinal neurons. Retinal photoreceptors synapse with bipolar neurons. When exposed to light, the release of the mediator (glutamate) from the photoreceptor decreases, which leads to hyperpolarization of the bipolar neuron membrane. From it, the nerve signal is transmitted to ganglion cells, the axons of which are fibers of the optic nerve. Signal transmission both from the photoreceptor to the bipolar neuron and from it to the ganglion cell occurs in a pulseless manner. A bipolar neuron does not generate impulses due to the extremely short distance over which it transmits a signal.

For 130 million photoreceptor cells, there are only 1 million 250 thousand ganglion cells, the axons of which form the optic nerve. This means that impulses from many photoreceptors converge (converge) through bipolar neurons to one ganglion cell. Photoreceptors connected to one ganglion cell form the receptive field of the ganglion cell. The receptive fields of different ganglion cells partially overlap each other. Thus, each ganglion cell summarizes the excitation arising in a large number of photoreceptors. This increases light sensitivity but degrades spatial resolution. Only in the center of the retina, in the area of the fovea, is each cone connected to one so-called dwarf bipolar cell, to which only one ganglion cell is also connected. This provides high spatial resolution here, but sharply reduces light sensitivity.

The interaction of neighboring retinal neurons is ensured by horizontal and amacrine cells, through the processes of which signals propagate that change synaptic transmission between photoreceptors and bipolar cells (horizontal cells) and between bipolar and ganglion cells (amacrine cells). Amacrine cells exert lateral inhibition between adjacent ganglion cells.

In addition to afferent fibers, the optic nerve also contains centrifugal, or efferent, nerve fibers that bring signals from the brain to the retina. It is believed that these impulses act on the synapses between the bipolar and ganglion cells of the retina, regulating the conduction of excitation between them.

Neural pathways and connections in the visual system. From the retina, visual information travels through the fibers of the optic nerve (II pair of cranial nerves) to the brain. The optic nerves from each eye meet at the base of the brain, where they form a partial decussation (chiasma). Here, part of the fibers of each optic nerve passes to the side opposite to its eye. Partial decussation of fibers provides each cerebral hemisphere with information from both eyes. These projections are organized in such a way that the occipital lobe of the right hemisphere receives signals from the right halves of each retina, and the left hemisphere receives signals from the left halves of the retinas.

After the optic chiasm, the optic nerves are called optic tracts. They are projected into a number of brain structures, but the main number of fibers comes to the thalamic subcortical visual center - the lateral, or external, geniculate body (NKT). From here, signals enter the primary projection area of the visual cortex (stiary cortex, or Brodmann area 17). The entire visual cortex includes several fields, each of which provides its own specific functions, but receives signals from the entire retina and generally maintains its topology, or retinotopy (signals from neighboring areas of the retina enter neighboring areas of the cortex).

Electrical activity of the centers of the visual system. Electrical phenomena in the retina and optic nerve. When exposed to light, electrical potentials are generated in the receptors and then in the neurons of the retina, reflecting the parameters of the active stimulus. The total electrical response of the retina to the action of light is called an electroretinogram (ERG). It can be recorded from the whole eye or directly from the retina. To do this, one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye or on the earlobe. On the electroretinogram, several characteristic waves are distinguished. Wave a reflects the excitation of the internal segments of photoreceptors (late receptor potential) and horizontal cells. Wave b occurs as a result of activation of glial (Müller) cells of the retina by potassium ions released during excitation of bipolar and amacrine neurons. Wave c reflects the activation of pigment epithelial cells, and wave d - horizontal cells.

The ERG clearly reflects the intensity, color, size and duration of action of the light stimulus. The amplitude of all ERG waves increases in proportion to the logarithm of the light intensity and the time during which the eye was in the dark. Wave d (response to switching off) is larger the longer the light is on. Since the ERG reflects the activity of almost all retinal cells (except ganglion cells), this indicator is widely used in the clinic of eye diseases for diagnosis and treatment monitoring for various retinal diseases.

Excitation of retinal ganglion cells leads to impulses being sent along their axons (optic nerve fibers) to the brain. The retinal ganglion cell is the first neuron of the “classical” type in the photoreceptor-brain circuit. Three main types of ganglion cells have been described: those that respond to light being turned on (on-response), to light being turned off (off-response), and to both (on-off-response).

The diameter of the receptive fields of ganglion cells in the center of the retina is much smaller than in the periphery. These receptive fields are circular in shape and concentrically constructed: a round excitatory center and a circular inhibitory peripheral zone, or vice versa. As the size of the light spot flashing in the center of the receptive field increases, the response of the ganglion cell increases (spatial summation). Simultaneous excitation of closely located ganglion cells leads to their mutual inhibition: the responses of each cell become smaller than with a single stimulation. This effect is based on lateral, or lateral, inhibition. The receptive fields of neighboring ganglion cells partially overlap, so that the same receptors can be involved in generating the responses of several neurons. Due to their circular shape, the receptive fields of the retinal ganglion cells produce a so-called point-by-point description of the retinal image: it is displayed as a very fine mosaic consisting of excited neurons

Electrical phenomena in the subcortical visual center and visual cortex. The pattern of excitation in the neural layers of the subcortical visual center - the external or lateral geniculate body (NCT), where the fibers of the optic nerve arrive, is in many ways similar to that observed in the retina. The receptive fields of these neurons are also round, but smaller than those in the retina. The neuronal responses generated in response to a flash of light are shorter here than in the retina. At the level of the external geniculate bodies, the interaction of afferent signals coming from the retina occurs with efferent signals from the visual area of the cortex, as well as through the reticular formation from the auditory and other sensory systems. These interactions ensure the selection of the most essential components of the sensory signal and the processes of selective visual attention.

The impulse discharges of the neurons of the lateral geniculate body travel along their axons to the occipital part of the cerebral hemispheres, where the primary projection area of the visual cortex (striate cortex, or field 17) is located. Here, much more specialized and complex information processing occurs than in the retina and the external geniculate bodies. Neurons of the visual cortex have not round, but elongated (horizontally, vertically, or in one of the oblique directions) receptive fields of small size. Thanks to this, they are able to select from a whole image individual fragments of lines with one or another orientation and location (orientation detectors) and selectively respond to them.

In each small area of the visual cortex, neurons with the same orientation and localization of receptive fields in the visual field are concentrated along its depth. They form a column of neurons running vertically through all layers of the cortex. The column is an example of a functional association of cortical neurons that perform a similar function. As the results of recent studies show, the functional unification of distant neurons in the visual cortex can also occur due to the synchrony of their discharges. Many neurons in the visual cortex selectively respond to certain directions of movement (directional detectors) or to a certain color, and some neurons respond best to the relative distance of the object from the eyes. Information about different features of visual objects (shape, color, movement) is processed in parallel in different parts of the visual area of the cerebral cortex.

To assess signal transmission at different levels of the visual system, recording of total evoked potentials (EPs) is often used, which in animals can be simultaneously removed from all parts, and in humans from the visual cortex using electrodes placed on the scalp.

Comparison of the retinal response (ERG) caused by a light flash and the EP of the cerebral cortex allows us to establish the localization of the pathological process in the human visual system.

Visual functions. Light sensitivity. Absolute visual sensitivity. For a visual sensation to occur, it is necessary that the light stimulus have a certain minimum (threshold) energy. The minimum number of light quanta required for the sensation of light to occur under dark adaptation conditions ranges from 8 to 47. It is calculated that one rod can be excited by only 1 light quantum. Thus, the sensitivity of the retinal receptors in the most favorable conditions of light perception is physically limiting. Single rods and cones of the retina differ slightly in light sensitivity, but the number of photoreceptors sending signals to one ganglion cell in the center and periphery of the retina is different. The number of cones in the receptive field in the center of the retina is approximately 100 times less than the number of rods in the receptive field in the periphery of the retina. Accordingly, the sensitivity of the rod system is 100 times higher than the cone system.

14.1.6. Interaction of sensory systems

The interaction of sensory systems occurs at the spinal, reticular, thalamic and cortical levels. The integration of signals in the reticular formation is especially wide. In the cerebral cortex, higher order signals are integrated. As a result of the formation of multiple connections with other sensory and nonspecific systems, many cortical neurons acquire the ability to respond to complex combinations of signals of different modalities. This is especially characteristic of nerve cells in associative areas bark cerebral hemispheres, which have high plasticity, which ensures their restructuring

properties in the process of continuous learning to recognize new stimuli. Intersensory (cross-modal) interaction at the cortical level creates conditions for the formation of a “scheme (or map) of the world” and continuous linking and coordination of the body’s own “body scheme” with it.

14.2. PARTICULAR PHYSIOLOGY OF SENSORY SYSTEMS

14.2.1. Visual system

Vision is evolutionarily adapted to the perception of electromagnetic radiation in a certain, very narrow part of its range (visible light). The visual system provides the brain with more than 90% of sensory information. Vision is a multi-link process that begins with the projection of an image onto the retina of a unique peripheral optical device - the eye. Then the photoreceptors are excited, the transmission and transformation of visual information occurs in the neural layers of the visual system, and visual perception ends with the decision about the visual image being made by the higher cortical parts of this system.

Structure and functions of the optical apparatus of the eye. The eyeball has a spherical shape, which makes it easier to rotate to point at the object in question. On the way to the photosensitive shell of the eye (retina), light rays pass through several transparent media - the cornea, lens and vitreous body. A certain curvature and refractive index of the cornea and, to a lesser extent, the lens determine the refraction of light rays inside the eye (Fig. 14.2).

The refractive power of any optical system is expressed in diopters (D). One diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of a healthy eye is 59D when viewing distant objects and 70.5D when viewing near objects. To schematically represent the projection of the image of an object onto the retina, you need to draw lines from its ends through the nodal point (7 mm behind the cornea

shells). The image on the retina is sharply reduced and turned upside down and from right to left (Fig. 14.3).

Accommodation. Accommodation is the adaptation of the eye to clearly seeing objects at different distances. To see an object clearly, it is necessary that it be focused on the retina, that is, that rays from all points on its surface are projected onto the surface of the retina (Fig. 14.4). When we look at distant objects (A), their image (a) is focused on the retina and they are visible clearly. But the image (b) of nearby objects (B) is blurry, since the rays from them are collected behind the retina. The main role in accommodation is played by the lens, which changes its curvature and, consequently, its refractive power. When viewing close objects, the lens becomes more convex (see Fig. 14.2), due to which rays diverging from any point of the object converge on the retina. The mechanism of accommodation is the contraction of the ciliary muscles, which change the convexity of the lens. The lens is enclosed in a thin transparent capsule, which is always stretched, i.e. flattened, by the fibers of the ciliary band (ligament of Zinn). Contraction of the smooth muscle cells of the ciliary body reduces the traction of the zonules of Zinn, which increases the convexity of the lens due to its elasticity. The ciliary muscles are innervated by parasympathetic fibers of the oculomotor nerve. The introduction of atropine into the eye causes a disruption in the transmission of excitation to this muscle and limits the accommodation of the eye when examining close objects. On the contrary, parasympathomimetic substances - pilocarpine and eserine - cause contraction of this muscle.

For a normal young person's eye, the farthest point of clear vision lies at infinity. He examines distant objects without any strain of accommodation, i.e., without contraction

ciliary muscle. The closest point of clear vision is 10 cm from the eye.

Presbyopia. The lens loses elasticity with age, and when the tension of the zonules of Zinn changes, its curvature changes little. Therefore, the nearest point of clear vision is no longer located at a distance of 10 cm from the eye, but moves away from it. Nearby objects are poorly visible. This condition is called senile farsightedness, or presbyopia. Elderly people are forced to use glasses with biconvex lenses.

Refractive errors eyes. The two main refractive errors of the eye - myopia, or myopia, and farsightedness, or hypermetropia - are caused not by insufficiency of the refractive media of the eye, but by a change in the length of the eyeball (Fig. 14.5, A).

Myopia. If the longitudinal axis of the eye is too long, then rays from a distant object will be focused not on the retina, but in front of it, in the vitreous body (Fig. 14.5, B). Such an eye is called myopic or myopic. To see clearly into the distance, it is necessary to place concave glasses in front of myopic eyes, which will push the focused image onto the retina (Fig. 14.5, B).

Farsightedness. The opposite of myopia is farsightedness, or hypermetropia. In the farsighted eye (Fig. 14.5, D), the longitudinal axis of the eye is shortened, and therefore rays from a distant object are focused not on the retina, but behind it. This lack of refraction can be compensated for by an accommodative effort, i.e., an increase in the convexity of the lens. Therefore, a farsighted person strains the accommodative muscle, examining not only close, but also distant objects. When viewing close objects, the accommodative efforts of far-sighted people

days are insufficient. Therefore, to read, farsighted people must wear glasses with biconvex lenses that enhance the refraction of light (Fig. 14.5, E). Hypermetropia should not be confused with senile farsightedness. The only thing they have in common is that it is necessary to use glasses with biconvex lenses.

Astigmatism. Refractive errors also include astigmatism, i.e., unequal refraction of rays in different directions (for example, along the horizontal and vertical meridian). Astigmatism is not due to the strictly spherical surface of the cornea. With severe astigmatism, this surface can approach cylindrical, which is corrected by cylindrical glasses that compensate for the imperfections of the cornea.

Pupil and pupillary reflex. The pupil is the hole in the center of the iris through which light rays pass into the eye. The pupil sharpens the image on the retina, increasing the depth of field of the eye. By transmitting only the central rays, it improves the image on the retina also by eliminating spherical aberration. If you cover your eye from the light and then open it, the pupil, which dilated during darkening, quickly narrows (“pupillary reflex”). The muscles of the iris change the size of the pupil, regulating the amount of light entering the eye. So, in very bright light the pupil has a minimum diameter (1.8 mm), in average daylight it expands (2.4 mm), and in the dark the dilation is maximum (7.5 mm). This leads to a deterioration in the quality of the retinal image, but increases the sensitivity of vision. The maximum change in the diameter of the pupil changes its area by approximately 17 times. The luminous flux changes by the same amount. There is a logarithmic relationship between lighting intensity and pupil diameter. The reaction of the pupil to changes in illumination is adaptive in nature, since it stabilizes the illumination of the retina in a small range.

In the iris there are two types of muscle fibers surrounding the pupil: ring (m. sphincter iridis), innervated by parasympathetic fibers of the oculomotor nerve, and radial (m. dilatator iridis), innervated by sympathetic nerves. Contraction of the former causes constriction, contraction of the latter causes dilation of the pupil. Accordingly, acetylcholine and eserine cause constriction, and adrenaline causes pupil dilation. The pupils dilate during pain, during hypoxia, and also during emotions that increase the excitation of the sympathetic system (fear, rage). Pupil dilation is an important symptom of a number of pathological conditions, such as pain shock and hypoxia.

Structure and functions of the retina. The retina is the inner light-sensitive layer of the eye. It has a complex multilayer structure (Fig. 14.6). There are two types of secondary sensory photoreceptors, different in their functional significance (rod and cone) and several types of nerve cells. Stimulation of the photoreceptors activates the first retinal nerve cell (bipolar neuron). Excitation of bipolar neurons activates retinal ganglion cells, which transmit their impulse signals to the subcortical visual centers. Horizontal and amacrine cells are also involved in the processes of transmitting and processing information in the retina. All of the listed retinal neurons with their processes form nervous apparatus of the eye, which not only transmits information to the visual centers of the brain, but also participates in its analysis and processing. Therefore, the retina is called the part of the brain located in the periphery.

The place where the optic nerve exits the eyeball, the optic disc, is called the blind spot. It does not contain photoreceptors and is therefore insensitive to light. We do not feel the presence of a “hole” in the retina.

Let us consider the structure and functions of the layers of the retina, proceeding from the outer (back, farthest from the pupil) layer of the retina to the inner (located closer to the pupil) layer.

Pigment layer. This layer is formed by a single row of epithelial cells containing a large number of different intracellular organelles, including melanosomes, which give this layer its black color. This pigment, also called shielding pigment, absorbs the light reaching it, thereby preventing it from being reflected and scattered, which promotes clarity of visual perception. Pigment epithelial cells have numerous processes that tightly surround the light-sensitive outer segments of rods and cones. Pigment epithelium plays a critical role in a number of functions, including the resynthesis (regeneration) of visual pigment after its bleaching, phagocytosis and digestion of debris from the outer segments of rods and cones, in other words, in the mechanism of constant renewal of the outer segments of visual cells, in protecting visual cells from the danger of light damage, as well as in transporting oxygen and other substances they need to the photoreceptors. It should be noted that the contact between pigment epithelial cells and photoreceptors is quite weak. It is in this place that retinal detachment occurs, a dangerous eye disease. Retinal detachment leads to visual impairment not only due to its displacement from the place of optical focusing of the image, but also due to degeneration of the receptors due to disruption of contact with the pigment epithelium, which leads to a serious disruption of the metabolism of the receptors themselves. Metabolic disorders are aggravated by the fact that the delivery of nutrients from the capillaries is disrupted

choroid, and the photoreceptor layer itself does not contain capillaries (avascularized).

Photoreceptors. Adjacent to the pigment layer from the inside is a layer of photoreceptors: rods and cones. In the retina of each human eye there are 6-7 million cones and 110-123 million rods. They are distributed unevenly in the retina. The central fovea of the retina (fovea centralis) contains only cones (up to 140 thousand per 1 mm2). Towards the periphery of the retina, their number decreases, and the number of rods increases, so that at the far periphery there are only rods. Cones function in high light conditions, they provide daylight and color vision; the much more light-sensitive rods are responsible for twilight vision.

Impaired rod function, which occurs when there is a lack of vitamin A in food, causes a twilight vision disorder - the so-called night blindness: a person becomes completely blind at dusk, but vision remains normal during the day. On the contrary, when the cones are damaged, photophobia occurs: a person sees in dim light, but goes blind in bright light. In this case, complete color blindness may develop - achromasia.

The structure of a photoreceptor cell. A photoreceptor cell - rod or cone - consists of a light-sensitive outer segment containing visual pigment, an inner segment, a connecting stalk, a nuclear part with a large nucleus and a presynaptic ending. The rod and cone of the retina face their light-sensitive outer segments towards the pigment epithelium, i.e. in the direction opposite to light. U In humans, the outer segment of the photoreceptor (rod or cone) contains about a thousand photoreceptor disks. The outer segment of the rod is much longer than the cone and contains more visual pigment. This partly explains the higher sensitivity of the rod to light: rod

can excite just one quantum of light, but more than a hundred quanta are required to activate a cone.

The inner segment of the photoreceptor is connected to the outer segment by a modified cilium, which contains nine pairs of microtubules. The inner segment contains a large nucleus and the entire metabolic apparatus of the cell, including mitochondria, which provide the energy needs of the photoreceptor, and a protein synthesis system, which ensures the renewal of the membranes of the outer segment. Here the synthesis and incorporation of visual pigment molecules into the photoreceptor membrane of the disc occurs. In an hour, on average, three new discs are re-formed at the border of the inner and outer segments. Then they slowly (in humans, about 2-3 weeks) move from the base of the outer segment of the rod to its apex. Eventually, the apex of the outer segment, containing up to hundreds of now old discs, breaks off and is phagocytosed by the cells of the pigment layer. This is one of the most important mechanisms for protecting photoreceptor cells from molecular defects that accumulate during their light life.

The outer segments of the cones are also constantly renewed, but at a slower rate. Interestingly, there is a daily rhythm of renewal: the tips of the outer segments of the rods mainly break off and are phagocytosed in the morning and daytime, and the tips of the cones in the evening and night.

The presynaptic terminal of the receptor contains a synaptic ribbon, around which there are many synaptic vesicles containing glutamate.

Visual pigments. The rods of the human retina contain the pigment rhodopsin, or visual purple, the maximum absorption spectrum of which is in the region of 500 nanometers (nm). The outer segments of the three types of cones (blue-, green- and red-sensitive) contain three types of visual pigments, the maximum absorption spectra of which are in the blue (420 nm), green (531 nm) and red (558 nm) parts of the spectrum. The red cone pigment is called iodopsin. The visual pigment molecule is relatively small (with a molecular weight of about 40 kilodaltons), consists of a larger protein part (opsin) and a smaller chromophore (retinal, or vitamin A aldehyde). Retinal can be found in various

different spatial configurations, i.e., isomeric forms, but only one of them, the 11-cis-isomer of retinal, acts as the chromophore group of all known visual pigments. The source of retinal in the body is carotenoids, so their deficiency leads to vitamin A deficiency and, as a consequence, to insufficient rhodopsin resynthesis, which in turn causes impaired twilight vision, or “night blindness.” Molecular physiology of photoreception. Let us consider the sequence of changes in molecules in the outer segment of the rod, responsible for its excitation (Fig. 14.7, A). When a quantum of light is absorbed by a molecule of visual pigment (rhodopsin), instant isomerization of its chromophore group occurs in it: 11-cis-retinal is straightened and converted into full-trans-retinal. This reaction lasts about 1 ps (1 -12 s). Light acts as a trigger, or trigger, factor that initiates the photoreception mechanism. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes discolored and passes into the state of metarhodopsin II. As a result, the visual pigment molecule is

acquires the ability to interact with another protein - the membrane-bound guanosine triphosphate-binding protein transducin (T). In complex with metarhodopsin II, transducin enters an active state and exchanges guanosine diphosphate (GDP) bound to it in the dark for guanosine triphosphate (GTP). Metaradopsin II is capable of activating about 500-1000 molecules of trans-ducin, which leads to an increase in the light signal.

Each activated transducin molecule associated with a GTP molecule activates one molecule of another near-membrane protein - the phosphodiesterase enzyme (PDE). Activated PDE destroys cyclic guanosine monophosphate (cGMP) molecules at a high speed. Each activated PDE molecule destroys several thousand cGMP molecules - this is another step in signal amplification in the photoreception mechanism. The result of all the described events caused by the absorption of a light quantum is a drop in the concentration of free cGMP in the cytoplasm of the outer segment of the receptor. This in turn leads to the closure of ion channels in the plasma membrane of the outer segment, which were open in the dark and through which Na + and Ca 2+ entered the cell. The ion channel closes due to the fact that, due to a drop in the concentration of free cGMP in the cell, cGMP molecules that were bound to it in the dark and kept it open leave the channel.

A decrease or cessation of entry into the outer segment of Na + leads to hyperpolarization of the cell membrane, i.e., the appearance of a receptor potential on it. In Fig. Figure 14.7, B shows the directions of ionic currents flowing through the plasma membrane of the photoreceptor in the dark. Concentration gradients of Na + and K + are maintained on the plasma membrane of the rod by the active operation of the sodium-potassium pump, localized in the membrane of the inner segment.

tion of free calcium in the cytoplasm due to the closure of the membrane ion channel and the constant operation of the exchanger protein, which releases calcium from the cell. As a result of all this, the concentration of cGMP inside the cell increases and cGMP again binds to the ion channel of the plasma membrane, opening it. Through the open channel, Na + and Ca 2+ begin to enter the cell again, depolarizing the receptor membrane and transferring it to a “dark” state. The release of the transmitter from the presynaptic ending of the depolarized receptor is again accelerated.

Retinal neurons. Retinal photoreceptors are synaptically connected with bipolar neurons (see Fig. 14.6, B). When exposed to light, the release of the mediator (glutamate) from the photoreceptor decreases, which leads to hyperpolarization of the bipolar neuron membrane. From it, the nerve signal is transmitted to ganglion cells, the axons of which are fibers of the optic nerve. Signal transmission both from the photoreceptor to the bipolar neuron and from it to the ganglion neuron cell occurs in a non-pulse way. A bipolar neuron does not generate impulses due to the extremely short distance over which it transmits a signal.

Neural pathways and connections V visual system. From the retina, visual information along the fibers of the optic nerve (II pair

cranial nerves) rushes to the brain. The optic nerves from each eye meet at the base of the brain, where they form a partial decussation (chiasma). Here, part of the fibers of each optic nerve passes to the side opposite to its eye. Partial decussation of fibers provides each cerebral hemisphere with information from both eyes. These projections are organized in such a way that the occipital lobe of the right hemisphere receives signals from the right halves of each retina, and the left hemisphere receives signals from the left halves of the retinas.

After the optic chiasm, the optic nerves are called optic tracts. They are projected into a number of brain structures, but the main number of fibers comes to the thalamic subcortical visual center - the lateral, or external, geniculate body (NKT). From here, signals enter the primary projection area of the visual cortex (striate cortex, or Brodmann area 17). The entire visual cortex includes several fields, each of which provides its own specific functions, but receives signals from the entire retina and generally maintains its topology, or retinotopy (signals from neighboring areas of the retina enter neighboring areas of the cortex).

Electrical activity of the centers of the visual system.Elektric phenomena in the retina and optic nerve. When exposed to light, electrical potentials are generated in the receptors and then in the neurons of the retina, reflecting the parameters of the current stimulus.

The total electrical response of the retina to light is called an electroretinogram (ERG). It can be recorded from the whole eye or directly from the retina. To do this, one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye or on the earlobe. On the electroretinogram, several characteristic waves are distinguished (Fig. 14.8). Wave A reflects the excitation of the internal segments of photoreceptors (late receptor potential) and horizontal cells. Wave b arises as a result of activation of glial (Müller) cells of the retina by potassium ions released during excitation of bipolar and amacrine neurons. Wave c reflects the activation of pigment epithelial cells, and wave d - horizontal cells.

The ERG clearly reflects the intensity, color, size and duration of action of the light stimulus. The amplitude of all ERG waves increases in proportion to the logarithm of the light intensity and the time during which the eye was in the dark. Wave d ( response to switching off) is greater the longer the light is on. Since the ERG reflects the activity of almost all retinal cells (except ganglion cells), this indicator is widely used in the clinic of eye diseases for diagnosis and treatment monitoring for various retinal diseases.

Excitation of retinal ganglion cells leads to the fact that along their axons (optic nerve fibers) the brain rushes.

impulses appear. The retinal ganglion cell is the first neuron of the “classical” type in the photoreceptor-brain circuit. Three main types of ganglion cells have been described: those that respond to light being turned on (op-response), to light being turned off (off-response), and to both (on-off-response) (Fig. 14.9).

Simultaneous excitation of closely located ganglion cells leads to their mutual inhibition: the responses of each cell become smaller than with a single stimulation. This effect is based on lateral, or lateral, inhibition. The receptive fields of neighboring ganglion cells partially overlap, so that the same receptors can be involved in generating the responses of several neurons. Due to their circular shape, the receptive fields of retinal ganglion cells produce what is called a point-by-point description of the retinal image: it is displayed as a very fine mosaic of excited neurons.

Electrical phenomena in the subcortical visual center andvisual cortex. The pattern of excitation in the neural layers of the subcortical visual center - the external or lateral geniculate body (NCT), where the fibers of the optic nerve arrive, is in many ways similar to that observed in the retina. The receptive fields of these neurons are also round, but smaller than those in the retina. The neuronal responses generated in response to a flash of light are shorter here than in the retina. At the level of the external geniculate bodies, the interaction of afferent signals coming from the retina occurs with efferent signals from the visual area of the cortex, as well as through the reticular formation from the auditory and other sensory systems. These interactions ensure the selection of the most essential components of the sensory signal and the processes of selective visual attention.

To assess signal transmission at different levels of the visual system, recording of total evoked potentials (EPs) is often used, which in animals can be simultaneously removed from all parts, and in humans - from the visual cortex using electrodes placed on the scalp (Fig. 14.10).

Comparison of the retinal response (ERG) caused by a light flash and the EP of the cerebral cortex allows us to establish the localization of the pathological process in the human visual system.

Visual functions. Light sensitivity. Absolute visual sensitivity. For a visual sensation to occur, it is necessary that the light stimulus have a certain minimum (threshold) energy. The minimum number of light quanta required to produce a sensation of light

that, under conditions of dark adaptation, ranges from 8 to 47. It is calculated that one rod can be excited by only 1 quantum of light. Thus, the sensitivity of the retinal receptors in the most favorable conditions of light perception is physically limiting. Single rods and cones of the retina differ slightly in light sensitivity, but the number of photoreceptors sending signals to one ganglion cell in the center and periphery of the retina is different. The number of cones in the receptive field in the center of the retina is approximately 100 times less than the number of rods in the receptive field in the periphery of the retina. Accordingly, the sensitivity of the rod system is 100 times higher than the cone system.

Visual adaptation. When moving from darkness to light, temporary blindness occurs, and then the sensitivity of the eye gradually decreases. This adaptation of the visual sensory system to bright light conditions is called light adaptertion. Reverse phenomenon (dark adaptation) observed when moving from a bright room to an almost dark one. At first, a person sees almost nothing due to reduced excitability of photoreceptors and visual neurons. Gradually, the contours of objects begin to emerge, and then their details also differ, as the sensitivity of photoreceptors and visual neurons in the dark gradually increases.

The increase in light sensitivity while in the dark occurs unevenly: in the first 10 minutes it increases tens of times, and then within an hour - tens of thousands of times. The restoration of visual pigments plays an important role in this process. Cone pigments in the dark are restored faster than rod rhodopsin, so in the first minutes of being in the dark, adaptation is due to processes in the cones. This first period of adaptation does not lead to large changes in the sensitivity of the eye, since the absolute sensitivity of the cone apparatus is small.

The next period of adaptation is due to the restoration of rod rhodopsin. This period ends only at the end of the first hour in the dark. The restoration of rhodopsin is accompanied by a sharp (100,000-200,000 times) increase in the sensitivity of rods to light. Due to the maximum sensitivity in the dark only rods, a dimly lit object is visible only in peripheral vision.

The light sensitivity of the eye also depends on the influences of the central nervous system. Irritation of certain areas of the reticular formation of the brain stem increases the frequency of impulses in the fibers of the optic nerve. The influence of the central nervous system on the adaptation of the retina to light is also manifested in the fact that illumination of one eye reduces the light sensitivity of the unilluminated eye. Sensitivity to light is also influenced by auditory, olfactory and gustatory signals.

Differential visual sensitivity. If additional illumination is applied to an illuminated surface whose brightness I (dl), then, according to the law

PARTICULAR PHYSIOLOGY OF SENSORY SYSTEMS

Visual system

Vision is evolutionarily adapted to the perception of electromagnetic radiation in a certain, very narrow part of its range (visible light). The visual system provides the brain with more than 95% of sensory information. Vision is a multi-link process that begins with the projection of an image onto the retina of a unique peripheral optical device - the eye. Then the photoreceptors are excited, the transmission and transformation of visual information occurs in the neural layers of the visual system, and visual perception ends with the decision about the visual image being made by the higher cortical parts of this system.

Structure and functions of the optical apparatus of the eye. The eyeball has a spherical shape, which makes it easier to rotate to point at the object in question. On the way to the photosensitive shell of the eye (retina), light rays pass through several transparent media - the cornea, lens and vitreous body. A certain curvature and refractive index of the cornea and, to a lesser extent, the lens determine the refraction of light rays inside the eye (Fig. 14.2).

The refractive power of any optical system is expressed in diopters (D). One diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of a healthy eye is 59D when viewing distant objects and 70.5D when viewing near objects. To schematically represent the projection of an image of an object onto the retina, you need to draw lines from its ends through the nodal point (7 mm behind the cornea). The image on the retina is sharply reduced and turned upside down and from right to left

For a normal young person's eye, the farthest point of clear vision lies at infinity. He examines distant objects without any strain of accommodation, that is, without contraction of the ciliary muscle. The closest point of clear vision is 10 cm from the eye.

Presbyopia. The lens loses elasticity with age, and when the tension of the zonules of Zinn changes, its curvature changes little. Therefore, the nearest point of clear vision is no longer located at a distance of 10 cm from the eye, but moves away from it. Nearby objects are poorly visible. This condition is called senile farsightedness, or presbyopia. Elderly people are forced to use glasses with biconvex lenses.

Refractive errors of the eye. The two main refractive errors of the eye - myopia, or myopia, and farsightedness, or hypermetropia - are caused not by insufficiency of the refractive media of the eye, but by a change in the length of the eyeball (Fig. 14.5, A).

Therefore, to read, farsighted people must wear glasses with biconvex lenses that enhance the refraction of light (Fig. 14.5, E). Hypermetropia should not be confused with senile farsightedness. The only thing they have in common is that it is necessary to use glasses with biconvex lenses.

In the iris there are two types of muscle fibers surrounding the pupil: circular (m. sphincter iridis), innervated by parasympathetic fibers of the oculomotor nerve, and radial (m. dilatator iridis), innervated by sympathetic nerves. Contraction of the former causes constriction, contraction of the latter causes dilation of the pupil. Accordingly, acetylcholine and eserine cause constriction, and adrenaline causes pupil dilation. The pupils dilate during pain, during hypoxia, and also during emotions that increase the excitation of the sympathetic system (fear, rage). Pupil dilation is an important symptom of a number of pathological conditions, such as pain shock and hypoxia.

In healthy people, the pupil sizes of both eyes are the same. When one eye is illuminated, the pupil of the other also narrows; such a reaction is called friendly. In some pathological cases, the pupil sizes of both eyes are different (anisocoria). Structure and functions of the retina. The retina is the inner light-sensitive layer of the eye. It has a complex multilayer structure

There are two types of secondary sensory photoreceptors, different in their functional significance (rod and cone) and several types of nerve cells. Stimulation of the photoreceptors activates the first retinal nerve cell (bipolar neuron). Excitation of bipolar neurons activates retinal ganglion cells, which transmit their impulse signals to the subcortical visual centers. Horizontal and amacrine cells are also involved in the processes of transmitting and processing information in the retina. All of the listed retinal neurons with their processes form the nervous apparatus of the eye, which not only transmits information to the visual centers of the brain, but also participates in its analysis and processing. Therefore, the retina is called the part of the brain located in the periphery.

Pigment layer. This layer is formed by a single row of epithelial cells containing a large number of different intracellular organelles, including melanosomes, which give this layer its black color. This pigment, also called shielding pigment, absorbs the light that reaches it, thereby preventing reflection and scattering, which promotes clarity of visual perception. Pigment epithelial cells have numerous processes that tightly surround the light-sensitive outer segments of rods and cones. Pigment epithelium plays a critical role in a number of functions, including the resynthesis (regeneration) of visual pigment after its bleaching, phagocytosis and digestion of debris from the outer segments of rods and cones, in other words, in the mechanism of constant renewal of the outer segments of visual cells, in protecting visual cells from the danger of light damage, as well as in transporting oxygen and other substances they need to the photoreceptors. It should be noted that the contact between pigment epithelial cells and photoreceptors is quite weak. It is in this place that retinal detachment occurs, a dangerous eye disease. Retinal detachment leads to visual impairment not only due to its displacement from the place of optical focusing of the image, but also due to degeneration of the receptors due to disruption of contact with the pigment epithelium, which leads to a serious disruption of the metabolism of the receptors themselves. Metabolic disorders are aggravated by the fact that the delivery of nutrients from the capillaries of the choroid is disrupted, and the photoreceptor layer itself does not contain capillaries (avascularized).

Photoreceptors. Adjacent to the pigment layer from the inside is a layer of photoreceptors: rods and cones1. In the retina of each human eye there are 6-7 million cones and 110-123 million rods. They are distributed unevenly in the retina. The central fovea of the retina (fovea centralis) contains only cones (up to 140 thousand per 1 mm2). Towards the periphery of the retina, their number decreases, and the number of rods increases, so that at the far periphery there are only rods. Cones function in high light conditions; they provide daylight. and color vision; the much more light-sensitive rods are responsible for twilight vision.

Impaired rod function, which occurs when there is a lack of vitamin A in food, causes a twilight vision disorder - the so-called night blindness: a person becomes completely blind at dusk, but vision remains normal during the day. On the contrary, when the cones are damaged, photophobia occurs: a person sees in dim light, but goes blind in bright light. In this case, complete color blindness may develop - achromasia.

The inner segment of the photoreceptor is connected to the outer segment by a modified cilium, which contains nine pairs of microtubules. The inner segment contains a large nucleus and the entire metabolic apparatus of the cell, including mitochondria, which provide the energy needs of the photoreceptor, and a protein synthesis system, which ensures the renewal of the membranes of the outer segment. Here the synthesis and incorporation of visual pigment molecules into the photoreceptor membrane of the disc occurs. In an hour, on average, three new discs are re-formed at the border of the inner and outer segments. Then they slowly (in humans, about 2-3 weeks) move from the base of the outer segment of the rod to its apex. Eventually, the apex of the outer segment, containing up to a hundred now old discs, breaks off and is phagocytosed by the cells of the pigment layer. This is one of the most important mechanisms for protecting photoreceptor cells from molecular defects that accumulate during their light life.

The presynaptic terminal of the receptor contains a synaptic ribbon, around which there are many synaptic vesicles containing glutamate.

Retinal can be found in various spatial configurations, i.e., isomeric forms, but only one of them, the 11-cis-isomer of retinal, acts as the chromophore group of all known visual pigments. The source of retinal in the body is carotenoids, so their deficiency leads to vitamin A deficiency and, as a consequence, to insufficient rhodopsin resynthesis, which in turn causes impaired twilight vision, or “night blindness.” Molecular physiology of photoreception. Let us consider the sequence of changes in molecules in the outer segment of the rod, responsible for its excitation (Fig. 14.7, A). When a quantum of light is absorbed by a molecule of visual pigment (rhodopsin), instant isomerization of its chromophore group occurs in it: 11-cis-retinal is straightened and converted into completely trans-retinal. This reaction lasts about 1 ps (1--12 s). Light acts as a trigger, or trigger, factor that initiates the photoreception mechanism. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes discolored and passes into the state of metarhodopsin II.

A decrease or cessation of entry into the outer segment of Na+ leads to hyperpolarization of the cell membrane, i.e., the appearance of a receptor potential on it. In Fig. Figure 14.7, B shows the directions of ionic currents flowing through the plasma membrane of the photoreceptor in the dark. Concentration gradients of Na+ and K+ are maintained on the plasma membrane of the rod by the active work of the sodium-potassium pump, localized in the membrane of the inner segment.

Retinal neurons. Retinal photoreceptors are synaptically connected with bipolar neurons (see Fig. 14.6, B). When exposed to light, the release of the mediator (glutamate) from the photoreceptor decreases, which leads to hyperpolarization of the bipolar neuron membrane. From it, the nerve signal is transmitted to ganglion cells, the axons of which are fibers of the optic nerve. Signal transmission both from the photoreceptor to the bipolar neuron and from it to the ganglion cell occurs in a pulseless manner. A bipolar neuron does not generate impulses due to the extremely short distance over which it transmits a signal.

The total electrical response of the retina to light is called an electroretinogram (ERG). It can be recorded from the whole eye or directly from the retina. To do this, one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye or on the earlobe. On the electroretinogram, several characteristic waves are distinguished (Fig. 14.8). Wave a reflects the excitation of the internal segments of photoreceptors (late receptor potential) and horizontal cells. Wave b occurs as a result of activation of glial (Müller) cells of the retina by potassium ions released during excitation of bipolar and amacrine neurons. Wave c reflects the activation of pigment epithelial cells, and wave d - horizontal cells.

To assess signal transmission at different levels of the visual system, recording of total evoked potentials (EPs) is often used, which in animals can be simultaneously removed from all parts, and in humans - from the visual cortex using electrodes placed on the scalp (Fig. 14.10).

Comparison of the retinal response (ERG) caused by a light flash and the EP of the cerebral cortex allows us to establish the localization of the pathological process in the human visual system.

Visual adaptation. When moving from darkness to light, temporary blindness occurs, and then the sensitivity of the eye gradually decreases. This adaptation of the visual sensory system to bright light conditions is called light adaptation. The opposite phenomenon (dark adaptation) is observed when moving from a light room to an almost unlit room. At first, a person sees almost nothing due to reduced excitability of photoreceptors and visual neurons. Gradually, the contours of objects begin to emerge, and then their details also differ, as the sensitivity of photoreceptors and visual neurons in the dark gradually increases.

The increase in light sensitivity while in the dark occurs unevenly: in the first 10 minutes it increases tens of times, and then within an hour - tens of thousands of times. “An important role in this process is played by the restoration of visual pigments. Cone pigments in the dark are restored faster than rod rhodopsin, therefore, in the first minutes of being in the dark, adaptation is due to processes in the cones. This first period of adaptation does not lead to large changes in the sensitivity of the eye, since the absolute sensitivity of the cone the device is small.

A significant role in adaptation, in addition to visual pigments, is played by changes (switching) of connections between the elements of the retina. In the dark, the area of the excitatory center of the receptive field of the ganglion cell increases due to the weakening or removal of horizontal inhibition. This increases the convergence of photoreceptors onto bipolar neurons and bipolar neurons onto the ganglion cell. As a result, due to spatial summation at the periphery of the retina, light sensitivity in the dark increases. The light sensitivity of the eye also depends on the influences of the central nervous system. Irritation of certain areas of the reticular formation of the brain stem increases the frequency of impulses in the fibers of the optic nerve. The influence of the central nervous system on the adaptation of the retina to light is also manifested in the fact that illumination of one eye reduces the light sensitivity of the unilluminated eye. Sensitivity to light is also influenced by auditory, olfactory and gustatory signals.

Differential visual sensitivity. If additional illumination (dI) is applied to an illuminated surface, the brightness of which I, then, according to Weber’s law, a person will notice a difference in illumination only if dI/I = K, where K is a constant equal to 0.01-0.015. The dI/I value is called the differential threshold of light sensitivity. The dI/I ratio is constant under different illumination and means that to perceive the difference in illumination of two surfaces, one of them must be 1-1.5% brighter than the other.

Brightness contrast. Mutual lateral inhibition of visual neurons underlies the overall, or global, luminance contrast. Thus, a gray strip of paper lying on a light background appears darker than the same strip lying on a dark background. The reason is that the light background excites many neurons in the retina, and their excitation inhibits the cells activated by the strip. Therefore, against a brightly lit background, the gray stripe appears darker than against a black background. Lateral inhibition acts most strongly between closely spaced neurons, producing local contrast. There is an apparent increase in the difference in brightness at the border of surfaces of different illumination. This effect is also called edge enhancement: at the border of a bright field and a dark surface, two additional lines can be seen (an even brighter line at the border of a bright field and a very dark line at the border of a dark surface).

Blinding brightness of light. Light that is too bright causes an unpleasant feeling of being blinded. The upper limit of blinding brightness depends on the adaptation of the eye: the longer the dark adaptation, the lower the brightness of the light causes blinding. If very bright (dazzle) objects come into the field of view, they impair the discrimination of signals in a significant part of the retina (on a night road, drivers are blinded by the headlights of oncoming cars). For delicate visual work (long reading, assembling small parts, surgical work), you should use only diffused light that does not blind the eyes.

Inertia of vision, merging of flickers and sequential images. The visual sensation does not appear instantly. Before a sensation occurs, multiple transformations and signal transmission must occur in the visual system. The time of “inertia of vision” required for the occurrence of a visual sensation is on average 0.03-0.1 s. This sensation also does not disappear immediately after the irritation has stopped; it persists for some time. If you move a bright point (for example, a burning match) through the air in the dark, then we will see not a moving point, but a luminous line. Light stimuli that quickly follow one after another merge into one continuous sensation.

The minimum frequency of repetition of light stimuli (for example, flashes of light), at which the fusion of individual sensations occurs, is called the critical frequency of flicker fusion. Cinema and television are based on this property of vision: we do not see gaps between individual frames ("/24 s in cinema), since the visual sensation from one frame continues until the appearance of another. This provides the illusion of continuity of the image and its movement.

Sensations that continue after the cessation of stimulation are called sequential images. If you look at a lamp that is turned on and close your eyes, it will still be visible for some time. If, after fixing your gaze on an illuminated object, you turn your gaze to a light background, then for some time you can see a negative image of this object, that is, its light parts are dark, and its dark parts are light (negative sequential image). The reason is that excitation from an illuminated object locally inhibits (adapts) certain areas of the retina; If you then turn your gaze to a uniformly illuminated screen, its light will more strongly excite those areas that were not previously excited.

Color vision. The entire spectrum of electromagnetic radiation visible to us lies between short-wavelength (wavelength from 400 nm) radiation, which we call violet, and long-wavelength radiation (wavelength up to 700 nm), called red. The remaining colors of the visible spectrum (blue, green, yellow, orange) have intermediate wavelengths. Mixing the rays of all colors gives white. It can also be obtained by mixing two so-called paired complementary colors: red and blue, yellow and blue. If you mix three primary colors - red, green and blue, then any colors can be obtained.

Theories of color perception. The most widely accepted is the three-component theory (G. Helmholtz), according to which color perception is provided by three types of cones with different color sensitivities. Some of them are sensitive to red, others to green, and others to blue. Every color affects all three color-sensing elements, but to varying degrees. This theory was directly confirmed in experiments where the absorption of radiation of different wavelengths in single cones of the human retina was measured with a microspectrophotometer.

According to another theory proposed by E. Hering, cones contain substances that are sensitive to white-black, red-green and yellow-blue radiation. In experiments where a microelectrode was used to record impulses from the retinal ganglion cells of animals illuminated with monochromatic light, it was found that discharges of the majority of neurons (dominators) occur when exposed to any color. In other ganglion cells (modulators), impulses occur when illuminated with only one color. 7 types of modulators have been identified that respond optimally to light of different wavelengths (from 400 to 600 nm).

Many so-called color-opposite neurons are found in the retina and visual centers. The effect on the eye of radiation in some part of the spectrum excites them, and in other parts of the spectrum inhibits them. It is believed that such neurons encode color information most efficiently.

Consistent color images. If you look at a colored object for a long time and then move your gaze to white paper, then the same object is seen painted in a complementary color. The reason for this phenomenon is color adaptation, i.e., a decrease in sensitivity to this color. Therefore, the one that acted on the eye before is subtracted from the white light, and a sensation of additional color arises.

Color blindness. Partial color blindness was described at the end of the 18th century. D. Dalton, who himself suffered from it (therefore, the anomaly of color perception was called color blindness). Color blindness occurs in 8% of men and much less frequently in women: its occurrence is associated with the absence of certain genes on the unpaired X chromosome in men. To diagnose color blindness, which is important in professional selection, polychromatic tables are used. People suffering from this disease cannot be full-fledged drivers of transport, since they cannot distinguish the color of traffic lights and road signs. There are three types of partial color blindness: protanopia, deuteranopia and tritanopia. Each of them is characterized by the lack of perception of one of the three primary colors.

People suffering from protanopia (“red-blind”) do not perceive the color red; blue-blue rays seem colorless to them. People suffering from deuteranopia (“green-blind”) cannot distinguish green colors from dark red and blue. With tritanopia, a rare color vision anomaly, blue and violet light is not perceived.

All of the listed types of partial color blindness are well explained by the three-component theory of color perception. Each type of blindness is the result of the absence of one of the three cone color-perceiving substances. There is also complete color blindness - achromasia, in which, as a result of damage to the cone apparatus of the retina, a person sees all objects only in different shades of gray.

Perception of space. Visual acuity. Visual acuity is the maximum ability of the eye to distinguish individual details of objects.

Visual acuity is determined by the shortest distance between two points that the eye distinguishes, that is, sees separately and not together. A normal eye distinguishes two points visible at an angle of 1". The yellow spot has the maximum visual acuity. To the periphery of it, visual acuity is much lower (Fig. 14.11). Visual acuity is measured using special tables, which consist of several rows of letters or open circles of various sizes.Visual acuity, determined from the table, is usually expressed in relative values, with normal acuity taken as 1. There are people who have hyperacuity of vision (visus more than 2).

Line of sight. If you fix your gaze on a small object, its image is projected onto the macula of the retina. In this case, we see the object with central vision. Its angular size in humans is 1.5-2°. Objects whose images fall on the rest of the retina are perceived by peripheral vision. The space visible to the eye when the gaze is fixed at one point is called the visual field. The boundary of the field of view is measured using the perimeter. The boundaries of the visual field for colorless objects are 70° downward, 60° upward, 60° inward, and 90° outward. The visual fields of both eyes in humans partially coincide, which is of great importance for the perception of depth of space. The fields of view for different colors are not the same and are smaller than for black and white objects.

Distance estimation. Perception of the depth of space and estimation of the distance to an object are possible both with vision with one eye (monocular vision) and with two eyes (binocular vision). In the second case, the distance estimate is much more accurate. The phenomenon of accommodation is of some importance in assessing close distances with monocular vision. For assessing distance, it is also important that the image of an object on the retina is larger, the closer it is. The role of eye movement for vision. When looking at any objects, the eyes move. Eye movements are carried out by 6 muscles attached to the eyeball slightly anterior to its equator. These are 2 oblique and 4 rectus muscles - external, internal, superior and inferior. The movement of the two eyes occurs simultaneously and in a friendly manner. When looking at close objects, it is necessary to bring them together (convergence), and when looking at distant objects, it is necessary to separate the visual axes of the two eyes (divergence). The important role of eye movements for vision is also determined by the fact that for the brain to continuously receive visual information, image movement on the retina is necessary. As already mentioned, impulses in the optic nerve occur when the light image is turned on and off. With continued exposure to light on the same photoreceptors, the impulse in the fibers of the optic nerve quickly stops and the visual sensation with motionless eyes and objects disappears after 1-2 s. To prevent this from happening, the eye, when examining any object, produces continuous jumps (saccades) that are not felt by humans. As a result of each jump, the image on the retina shifts from one photoreceptor to a new one, again causing impulses in the ganglion cells. The duration of each jump is equal to hundredths of a second, and its amplitude does not exceed 20°. The more complex the object in question, the more complex the trajectory of eye movement. They seem to trace the contours of the image, lingering on its most informative areas (for example, in the face - these are the eyes). In addition, the eye continuously trembles and drifts (slowly moves from the point of fixation of the gaze), which is also important for visual perception.

Binocular vision. When looking at any object, a person with normal vision does not have the sensation of two objects, although there are two images on two retinas. The images of all objects fall on the so-called corresponding, or corresponding, areas of the two retinas, and in human perception these two images merge into one. Apply light pressure on one eye from the side: you will immediately begin to see double because the alignment of the retinas is disrupted. If you look at a close object, converging your eyes, then the image of some more distant point falls on non-identical (disparate) points of the two retinas. Disparity plays a big role in estimating distance and, therefore, in seeing the depth of the relief. A person is able to notice a change in depth, creating a shift in the image on the retinas of several arc seconds. Binocular fusion, or the combining of signals from the two retinas into a single neural image, occurs in the primary visual cortex.

Estimation of the size of an object. The size of an object is estimated as a function of the size of the image on the retina and the distance of the object from the eye. In cases where it is difficult to estimate the distance to an unfamiliar object, gross errors in determining its size are possible.

Photochemical processes in the retina associated with the transformation of a number of substances in light or in the dark. As mentioned above, the outer segments of receptor cells contain pigments. Pigments are substances that absorb a certain portion of light rays and reflect the remaining rays. The absorption of light rays occurs by a group of chromophores that are contained in visual pigments. This role is played by aldehydes of vitamin A alcohols.

Cone visual pigment, iodopsin ( jodos - violet) consists of the protein photopsin (photos - light) and 11-cis-retinal, rod pigment - rhodopsin ( rodos - purple) - from the scotopsin protein ( scotos - darkness) and also 11-cis retinal. Thus, the difference between the pigments of receptor cells lies in the characteristics of the protein part. The processes that occur in rods have been studied in more detail,

Rice. 12.10. Diagram of the structure of cones and rods

therefore, the subsequent analysis will concern them specifically.

Photochemical processes occurring in rods in the light

Under the influence of a quantum of light absorbed by rhodopsin, photoisomerization of the chromophore part of rhodopsin occurs. This process comes down to a change in the shape of the molecule; a bent 11-cis-retinal molecule turns into a straightened all-trans-retinal molecule. The process of detaching scotopsin begins. The pigment molecule becomes discolored. At this stage, the bleaching of the rhodopsin pigment ends. The decolorization of one molecule contributes to the closure of 1,000,000 pores (Na + channels) (Hubel).

Photochemical processes occurring in rods in the dark

The first stage is the resynthesis of rhodopsin - the transition of all-trans-retinal to 11-cis-retinal. This process requires metabolic energy and the enzyme retinal isomerase. Once 11-cis-retinal is formed, it combines with the scotopsin protein, resulting in the formation of rhodopsin. This form of rhodopsin is stable to the action of the next quantum of light (Fig. 12.11). Part of rhodopsin is subject to direct regeneration, part of retinal1 in the presence of NADH is reduced by the enzyme alcohol dehydrogenase to vitamin A1, which, accordingly, interacts with scotopsin to form rhodopsin.

If a person does not receive vitamin A for a long time (months), he develops night blindness, or hemeralopia. It can be treated - within an hour after the injection of vitamin A, it disappears. Retinal molecules are aldehydes, which is why they are called retinalums, and group vitamins

Rice. 12.11. Photochemical and electrical processes in the retina

Group A - alcohols, which is why they are called retinol. For the formation of rhodopsin with the participation of vitamin A, it is necessary that 11-cis-retinal is converted into 11-trans-retinol.

Electrical processes in the retina

peculiarities:

1. Photoreceptor MP is very low (25-50 mV).

2. In the world in the outer segment, Na + - channels close, and in the dark they open. Accordingly, hyperpolarization occurs in photoreceptors in light, and depolarization occurs in darkness. Closure of the Na + channels of the outer segment causes hyperpolarization by K + strum, that is, the appearance of an inhibitory receptor potential (up to 70-80 mV) (Fig. 12.12). As a result of hyperpolarization, the release of the inhibitory transmitter, glutamate, decreases or stops, which promotes the activation of bipolar cells.

3. In the dark: N a + -channels of the outer segments open. Na + enters the outer segment and depolarizes the photoreceptor membrane (up to 25-50 mV). Depolarization of the photoreceptor leads to the appearance of an excitatory potential and increases the release of the photoreceptor mediator glutamate, which is an inhibitory mediator, so the activity of bipolar cells will be inhibited. Thus, cells of the second functional layer of the retina, when exposed to light, can activate cells of the next layer of the retina, that is, ganglion cells.

The role of cells of the second functional layer

bipolar cells, like receptor (rods and cones) and horizontal, they do not generate action potentials, but only local potentials. Synapses between receptor and bipolar cells are of two types - excitatory and inhibitory, therefore the local potentials produced by them can be both depolarizing - excitatory and hyperpolarizing - inhibitory. Bipolar cells receive inhibitory synapses from horizontal cells (Fig. 12.13).

Horizontal cells are excited by the action of receptor cells, but they themselves inhibit bipolar cells. This type of inhibition is called lateral (see Fig. 12.13).

Amacrine cells - the third type of cells of the second functional layer of the retina. they are activated

Rice. 12.12. The influence of darkness (A) and light (B) on the transport of Nα* ions in photoreceptor cells of the retina:

The outer segment channels are open in the dark due to cGMP (A). When exposed to light, 5-HMP partially closes them (B). This leads to hyperpolarization of the synaptic endings of photoreceptors (a - depolarization b - hyperpolarization)

bipolar cells, and they inhibit ganglion cells (see Fig. 3.13). It is believed that there are more than 20 types of amacrine cells and, accordingly, they secrete a large number of different mediators (GABA, glycine, dopamine, indoleamine, acetylcholine, etc.). The reactions of these cells are also varied. Some react to the light being turned on, others to the light being turned off, others to the movement of a spot across the retina, and the like.

The role of the third functional layer of the retina

Ganglion cells - the only classical retinal neurons that always generate action potentials; they are located in the last functional layer of the retina, have a constant background activity with a frequency of 5 to 40 per 1 minute (Guyton). Everything that happens in the retina between different cells affects the ganglion cells.

They receive signals from bipolar cells, in addition, they are inhibited by amacrine cells. The influence from bipolar cells is twofold depending on whether the local potential arises in the bipolar cells. If there is depolarization, then such a cell will activate the ganglion cell and the frequency of action potentials in it will increase. If the local potential in a bipolar cell is hyperpolarizing, then the effect on ganglion cells will be the opposite, that is, a decrease in the frequency of its background activity.

Thus, due to the fact that most retinal cells produce only local potentials and conduction in ganglion cells is electrotonic, this makes it possible to estimate the intensity of illumination. Action potentials that operate on an all-or-nothing basis would not provide this.

In ganglion cells, as in bipolar and horizontal cells, there are receptor sites. Receptor sites are a collection of receptors that send signals to this cell through one or more synapses. The receptor sites of these cells have a concentric shape. They distinguish between a center and a periphery with antagonistic interaction. The size of the receptor sites of ganglion cells can vary depending on which part of the retina sends signals to them; they will be fewer receptors in the fovea compared to signals from the periphery of the retina.

Rice. 12.13. Scheme of functional connections of retinal cells:

1 - layer of photoreceptors;

2 - layer of bipolar, horizontal, amacrine cells;

3 - layer of ganglion cells;

Black arrows - inhibitory effect, white - excitatory

Ganglion cells with an “on” center are activated when the center is illuminated, and inhibited when the periphery is illuminated. On the contrary, ganglion cells with an “off” center are inhibited when the center is illuminated, and when the periphery is illuminated, they are activated.

By changing the frequency of ganglion cell impulses, the influence on the next level of the visual sensory system will change.

It has been established that ganglion neurons are not just the last link in the transmission of signals from retinal receptors to brain structures. A third visual pigment was discovered in them - melanopsin! It plays a key role in ensuring the body’s circadian rhythms associated with changes in lighting, it affects the synthesis of melatonin, and is also responsible for the reflex reaction of the pupils to light.

In experimental mice, the absence of the gene responsible for the synthesis of melanopsin leads to a pronounced disruption of circadian rhythms, a decrease in the intensity of the pupillary reaction to light, and inactivation of rods and cones - to its disappearance altogether. The axons of ganglion cells, which contain melanopsin, are directed to the suprachiasmatic nuclei of the hypothalamus.

The brain receives more than 90% of sensory information through the organ of vision. From the entire spectrum of electromagnetic radiation, the photoreceptors of the retina register only waves with a length from 400 to 800 nm. The physiological role of the eye as an organ of vision is twofold. First, it is an optical instrument that collects light from environmental objects and projects their images onto the retina. Second, photoreceptors in the retina convert optical images into neural signals that are transmitted to the visual cortex.

Organ of vision(Figure 10-1) includes eyeball, connected through the optic nerve to the brain, protective apparatus(including eyelids and lacrimal glands) and motion apparatus(striated oculomotor muscles). Eyeball. The wall of the eyeball is formed by membranes: in the anterior part there are conjunctiva And cornea, in the back - retina, choroid And sclera. The cavity of the eyeball occupies vitreous body. Anterior to the vitreous body is a biconvex lens Between the cornea and the lens there are

Fig.10-1. Eyeball.Inset: pupillary reflex

aqueous humor front camera(between the posterior surface of the cornea and the iris with the pupil) and rear camera eyes (between the iris and lens).

Protective apparatus of the eye. Long eyelashes the upper eyelid protects the eye from dust; The blink reflex (blinking) occurs automatically. Eyelids contain meibomian glands, thanks to which the edges of the eyelids are always moisturized. Conjunctiva- thin mucous membrane - lines both the inner surface of the eyelids and the outer surface of the eyeball. Lacrimal gland secretes tear fluid that irrigates the conjunctiva.

Retina

A diagram of the visual part of the retina is shown in Fig. 10-2. At the posterior edge of the optical axis of the eye, the retina has a rounded yellow spot about 2 mm in diameter (Fig. 10-2, inset). Fossa fovea- the depression in the middle part of the macula is the place of best perception. Optic nerve exits the retina medial to the macula. Here it is formed optic disc (blind spot), not perceiving light. In the center of the disk there is a depression in which the vessels supplying the retina are visible. In the visual retina, starting from the outermost - pigment (prevents the reflection and scattering of light passing through the entire thickness of the retina, see arrow in Fig. 10-2) and to the innermost - layer of nerve fibers (axons of ganglion neurons) of the optic nerve, the following are distinguished: layers.

External nuclear The layer contains the nucleated parts of photoreceptor cells - cones and rods. Cones concentrated in the macula area. The eyeball is organized in such a way that the central part of the light spot from the visualized object falls on the cones. Along the periphery of the macula are located sticks.

Outer mesh. Here contacts are made between the internal segments of rods and cones with the dendrites of bipolar cells.

Internal nuclear. Here are located bipolar cells, connecting rods and cones with ganglion cells, as well as horizontal and amacrine cells.

Inner mesh. In it, bipolar cells contact ganglion cells, and amacrine cells act as interneurons.

Ganglion layer contains cell bodies of ganglion neurons.

Rice. 10-2. Retina(B - bipolar cells; D - ganglion cells; mountains - horizontal cells; A - amacrine cells). Inset- ocular fundus

The general scheme of information transmission in the retina is as follows: receptor cell, bipolar cell, ganglion cell, and at the same time, amacrine cell - ganglion cell, axons of ganglion cells. The optic nerve exits the eye in an area visible through an ophthalmoscope as optic disc(Figure 10-2, inset). Photoreceptor cells(Fig. 10-3 and 10-5B) - rods and cones. The peripheral processes of photoreceptor cells consist of outer and inner segments connected by a cilium.

Outer segment has many flattened closed disks (duplicates of cell membranes) containing visual pigments: rhodopsin(absorption maximum - 505 nm) - in sticks: red(570 nm), green(535 nm) and blue(445 nm) pigments - in cones. The outer segment of rods and cones consists of regular membrane formations - disks(Figure 10-3, right). Each photoreceptor contains more than 1000 disks.

Internal segment filled with mitochondria and contains a basal body, from which 9 pairs of microtubules extend into the outer segment.

Central vision and visual acuity realized by cones.

Peripheral vision, and night vision And perception of moving objects- functions of sticks.

OPTICS OF THE EYE

The eye has a system of lenses with different curvatures and different refractive indices of light rays (Fig. 10-4.1), including

Fig.10-3. Retinal photoreceptors.The outer segments are enclosed in a rectangle

There are four refractive media between: O air and the anterior surface of the cornea; About the posterior surface of the cornea and the aqueous humor of the anterior chamber; About the aqueous humor of the anterior chamber and the lens; About the posterior surface of the lens and the vitreous body.

Refractive power. For practical calculations of the refractive power of the eye, the concept of the so-called “reduced eye” is used, when all refractive surfaces are algebraically added and considered as one lens. In such a reduced eye with a single refractive surface, the central point of which is located 17 mm anterior to the retina, the total refractive power is 59 diopters when the lens is adapted for viewing distant objects. The refractive power of any optical system is expressed in diopters (D): 1 diopter is equal to the refractive power of a lens with a focal length of 1 meter.

Accommodation- adaptation of the eye to clearly seeing objects located at different distances. The main role in the process of accommodation belongs to the lens, which can change its curvature. In young people, the refractive power of the lens can increase from 20 to 34 diopters. In this case, the lens changes shape from moderately convex to significantly convex. The mechanism of accommodation is illustrated in Fig. 10-4, II.

Fig.10-4. OPTICS OF THE EYE. I The eye as an optical system. II Accommodation mechanism. A is a distant object. B - nearby object. III Refraction. IV Visual fields. The broken line outlines the visual field of the left eye, the solid line outlines the visual field of the right eye. The light (heart-shaped) area in the center is the binocular vision zone. The colored areas on the left and right are monocular vision fields)

When looking at distant objects (A), the ciliary muscles relax, the suspensory ligament stretches and flattens the lens, giving it a disc-shaped shape. When looking at close objects (B), a more significant curvature of the lens is required for full focusing, so the SMCs of the ciliary body contract, the ligaments relax, and the lens, due to its elasticity, becomes more convex. Visual acuity- the accuracy with which the object is visible; theoretically, the object should be of such a size that it could stimulate one rod or cone. Both eyes work together (binocular vision) to transmit visual information to the visual centers of the cerebral cortex, where the visual image is evaluated in three dimensions.

Pupillary reflex. The pupil, a round hole in the iris, changes in size very quickly depending on the amount of light falling on the retina. The pupil lumen can vary from 1 mm to 8 mm. This gives the pupil the properties of a diaphragm. The retina is very sensitive to light (Figure 10-1, inset), and too much light (A) distorts colors and irritates the eye. By changing the lumen, the pupil regulates the amount of light entering the eye. Bright light causes an unconditional reflex autonomic reaction that closes in the midbrain: the sphincter of the pupil (1) in the iris of both eyes contracts, and the dilator of the pupil (2) relaxes, as a result the diameter of the pupil decreases. Poor lighting (B) causes both pupils to dilate so that enough light can reach the retina and excite the photoreceptors.

Friendly pupil reaction. In healthy people, the pupils of both eyes are the same size. Lighting one eye causes the pupil of the other eye to constrict. This reaction is called a friendly pupil reaction. In some diseases, the pupil sizes of both eyes are different (anisocoria).

Depth of focus. The pupil enhances the clarity of the image on the retina by increasing the depth of field. In bright light, the pupil has a diameter of 1.8 mm, in average daylight illumination - 2.4 mm, in the dark the pupil dilation is maximum - 7.5 mm. Pupil dilation in the dark degrades the quality of the retinal image. There is a logarithmic relationship between the pupil diameter and lighting intensity. The maximum increase in pupil diameter increases its area by 17 times. The light flux entering the retina increases by the same amount.

Focus control. Lens accommodation is regulated by a negative feedback mechanism, automatically adjusting the focal power of the lens for the highest visual acuity. When the eyes are fixed on a distant object and must immediately change their fixation to a near object, accommodation of the lens occurs within a fraction of a second, providing better visual acuity. If the point of fixation unexpectedly changes, the lens always changes its refractive power in the desired direction. In addition to the autonomic innervation of the iris (pupillary reflex), the following points are important for focusing control.

❖ Chromatic aberration. Red rays focus later than blue rays because the lens refracts blue rays

stronger than red ones. The eyes are able to determine which of these two types of rays is in better focus and send information to the accommodative mechanism with instructions to make the lens stronger or weaker.

❖ Spherical aberration. By transmitting only the central rays, the pupil eliminates spherical aberration.

❖ Convergence of eyes when fixating on a close object. The neural mechanism that causes convergence simultaneously signals an increase in the refractive power of the lens.

❖ Degree of lens accommodation oscillates constantly but slightly twice per second, allowing the lens to respond more quickly to establish focus. The visual image becomes clearer when the oscillations of the lens enhance changes in the desired direction; clarity decreases when the power of the lens changes in the wrong direction.

❖ Areas of the cerebral cortex those that control accommodation interact with the neural structures that control the fixation of the eyes on a moving object. The final integration of visual signals occurs in Brodmann's areas 18 and 19, then motor signals are transmitted to the ciliary muscle through the brain stem and Edinger-Westphal nuclei.

❖ Point of closest vision- the ability to clearly see a nearby object in focus - becomes distant during life. At the age of ten, it is approximately 9-10 cm and moves away to 83 cm at the age of 60 years. This regression of the point of closest vision occurs as a result of decreased elasticity of the lens and loss of accommodation.

Presbyopia. As a person gets older, the lens grows, becomes thicker and less elastic. The ability of the lens to change its shape also decreases. The power of accommodation drops from 14 diopters in a child to less than 2 diopters in a person aged 45 to 50 years and to 0 at age 70 years. Thus, the lens loses its ability to accommodate, and this condition is called presbyopia (senile farsightedness). When a person reaches a state of presbyopia, each eye remains at a constant focal length; this distance depends on the physical characteristics of each individual's eyes. Therefore, older people are forced to use glasses with biconvex lenses.

Refractive errors. Emmetropia(normal vision, Fig. 10-4,III) corresponds to a normal eye if parallel rays from distant objects are focused on the retina when the ciliary

the muscle is completely relaxed. This means that the emmetropic eye can see all distant objects very clearly and easily transition (through accommodation) to clear vision of nearby objects.

❖ Hypermetropia(farsightedness) may be caused by an eyeball that is too short or, in more rare cases, by the fact that the eye has a lens that is too inelastic. In the farsighted eye, the longitudinal axis of the eye is shorter, and the beam from distant objects is focused behind the retina (Fig. 10-4, III). This lack of refraction is compensated by a farsighted person by accommodative effort. A farsighted person strains the accommodative muscle when looking at distant objects. Attempts to look at nearby objects cause excessive strain on accommodation. To work with close objects and read, farsighted people should use glasses with biconvex lenses.

❖ Myopia(myopia) represents the case when the ciliary muscle is completely relaxed, and light rays from a distant object are focused in front of the retina (Fig. 10-4,III). Myopia occurs either as a result of the eyeball being too long, or as a result of the high refractive power of the eye's lens. There is no mechanism by which the eye could reduce the refractive power of the lens when the ciliary muscle is completely relaxed. However, if an object is close to the eyes, a nearsighted person can use the mechanism of accommodation to focus the object clearly on the retina. Therefore, a nearsighted person is limited only to the clear point of “far vision.” For clear distance vision, a nearsighted person needs to use glasses with biconcave lenses.

❖ Astigmatism- unequal refraction of rays in different directions, caused by different curvature of the spherical surface of the cornea. Accommodation of the eye is unable to overcome astigmatism, because the curvature of the lens changes equally during accommodation. To compensate for deficiencies in corneal refraction, special cylindrical lenses are used.

Visual field and binocular vision

❖ Visual field each eye is part of the external space visible to the eye. In theory it should be round, but in reality it is cut medially by the nose and the upper edge of the eye socket! (Fig. 10-4,IV). Mapping

visual field is important for neurological and ophthalmological diagnostics. The circumference of the visual field is determined using the perimeter. One eye closes and the other fixates on the central point. By moving a small target along the meridians towards the center, the points are marked when the target becomes visible, thus describing the visual field. In Fig. 10-4,IV, the central visual fields are outlined along a tangent line with solid and dotted lines. White areas outside the lines are a blind spot (physiological scotoma).

❖ Binocular vision. The central part of the visual fields of the two eyes completely coincides; therefore, any area in this visual field is covered by binocular vision. Impulses coming from two retinas, excited by light rays from an object, merge into one image at the level of the visual cortex. The points on the retina of both eyes where the image must fall in order for it to be perceived binocularly as a single object are called corresponding points. Light pressure on one eye causes double vision due to misalignment of the retinas.

❖ Depth of vision. Binocular vision plays an important role in determining the depth of vision based on the relative sizes of objects, their reflections, and their movement relative to each other. In fact, depth perception is also a component of monocular vision, but binocular vision adds clarity and proportionality to depth perception.

FUNCTIONS OF THE RETINA

Photoreception

The discs of photoreceptor cells contain visual pigments, including rod rhodopsin. Rhodopsin (Fig. 10-5A) consists of a protein part (opsin) and a chromophore - 11-cis-retinal, which under the influence of photons turns into trance-retinal (photoisomerization). When light quanta hit the outer segments in photoreceptor cells, the following events occur sequentially (Fig. 10-5B): activation of rhodopsin as a result of photoisomerization - catalytic activation of G protein (G t, transducin) by rhodopsin - activation of phosphodiesterase upon binding to G t a - hydrolysis cGMP by cGMP phosphodiesterase - transition of cGMP-dependent Na+ channels from an open to a closed state - hyperpolarization of the plasma membrane of a photoreceptor cell - signal transmission to bipolar cells.

Rice. 10-5. RHODOPSIN AND ACTIVATION OF ION CHANNELS. A. Opsin molecule contains 7 transmembrane alpha-helical regions. The filled circles correspond to the localization of the most common molecular defects. Thus, in one of the mutations, glycine in the second transmembrane region at position 90 is replaced by asparagine, which leads to congenital night blindness. B. Transmembrane protein rhodopsin and its connection with G-protein (transducin) in the plasmalemma of the photoreceptor cell. Rhodopsin, excited by photons, activates the G protein. In this case, guanosine diphosphate bound to the α-CE of the G protein is replaced by GTP. The cleaved α-CE and β-CE act on phosphodiesterase and cause it to convert cGMP into guanosine monophosphate. This closes the Na+ channels, and Na+ ions cannot enter the cell, which leads to its hyperpolarization. R - rhodopsin; α, β and γ - G protein CE; A - agonist (in this case light quanta); E - phosphodiesterase effector enzyme. B. Diagram of a stick. In the outer segment there is a stack of discs containing the visual pigment rhodopsin. The disc membrane and the cell membrane are separated. Light (hv) activates rhodopsin (Rh*) in the discs, which closes the β+ channels in the cell membrane and reduces the entry of Na+ into the cell

Ionic basis of photoreceptor potentials

❖ In the dark Na The + channels of the membrane of the outer segments of rods and cones are open, and current flows from the cytoplasm of the inner segments into the membranes of the outer segments (Fig. 10-5B and 10-6,I). The current also flows into the synaptic terminal of the photoreceptor, causing a constant release of the neurotransmitter. Na+,K+-

Figure 10-6. ELECTRICAL REACTIONS OF THE RETINA. I. Photoreceptor response to illumination. II. Ganglion cell responses. Illuminated fields are shown in white. III. Local potentials of retinal cells. P - sticks; GC - horizontal cells; B - bipolar cells; AK - amacrine cells; G - ganglion cells

the pump located in the inner segment maintains ionic equilibrium by compensating the Na+ output with the K+ input. Thus, in the dark, ion channels are kept open and the flows into the cell of Na+ and Ca 2+ through open channels provide the appearance of current (dark current). ABOUT In the light those. when light excites the outer segment, the Na + channels close and a hyperpolarizing receptor potential. This potential, which appears on the membrane of the outer segment, extends to the synaptic ending of the photoreceptor and reduces the release of the synaptic transmitter - glutamate. This immediately leads to the appearance of APs in the axons of ganglion cells. This way

zom, hyperpolarization of the plasmalemma- a consequence of ion channel closure.

ABOUTReturn to original state. Light, which causes a cascade of reactions that lower the concentration of intracellular cGMP and leads to the closure of sodium channels, reduces the content of not only Na+, but also Ca2+ in the photoreceptor. As a result of a decrease in Ca 2 + concentration, the enzyme is activated guanylate cyclase, synthesizing cGMP, and the content of cGMP in the cell increases. This leads to inhibition of the functions of light-activated phosphodiesterase. Both of these processes - an increase in cGMP content and inhibition of phosphodiesterase activity - return the photoreceptor to its original state and open Na+ channels.

Light and dark adaptation

❖ Light adaptation. If a person is exposed to bright lighting for a long time, then a significant portion of the visual pigments are converted into retinal and opsin in the rods and cones. Most of the retinal is converted into vitamin A. All this leads to a corresponding decrease in the sensitivity of the eye, called light adaptation.

❖ Dark adaptation. On the contrary, if a person remains in the dark for a long time, then vitamin A is converted back into retinal, retinal and opsin form visual pigments. All this leads to increased sensitivity of the eye - dark adaptation.

Electrical responses of the retina

Various cells of the retina (photoreceptors, bipolar, horizontal, amacrine, as well as the dendritic zone of ganglion neurons) generate local potentials, but not PD (Fig. 10-6). Of all retinal cells PD arise only in the axons of ganglion cells. Total electrical potentials of the retina - electroretinogram(ERG). ERG is recorded as follows: one electrode is placed on the surface of the cornea, the other on the skin of the face. ERG has several waves associated with the excitation of various retinal structures and collectively reflects the intensity and duration of light exposure. ERG data can be used for diagnostic purposes in retinal diseases

Neurotransmitters. Retinal neurons synthesize acetylcholine, dopamine, Z-glutamic acid, glycine, γ-aminobutyric acid (GABA). Some neurons contain serotonin, its analogues (indolamines) and neuropeptides. Rods and cones in

synapses with bipolar cells secrete glutamate. Various amacrine cells secrete GABA, glycine, dopamine, acetylcholine and indoleamine, which have inhibitory effects. Neurotransmitters for bipolar and horizontal have not been identified.

Local potentials. The responses of rods, cones and horizontal cells are hyperpolarizing (Fig. 10-6, II), the responses of bipolar cells are either hyperpolarizing or depolarizing. Amacrine cells create depolarizing potentials.

Functional features of retinal cells

Visual images. The retina is involved in the formation of three visual images. First image formed under the influence of light at the level of photoreceptors, turns into second image at the level of bipolar cells, in ganglion neurons it is formed third image. Horizontal cells also take part in the formation of the second image, and amacrine cells are involved in the formation of the third.

Lateral inhibition- a way to enhance visual contrast. Lateral inhibition is the most important element of the activity of sensory systems, allowing the enhancement of contrast phenomena in the retina. In the retina, lateral inhibition is observed in all neural layers, but for horizontal cells it is their main function. Horizontal cells laterally synapse with the synaptic sites of rods and cones and with the dendrites of bipolar cells. A mediator is released at the ends of horizontal cells, which always has an inhibitory effect. Thus, lateral contacts of horizontal cells ensure the occurrence of lateral inhibition and the transmission of the correct visual pattern to the brain.

Receptive fields. In the retina, for every 100 million rods and 3 million cones, there are about 1.6 million ganglion cells. On average, 60 rods and 2 cones converge per ganglion cell. There are large differences between the peripheral and central retina in the number of rods and cones converging on ganglion neurons. At the periphery of the retina, photoreceptors associated with one ganglion cell form its receptive field. Overlapping the receptive fields of different ganglion cells allows for increased light sensitivity at low spatial resolution. As you approach the central fossa, the ratio of rods and

The cone ganglion cells become more organized, with only a few rods and cones per nerve fiber. In the area of the fovea, only cones remain (about 35 thousand), and the number of optic nerve fibers emerging from this area is equal to the number of cones. This creates a high degree of visual acuity compared to the relatively poor visual acuity in the periphery of the retina. In Fig. 10-6,II shows: on the left - diagrams of receptive fields illuminated in the center and along the periphery of the circle, on the right - diagrams of the frequency of APs arising in the axons of ganglion nerve cells in response to illumination. Under central illumination, the excited receptive field causes lateral inhibition along the periphery: in the upper figure on the right, the frequency of impulses in the center is much higher than at the edges. When the receptive field is illuminated along the edges of the circle, impulses are present at the periphery and absent in the center. Ganglion cells of different types. Ganglion cells at rest generate spontaneous potentials with a frequency of 5 to 40 Hz, which are superimposed by visual signals. Several types of ganglion neurons are known.

❖ W cells(perikaryon diameter<10 мкм, скорость проведения ПД 8 м/сек) составляют 40% от общего числа всех ганглиозных клеток. W-клетки имеют обширное рецептивное поле, они получают сигналы от палочек, передаваемые биполярными и амакринными клетками, и ответственны за сумеречное зрение.

❖ X cells(diameter 10-15 µm, conduction speed about 14 m/sec, 55%) have a small receptive field with discrete localization. They are responsible for the transmission of the visual image as such and all types of color vision.

❖ Y cells(diameter >35 µm, conduction velocity >50 m/sec, 5%) - the largest ganglion cells - have an extensive dendritic field and receive signals from various areas of the retina. Y cells respond to rapid changes in visual images, rapid movements in front of the eyes, and rapid changes in light intensity. These cells instantly signal to the central nervous system when a new visual image suddenly appears in any part of the visual field.

❖ on- and off-responses. Many ganglion neurons are excited by changes in light intensity. There are two types of responses: an on-response to turning on the light and an off-response to turning off the light. These different types of responses appear accordingly.

specifically from depolarized or hyperpolarized bipolars.

Color vision

Characteristics of color. Color has three main indicators: tone(shade), intensity And saturation. For each color there is additional(complementary) color which, when properly mixed with the original color, gives the appearance of white. Black color is the sensation created by the absence of light. The perception of white, any color of the spectrum, and even additional colors of the spectrum can be achieved by mixing red (570 nm), green (535 nm) and blue (445 nm) colors in various proportions. Therefore, red, green and blue - primary (primary) colors. The perception of color depends to some extent on the color of other objects in the visual field. For example, a red object will appear red if the field is illuminated with green or blue, and the same red object will appear pale pink or white if the field is illuminated with red.

Color perception- function of cones. There are three types of cones, each containing only one of three different (red, green and blue) visual pigments.

Trichromasia- the ability to distinguish any colors - is determined by the presence in the retina of all three visual pigments (for red, green and blue - primary colors). These fundamentals of the theory of color vision were proposed by Thomas Young (1802) and developed by Hermann Helmholtz.

NERVE PATHWAYS AND CENTERS

Visual pathways

The visual pathways are divided into old system where the midbrain and the base of the forebrain belong, and new system(to transmit visual signals directly to the visual cortex located in the occipital lobes). The new system is actually responsible for the perception of all visual images, color and all forms of conscious vision.

Main pathway to visual cortex(new system). Axons of ganglion cells in the optic nerves and (after the chiasm) in the optic tracts reach the lateral geniculate body (LCT, Fig. 10-7A). In this case, the fibers from the nasal half of the retina in the optic chiasm do not pass to the other side.

Figure 10-7. Visual pathways (A) and cortical centers (B). A. The areas of transection of the visual pathways are indicated by capital letters, and the visual defects that occur after transection are shown on the right. PP - optic chiasm. LCT - lateral geniculate body. KSHV - geniculate-spur fibers. B. The medial surface of the right hemisphere with the projection of the retina in the area of the calcarine sulcus

Well. In the left LCT (ipsilateral eye), fibers from the nasal half of the retina of the left eye and fibers from the temporal half of the retina of the right eye synaptically contact LCT neurons, the axons of which form the geniculate calcarine tract (optic radiance). The geniculate calcarine fibers pass to the primary visual cortex of the same side. The paths from the right eye are organized similarly.

Other ways(old system). Axons of retinal ganglion neurons also pass to some ancient areas of the brain: ❖ to the supracrossus nuclei of the hypothalamus (control and synchronization of circadian rhythms); ❖ in the tegmental nuclei (reflexive eye movements when focusing an object, activation of the pupillary reflex); ❖ in the superior colliculus (control of rapid directed movements of both eyes); ❖ in the LCT and surrounding areas (control of behavioral reactions).

Lateral geniculate body(LCT) is part of the new visual system, where all the fibers passing through the optic tract end. LCT performs the function of transmitting information

from the optic tract to the visual cortex, precisely preserving the topology (spatial location) of different levels of paths from the retina (Fig. 10-7B). Another function of the LCT is to control the amount of information reaching the cortex. Signals for the implementation of LCT input control enter the LCT in the form of feedback impulses from the primary visual cortex and from the reticular area of the midbrain.

Visual cortex

The primary visual receptive area is located on the corresponding side of the calcarine sulcus (Fig. 10-7B). Like other parts of the neocortex, the visual cortex consists of six layers, the fibers of the geniculate calcarine tract ending predominantly in layer IV neurons. This layer is divided into sublayers that receive fibers from ganglion cells of type Y and X. In the primary visual cortex (Brodmann area 17) and visual area II (area 18), the three-dimensional arrangement of objects, the size of objects, the detail of objects and their color, and movement are analyzed objects, etc.

Columns and stripes. The visual cortex contains several million vertical primary columns, each column has a diameter of 30 to 50 μm and contains about 1000 neurons. Neuronal columns form intertwined strips 0.5 mm wide.

Color columnar structures. Among the primary visual columns, secondary areas are distributed - column-like formations (“color clots”). “Color clumps” receive signals from adjacent columns and are specifically activated by color signals.

Interaction of visual signals from the two eyes. Visual signals entering the brain remain separate until they enter layer IV of the primary visual cortex. Signals from one eye enter the columns of each strip, and the same happens with signals from the other eye. During the interaction of visual signals, the visual cortex deciphers the location of two visual images, finds their corresponding points (points in the same areas of the retina of both eyes) and adapts the decoded information to determine the distance to objects.

Specialization of neurons. In the columns of the visual cortex there are neurons that perform very specific functions (for example, analysis of contrast (including color), boundaries and directions of lines of the visual image, etc.).

PROPERTIES OF THE VISUAL SYSTEM Eye movements

External muscles of the eyeball. Eye movements are carried out by six pairs of striated muscles (Fig. 10-8A), coordinated by the brain through the III, IV, VI pairs of cranial nerves. If the rectus lateralis muscle of one eye contracts, the rectus medialis muscle of the other eye contracts by the same amount. The rectus superioris muscles work together to move the eyes back so that you can look up. The rectus inferior muscles enable you to look down. The superior oblique muscle rotates the eye downward and outward, and the inferior oblique muscle rotates the eye upward and outward.

ABOUT Convergence. The simultaneous and conjugal movement of both eyes allows, when looking at close objects, to bring them together (convergence).

ABOUT Divergence. Looking at distant objects leads to the separation of the visual axes of both eyes (divergence).

ABOUT Diplopia. Since the bulk of the visual field is binocular, it is clear that a high degree of coordination of movements of both eyes is necessary to maintain the visual image on the core.

Figure 10-8. External eye muscles. A. Ocular muscles of the left eye. B. Types of eye movements

responding points of both retinas and thereby avoid double vision (diplopia).

Types of movements. There are 4 types of eye movements (Fig. 10-8B).

ABOUT Saccades- imperceptible rapid jumps (in hundredths of a second) of the eye, tracing the contours of the image. Saccadic movements maintain the retention of the image on the retina, which is achieved by periodically shifting the image across the retina, resulting in the activation of new photoreceptors and new ganglion cells.

ABOUT Smooth Followers eye movements following a moving object.

ABOUT Converging movement - bringing the visual axes towards each other when viewing an object close to the observer. Each type of movement is controlled separately by the nervous apparatus, but ultimately all influences end on the motor neurons that innervate the external muscles of the eye.

ABOUT Vestibular eye movements are a regulatory mechanism that appears when the receptors of the semicircular canals are excited and maintains gaze fixation during head movements.

Physiological nystagmus. Even in conditions when the subject tries to fix a stationary object with his gaze, the eyeball continues to perform spasmodic and other movements (physiological nystagmus). In other words, the neuromuscular apparatus of the eye takes on the function of holding the visual image on the retina, since an attempt to hold the visual image motionless on the retina leads to its disappearance from the field of vision. That is why the need to constantly keep an object in the field of view requires a constant and rapid shift of the visual image across the retina.

CRITICAL FLICKING FREQUENCY. The eye retains traces of light stimulation for some time (150-250 ms) after the light is turned off. In other words, the eye perceives intermittent light as continuous at certain intervals between flashes. The minimum repetition rate of light stimuli at which individual flickering sensations merge into a sensation of continuous light is the critical flickering fusion frequency (24 frames per second). Television and cinema are based on this phenomenon: a person does not notice the gaps between individual frames, since the visual sensation from one frame continues until the appearance of another. This creates the illusion of image continuity and movement.

Aqueous moisture

Aqueous humor is continuously produced and reabsorbed. The balance between the formation and reabsorption of aqueous humor regulates the volume and pressure of intraocular fluid. Every minute, 2 to 3 µl of aqueous humor is formed. This fluid flows between the ligaments of the lens and then through the pupil into the anterior chamber of the eye. From here, the fluid enters the angle between the cornea and the iris, penetrates between the network of trabeculae into Schlemm's canal and pours into the external veins of the eyeball. Normal intraocular pressure the average is 15 mm Hg. with fluctuations between 12 and 20 mm Hg. The level of intraocular pressure is maintained constant with fluctuations of ±2 mm and is determined by the resistance to outflow from the anterior chamber into Schlemm's canal when fluid moves between the trabeculae, in which there are passages of 1-2 μm.