Retina

The retina (retina, reticular membrane) is the peripheral part of the visual analyzer. It develops from the anterior part of the cerebral bladder, so it can be considered part of the brain, which is carried to the periphery.

The inner (sensitive) shell of the eyeball (tunica interna, s. Sensoria bulbi), or the retina, fits snugly from the inside to the vascular wall all the way from the point of exit of the optic nerve to the edge of the pupil. In the retina developing from the wall of the anterior cerebral bladder, two layers (leaf) are isolated: the outer pigment part (pars pigmentosa), and the intricate internal photosensitive, called the nerve part (pars nervosa). Accordingly, the functions vschlyayut large posterior visual part of the retina (pars optica retinae), containing sensitive elements - rod-shaped and cone-shaped visual cells (rods and cones), and smaller - the "blind" part of the retina, devoid of rods and cones. The "blind" part of the retina (pars ciliaris retinae) combines the ciliary part of the retina (pars ciliaris retinae) and the iris of the retina (pars iridica retinae). The border between the visual and "blind" parts is a well-visible serrated tooth on the preparation of the opened eyeball (og serrata). It corresponds to the place of the transition of the vascular membrane proper to the ciliary circle of the choroid.

In the retina, the outer pigment part and the internal photosensitive nerve part are distinguished.

The first layer of pigment epithelium belongs to the Bruch-choroid membrane. Pigment cells surround photoreceptors. The cells of the pigment layer phagocytore the rejecting outer segments of the photoreceptors, transport metabolites, salts, oxygen, nutrients from the choroid to the photoreceptors and back.

Bruch's membrane separates the retinal pigment epithelium from the chorio capillaries and, according to electron microscopy, consists of 5 elements:

• Basal lamina of retinal pigment epithelium.
• The inner collagen layer.
• Thick layer of elastic fibers.
• External collagen layer.
• Basal plate of the outer layer of chorio capillaries.

Pigment epithelium of the retina is a single layer of hexagonal cells, the villous apparatus of which, surrounding the outer segments of the photoreceptors, ensures their renewal. Cell density of retinal pigment epithelium in the fovea is lower, they are larger and contain more large melanosomes than cells of retinal pigment epithelium of other parts of the retina. The relationship between retinal pigment epithelium and photoreceptors is less dense than between the retinal pigment epithelium and the underlying Bruch membrane. The conditional space between the retinal pigment epithelium and the sensory retina is called subretinal. The absence of fluid in the subretinal space is supported in two ways:

• Retinal pigment epithelial cells and an intermediate, dense binding complex (zonula occludentes) form an external hematoretinal barrier, which prevents the extracellular fluid from escaping from the chorioceillaries into the subretial space.
• Active transport of ions and water from the subretinal space.

The second layer is formed by the outer segments of photoreceptors, rods and cones. Sticks have a very high light sensitivity, so they provide twilight vision. In addition, the rods constitute the bulk of the cells of the neuroepithelium and are located throughout the retina to the boundaries of its optical part, providing peripheral vision. The cones perform a more subtle function of the eye: central, shaped vision and color perception. The cones are located mainly in the region of the central fovea of the yellow spot.

The third layer. In the direction toward the periphery, the number of cones decreases, and the number of rods increases. In the central fovea are some cones, then the cones are found amongst the rods, and in the peripheral zone of the mesh shell, cones are absent. That's why the central fossa has the best sharpness of the shaped vision, and the field of vision for the colors is much narrower than white.

The third layer, the outer border membrane, is the band of intercellular adhesions. It is called the final membrane of Verchor, since the outer segments of the rods and cones pass through the nest into the subretipal space (the space between the cone and stigma layer and the retinal pigment epithelium), where they are surrounded by a substance rich in mucopolysaccharides.

The fourth layer, the outer nuclear layer, is formed by the nuclei of photoreceptors.

The fifth layer - the outer plexiform (or mesh) - occupies an intermediate position between the outer and inner nuclear layers.

The sixth layer consists of bipolar cells, each of which has two processes. The cells of this layer connect two neurons: the first with the third. The number of bipolar cells is less than the number of rods, so one bipolar cell is connected by several cells of rod-shaped elements, while each cone has its bipolar cage. The nuclei of bipolar cells form the middle nuclear layer of the mesh shell.

The seventh layer - the inner plexiform layer - separates the inner nuclear layer from the layer of ganglion cells and consists of a tangle of intricately branched and intertwining neuronal processes. It delimits the internal vascular part of the retina from the external avascular, depending on the choroidal circulation of oxygen and nutrients.

The eighth layer is formed by ganglion cells. They are located in the same row with gaps, except for the zone immediately around the central fossa, where the layer of ganglion cells lies in 3-4 rows, so in this region it is thicker than the rest. The nuclei of the gaplio cells constitute the inner nuclear layer of the retina, the ganglion cells of the retina, like other cells of the retina, have a typical structure. These are round cells, rich in protoplasm, with a round nucleus and a well-developed chromatin structure. The thickness of the layer of ganglion cells decreases markedly as it moves away from the central fossa to the periphery. Around the fossa this layer consists of five rows of ganglion cells or more. In this area, each photoconceptor has a direct relationship with the bipolar and ganglion cells.

The ninth layer consists of axons of ganglion cells forming the nerve.

The tenth layer - the internal border membrane - covers the retina surface from the inside. It is the main membrane formed by the bases of the processes of Muller's neuroglial cells. These cells pass through all layers of the retina, have giant dimensions and perform support and isolated functions, carry out active transport of metabolites at different levels of the retina, participate in the generation of bioelectric currents. These cells completely fill the gap between the neurons of the retina and serve to separate their receptive surfaces.

Landmarks

1. Macula is a rounded zone at the posterior pole of the eye with a diameter of about 5.5 mm. Histologically it contains more than one layer of ganglion cells and xanthophilic pigment.
2. Fovea - a depression on the inner surface of the retina in the center of the macula with a diameter of 1.5 mm (1 diameter of the optic disc). Ophthalmoscopically has the form of an oval light reflex, the boundaries of which are due to the thickening of the retina and the internal border membrane.
3. Foveola with a diameter of 0.35 mm forms the central part of the fovea. This is the thinnest part of the retina, devoid of ganglion cells and consisting only of cones.
4. The foveal avascular zone is located within the fovea, protruding beyond the borders of the foveola. The diameter of the foveal avascular zone is variable, and it can be accurately determined only by fluorescent angiography.
5. Pit is a small depression in the very center of the foveola, corresponding to the foveolar reflex. The absence of a reflex can sometimes be interpreted as an early symptom of the disease.

Intranuclear layers of the retina consist of fibrous structures, processes and synaptic formations of nerve cells, as well as cells of the glial framework of the retina, whose fibers pass perpendicular to the layers through the entire thickness of the retina: from the outer boundary membrane separating the nuclei of the neuroepithelium from rod and cone endings, separating the retina from the vitreous.

The total number of rods is about 130 million. They are receptors of twilight vision. For rods characterized by the presence of a relatively small nucleus, surrounded by a rim of the cytoplasm. The rod dendrites have a radial orientation and are located between the processes of retinal pigment epithelium. The dendrite consists of the outer and inner segments (parts) connected by a cilium. The outer segment has a cylindrical shape. It consists of numerous double membranes superimposed on each other with the formation of closed disks. In the membranes of the outer segments there is rhodopsin, a visual pigment. Axon sticks ends in the outer reticular retinal layer with synapses with bipolar cells.

The number of cones in the retina is 6-7 million. They are receptors for day (color) vision. Unlike sticks of a cone of a larger size (up to 75 microns in length), they have a larger core. The outer segment of cones is represented by half-discs formed as a result of intussusception of the plasmolemma. Membranes of cone discs contain another visual pigment - iodopsin. In the inner segment of the cones, there is a cluster of mitochondria with a lipid droplet located here (ellipsoid). The axon of cones also forms synapses with dendrites of bipolar neurons.

In the posterior part of the retina at the bottom of the eyeball, an ophthalmoscope can see an off-white spot with a diameter of about 1.7 mm - the optic disc (discus nervi optici) with a raised edge in the form of a roller and a small depression (excavatio disci) in the center. The disk is the place where the optic nerve fibers leave the eyeball. The optic nerve is surrounded by shells (continuation of the brain envelopes) and is directed towards the visual channel opening into the cranial cavity. These membranes form the outer and inner vagina of the optic nerve (vagina externa et vagina inlerna n. Optici). The area of the optic disc is due to the lack of light-sensitive visual cells (rods and cones) called a blind spot. In the center of the disc, the central artery entering the retina (a. Centralis retinae) is visible. The lateral to the optic disc is approximately 4 mm, which corresponds to the posterior pole of the eye, is a yellowish spot (macula) with a small depression - the central fovea (fovea centralis). The central fossa is the place of the best vision: only cones are concentrated here, and the sticks are absent.

Functions of the retina - the transformation of light stimulation into neural excitation and primary signal processing.

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