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Lecture notes, cheat sheets
Histology. Organ of vision
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 27. ORGANIZATION OF VIEW Sense organs are organs that perceive information from the environment, after which it is analyzed and human actions are corrected. The sense organs form sensory systems. The sensory system consists of three sections: 1) receptors. These are the peripheral nerve endings of the afferent nerves that receive information from the environment. Receptors include, for example, rods and cones in the organ of vision, neurosensory cells of the organ of Corti - in the organ of hearing, taste buds and buds of the tongue - in the organ of taste. 2) a pathway that includes the afferent processes of the neuron, along which the electrical impulse generated as a result of receptor stimulation is transmitted to the third section. 3) the cortical center of the analyzer. Organ of vision The organ of vision, like any analyzer, consists of three departments: 1) the eyeball, in which receptors are located - rods and cones; 2) conducting apparatus - the 2nd pair of cranial nerves - the optic nerve; 3) the cortical center of the analyzer, located in the occipital lobe of the cerebral cortex. Development of the organ of vision The rudiment of the eye appears in a 22-day-old embryo in the form of paired shallow intussusceptions - ophthalmic grooves in the forebrain. After the closure of the neuropores, the intussusceptions enlarge and optic vesicles form. Cells that are involved in the formation of the sclera and ciliary muscle are evicted from the neural crest, and also differentiate into endothelial cells and corneal fibroblasts. The eye vesicles are connected to the fetal brain by means of eye stalks. The eye vesicles come into contact with the ectoderm of the future facial part of the head and induce the development of the lens in it. Invagination of the optic vesicle wall leads to the formation of a two-layer optic cup. The outer layer of the eyecup forms the pigment layer of the retina. The inner layer forms the retina. The axons of differentiating ganglion cells grow into the optic stalk, after which they become part of the optic nerve. The choroid is formed from the mesenchymal cells surrounding the eye cup. The corneal epithelium develops from the ectoderm. The lens placode separates from the ectoderm and forms a lens vesicle, over which the ectoderm closes. With the development of the lens vesicle, the thickness of its walls changes, in connection with which a thin anterior epithelium and a complex of densely packed elongated spindle-shaped epithelial cells appear - lens fibers located on the posterior surface. The lens fibers elongate and fill the cavity of the vesicle. In the epithelial cells of the lens, proteins special for the lens are synthesized - crystallins. At the initial stages of lens differentiation, a small amount of alpha and beta crystallins are synthesized. As the lens develops, in addition to these two proteins, gamma crystallins begin to be synthesized. The structure of the eyeball The wall of the eyeball consists of three shells - outer - fibrous shell (in the back surface it is an opaque sclera, which in front of the eyeball passes into a transparent cornea), middle shell - vascular, inner shell - retina. The structure of the cornea The cornea is the anterior wall of the eyeball, transparent. Posteriorly, the transparent cornea passes into the opaque sclera. The boundary of their transition into each other is called the limb. On the surface of the cornea is a film consisting of the secret of the lacrimal and mucous glands, which includes lysozyme, lactoferrin and immunoglobulins. The surface of the cornea is covered with stratified squamous nonkeratinized epithelium. The anterior limiting membrane (or Bowman's membrane) is a layer having a thickness of 10 to 16 microns, not containing cells. The anterior limiting membrane consists of the ground substance, as well as thin collagen and reticular fibers that take part in maintaining the shape of the cornea. The corneal proper substance consists of regularly arranged collagen plates, flattened fibroblasts embedded in a matrix of complex sugars, including keratin and chondroetin sulfate. The posterior limiting membrane (or Descement's membrane) is a transparent layer of the cornea, it is located between the corneal own substance and the endothelium of the posterior surface of the cornea. This layer consists of collagen fibers of the seventh type and an amorphous substance. The corneal endothelium limits the anterior chamber of the eye in front. The structure of the sclera The sclera is the opaque outer layer of the eyeball. The sclera consists of dense strands of collagen fibers, between which are flattened fibroblasts. At the junction of the sclera and cornea, there are small, communicating with each other cavities, which together form the Schlemm canal (or venous sinus) of the sclera, which ensures the outflow of intraocular fluid from the anterior chamber of the eye. The sclera of an adult has a fairly high resistance to increased intraocular pressure. However, there are separate areas of thinning of the sclera, especially in the limbus. In children, the sclera is poorly resistant to stretching, therefore, with an increase in intraocular pressure, the size of the eyeball increases significantly. The thinnest place of the sclera is the region of the ethmoid sinus. The bundles of optic nerve fibers pass through the opening of the cribriform plate. The optic nerve fibers pass through holes in the lamina cribrosa. The structure of the choroid The main function of the choroid is to nourish the retina. The choroid consists of several layers - supravascular, choriocapillary and basal plates. The supravascular membrane is located on the border with the sclera and consists of loose fibrous connective tissue with numerous pigment cells. The choroid plate contains plexuses of arteries and veins, consists of loose connective tissue, in which pigment cells and smooth muscle fibers are located. The choriocapillary plate is formed by a plexus of sinusoidal capillaries. The basal plate is located on the border of the choroid and the retina. In front of the eye, the choroid forms the iris and the ciliary body. The structure of the iris The iris is a continuation of the choroid, located between the cornea and the lens, separating the anterior and posterior chambers of the eye. The iris consists of several layers - the endothelial (or anterior), vascular outer, and inner boundary layers, as well as the pigment layer. The endothelium is a continuation of the endothelium of the cornea. The outer and inner boundary layers have a similar structure, contain fibroblasts, melonocytes, immersed in the ground substance. The vascular layer is a loose fibrous connective tissue that contains numerous vessels and melanocytes. The posterior pigment layer passes into the two-layer retinal epithelium, which covers the ciliary body. The iris contains muscles that constrict and dilate the pupil. When the parasympathetic nerve fibers are irritated, the pupil constricts, and when the sympathetic nerves are irritated, it expands. The structure of the ciliary body In the region of the corner of the eye, the choroid thickens, forming the ciliary body. On the cut, it has the form of a triangle with its base turned into the anterior chamber of the eye. The ciliary body consists of muscle fibers - the ciliary muscle involved in the regulation of accommodation of the eye. Smooth muscle fibers located in the ciliary muscle run in three mutually perpendicular directions. Ciliary processes extend from the ciliary body towards the lens of the eye. They contain a mass of capillaries, covered with two layers of epithelium - pigment and ciliary secretory, which produces aqueous humor. The ligament of cinnamon is attached to the ciliary processes. When the ciliary muscle contracts, the zinn ligament relaxes and the lens convexity increases. The structure of the lens The lens is a biconvex lens. The anterior surface of the lens is formed by a single layer of cuboidal epithelium, which becomes higher towards the equator. There are slit-like junctions between the epithelial cells of the lens. The lens consists of thin lens fibers that make up its bulk and contain crystallins. Outside, the lens is covered with a capsule - a thick basement membrane with a significant content of reticular fibers. The chambers of the eye, the movement of intraocular fluid The eye has two chambers - anterior and posterior. The anterior chamber of the eye is a space bounded in front by the cornea, behind by the iris, and in the pupil area by the central part of the anterior surface of the lens. The depth of the anterior chamber of the eye is greatest in the central part, where it reaches 3 mm. The angle between the posterior surface of the peripheral part of the cornea and the anterior surface of the root of the iris is called the angle of the anterior chamber of the eye. It is located in the transition region of the sclera to the cornea, as well as the iris - to the ciliary body. The posterior chamber of the eye is the space behind the iris, bounded by the lens, ciliary and vitreous body. Intraocular fluid is formed in the posterior chamber of the eye from the capillaries and epithelium of the ciliary processes. From the posterior chamber of the eye between the iris and the lens, it passes into the anterior chamber. In terms of composition, intraocular fluid consists of blood plasma proteins, depolymerized hyaluronic acid, is hypertonic in relation to blood plasma and does not contain fibrinogen. From the elements of the iris, cornea and vitreous body, a trabecula is formed, which forms the posterior wall of the Schlemm's canal. It is extremely important for the outflow of moisture from the anterior chamber of the eye. From the trabecular meshwork, moisture flows into the canal of Schlemm and is then absorbed into the venous vessels of the eye. The balance between the formation and absorption of aqueous humor forms and determines the amount of intraocular pressure. A hematotissue barrier is formed between the blood and tissues of the eye. The cells of the ciliary epithelium are tightly interconnected by strong contacts and do not allow macromolecules to pass through. The structure of the vitreous body Between the lens and the retina there is a cavity filled with one of the transparent media of the eye - the vitreous body. According to its structure, the vitreous body is a gel consisting of water, collagen, the second, ninth and eleventh types, vitrein protein and hyaluronic acid. The vitreous body is enclosed in a vitreous membrane, which is an accumulation of collagen fibers that form the vitreous capsule. A canal passes through the vitreous body in the direction from the lens to the retina - the remnant of the embryonic system of the eye. Structure, functions of the retina The retina (or retina) is the inner lining of the eye. It consists of two sections - the visual, where the photoreceptors are located, and the blind. At the posterior edge of the optical axis of the eye, the retina has a round yellow spot about 2 mm in diameter. The central fovea of the retina is located in the middle part of the macula. This is the place of the best perception of the image by the eye. The optic nerve exits the retina medially to the macula, forming the optic papilla. There are no photoreceptors at the point of exit of the optic nerve in the retina, the perception of the image in this place of the retina does not occur, therefore it is called the blind spot. In the center of the optic nerve head, you can see a recess in which the retinal supply vessels exiting the optic nerve can be seen. The pigment layer of the retina is the outermost, facing the vitreous body, contains polygonal cells adjacent to the choroid. One cell of the pigment epithelium interacts with the outer segments of a dozen photoreceptor cells - rods and cones. The cells of the pigment epithelium contain reserves of vitamin A, participate in its transformations and transfer its derivatives to photoreceptors for the formation of visual pigment. The outer nuclear layer includes the nucleated parts of the photoreceptor cells. Cones are most concentrated in the area of the macula and provide color vision. In this case, the eyeball is arranged in such a way that the central part of the light displayed from any object falls on the cones. Along the periphery of the retina are rods, the main function of which is the perception of signals in twilight lighting. The outer reticular layer is the point of contact between the inner segments of rods and cones and the processes of bipolar cells. inner nuclear layer. The bodies of bipolar cells are located in this layer. Bipolar cells have two processes. With the help of one - short - they communicate between the bodies and photoreceptors, and with the help of long ones - with ganglion cells. Thus, bipolar cells are the link between photoreceptors and ganglion cells. This layer also contains horizontal and amacrine cells. The inner reticular layer is the layer in which the processes of bipolar and ganglion cells contact, while amacrine cells act as intercalary neurons. It is currently believed that one type of bipolar cell transmits information to 16 types of ganglion cells with the participation of 20 types of amacrine cells. The ganglion layer contains ganglion cell bodies. It has been established that many photoreceptor cells transmit a signal to one bipolar cell, and several bipolar cells to one ganglion cell, i.e., the number of cells in the layers of the retina gradually decreases, and the amount of information received by one cell increases. The photoreceptors in the retina include rods and cones. It has been established that cones are predominantly located in the region of the macula and the central fossa of the retina. In this case, one cone makes one connection with one bipolar cell, which ensures the reliability of the transmission of the visual signal. The photoreceptors contain the visual pigment. In rods it is rhodopsin, and in cones it is red, green and blue pigments. Photoreceptors have outer and inner segments. The outer segment contains the visual pigment and faces the choroid. The inner segment is filled with mitochondria and contains a basal body, from which 9 pairs of microtubules extend into the outer segment. The main function of the cones is the perception of color, while there are three types of visual pigment, the main function of the rods is the perception of the shape of an object. The theory of color vision was proposed in 1802 by Thomas Young. At the same time, color vision in humans in this theory was explained by the presence of three types of visual pigment. This ability to distinguish any color, determined by the presence of three types of cones in the retina, is called trichromasia. In humans, defects in color perception are possible, dichromasia from colors is not perceived by the photoreceptors of the retina. The structure of retinal neurons and glial cells Retinal neurons synthesize acetylcholine, dopamine, glycine, α-aminobutyric acid. Some neurons contain serotonin and its analogues. The layers of the retina contain horizontal and amacrine cells. Horizontal cells are located in the outer part of the inner nuclear layer, and the processes of these cells enter the area of synapses between photoreceptors and bipolar cells. Horizontal cells receive information from the cones and pass it on to the cones as well. Neighboring horizontal cells are interconnected by slot-like junctions. Amacrine cells are located in the inner part of the inner nuclear layer, in the area of synapses between bipolar and ganglion cells, while amacrine cells function as intercalary neurons. Bipolar cells respond to image contrast. Some of these cells respond more strongly to color than to black and white contrast. Some bipolar cells receive information mainly from rods, while others, on the contrary, receive information mainly from cones. In addition to neurons, the retina also contains large cells of radial glia - Müller cells. Their nuclei are located at the level of the central part of the inner nuclear layer. The outer processes of these cells end in villi, thus forming a boundary layer. The internal processes have an extension (or stalk) in the inner boundary layer at the border with the vitreous body. Glial cells play an important role in the regulation of retinal ion homeostasis. They reduce the concentration of potassium ions in the extracellular space, where their concentration significantly increases when irritated by light. The plasma membrane of the Müllerian cell in the region of the stem is characterized by high permeability to potassium ions leaving the cell. The Müllerian cell captures potassium from the outer layers of the retina and directs the flow of these ions through its stalk into the vitreous fluid. Mechanism of photoperception When a light quantum hits the outer segments of photoreceptor cells, the following reactions sequentially occur: activation of rhodopsin and photoisomerization, catalytic reaction of G-protein by rhodopsin, activation of phosphodiesterase upon binding to a protein, hydrolysis of cGMP, transition of cGMP-dependent sodium channels from an open state to a closed state, as a result resulting in hyperpolarization of the plasmolemma of the photoreceptor cell and signal transmission to bipolar cells. An increase in the activity of cGMP-phosphodiestrase reduces the concentration of cGMP, which leads to the closure of ion channels and hyperpolarization of the plasmolemma of the photoreceptor cell. This serves as a signal for a change in the nature of transmitter secretion in the synapse between the inner segment of the receptor cell and the dendrite of the bipolar cell. In the dark, ion channels in the cell membrane of receptor cells are maintained open by binding of ion channel proteins to cyclic GMP. The ducts of sodium and calcium ions into the cell through open channels provide a dark current. The structure of the lacrimal gland The lacrimal gland is an auxiliary apparatus of the eye. The gland is surrounded by a group of complex tubular-alveolar glands, the secretory sections are surrounded by myoepithelial cells. The secret of the gland (tear fluid) through 6-12 ducts enters the fornix of the conjunctiva. From the lacrimal sac through the nasolacrimal canal, the lacrimal fluid enters the lower nasal passage. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Female reproductive system >> Forward: Organs of taste and smell
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