|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
Lecture notes, cheat sheets
Histology. Muscle tissue. Cardiac and smooth muscle tissue
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 16. MUSCLE TISSUES. CARDIAC AND SMOOTH MUSCLE TISSUE Cardiac muscle tissue The structural and functional unit of the cardiac striated muscle tissue is the cardiomyocyte. Based on their structure and function, cardiomyocytes are divided into two main groups: 1) typical (or contractile) cardiomyocytes, which together form the myocardium; 2) atypical cardiomyocytes that make up the conduction system of the heart. A contractile cardiomyocyte is an almost rectangular cell 50–120 µm long and 15–20 µm wide, usually with one nucleus in the center. Covered on the outside by a basal plate. In the sarcoplasm of the cardiomyocyte, myofibrils are located on the periphery of the nucleus, and between them and near the nucleus there are a large number of mitochondria - sarcosomes. Unlike skeletal muscles, myofibrils of cardiomyocytes are not separate cylindrical formations, but, in essence, a network consisting of anastomosing myofibrils, since some myofilaments seem to split off from one myofibril and continue obliquely into another. In addition, the dark and light disks of neighboring myofibrils are not always located at the same level, and therefore the transverse striation in cardiomyocytes is practically not pronounced compared to striated muscle tissue. The sarcoplasmic reticulum, covering the myofibrils, is represented by dilated anastomosing tubules. Terminal tanks and triads are absent. T-tubules are present, but they are short, wide, and are formed not only by depressions in the plasmalemma, but also in the basal lamina. The mechanism of contraction in cardiomyocytes practically does not differ from the striated skeletal muscles. Contractile cardiomyocytes, connecting end-to-end with each other, form functional muscle fibers, between which there are numerous anastomoses. Due to this, a network (functional syncytium) is formed from individual cardiomyocytes. The presence of such slit-like contacts between cardiomyocytes ensures their simultaneous and friendly contraction, first in the atria, and then in the ventricles. The contact areas of neighboring cardiomyocytes are called intercalated discs. In fact, there are no additional structures between cardiomyocytes. Intercalated discs are sites of contact between the cytolemmas of adjacent cardiomyocytes, including simple, desmosomal, and slit-like junctions. Intercalated discs are divided into transverse and longitudinal fragments. In the region of transverse fragments, there are extended desmosomal junctions; actin filaments of sarcomeres are attached to the same place on the inner side of the plasmolemma. Slot-like contacts are localized in the region of longitudinal fragments. Through the intercalated disks, both mechanical, metabolic, and functional connections of cardiomyocytes are provided. The contractile cardiomyocytes of the atria and the ventricle differ somewhat in morphology and function. Atrial cardiomyocytes in the sarcoplasm contain fewer myofibrils and mitochondria, T-tubules are almost not expressed in them, and instead of them, vesicles and caveolae, analogues of T-tubules, are detected in a large number under the plasmolemma. In the sarcoplasm of atrial cardiomyocytes, at the poles of the nuclei, specific atrial granules are localized, consisting of glycoprotein complexes. Released from cardiomyocytes into the blood of the atria, these biologically active substances affect the level of pressure in the heart and blood vessels, and also prevent the formation of intra-atrial thrombi. Thus, atrial cardiomyocytes have contractile and secretory functions. In ventricular cardiomyocytes, contractile elements are more pronounced, and secretory granules are absent. Atypical cardiomyocytes form the conduction system of the heart, which includes the following structural components: 1) sinus node; 2) atrioventricular node; 3) atrioventricular bundle (His bundle) - trunk, right and left legs; 4) terminal branching of the legs (Purkinje fibers). Atypical cardiomyocytes provide the generation of biopotentials, their behavior and transmission to contractile cardiomyocytes. In morphology, atypical cardiomyocytes differ from typical ones: 1) they are larger - 100 microns, thickness - up to 50 microns; 2) the cytoplasm contains few myofibrils, which are randomly arranged, which is why atypical cardiomyocytes do not have transverse striation; 3) the plasmalemma does not form T-tubules; 4) in the intercalated discs between these cells, there are no desmosomes and gap-like junctions. Atypical cardiomyocytes of different parts of the conducting system differ from each other in structure and function and are divided into three main varieties: 1) P-cells - pacemakers - type I pacemakers; 2) transitional - type II cells; 3) cells of the bundle of His and Purkinje fibers - type III cells. Type I cells are the basis of the sinoatrial node, and are also contained in a small amount in the atrioventricular node. These cells are able to independently generate bioelectric potentials with a certain frequency, as well as transmit them to type II cells with subsequent transmission to type III cells, from which biopotentials are distributed to contractile cardiomyocytes. The sources of development of cardiomyocytes are myoepicardial plates, which are certain areas of visceral splanchiotomes. Innervation of cardiac muscle tissue. Contractile cardiomyocytes receive biopotentials from two sources: 1) from the conducting system (primarily from the sinoatrial node); 2) from the autonomic nervous system (from its sympathetic and parasympathetic parts). Regeneration of cardiac muscle tissue. Cardiomyocytes regenerate only according to the intracellular type. Proliferation of cardiomyocytes is not observed. There are no cambial elements in cardiac muscle tissue. If significant areas of the myocardium are damaged (for example, necrosis of significant areas in myocardial infarction), the defect is restored due to the growth of connective tissue and the formation of a scar - plastic regeneration. At the same time, the contractile function of this area is absent. The defeat of the conducting system is accompanied by the appearance of rhythm and conduction disturbances. Smooth muscle tissue of mesenchymal origin It is localized in the walls of hollow organs (stomach, intestines, respiratory tract, organs of the genitourinary system) and in the walls of blood and lymphatic vessels. The structural and functional unit is the myocyte - a spindle-shaped cell, 30 - 100 microns long (up to 500 microns in the pregnant uterus), 8 microns in diameter, covered with a basal plate. In the center of the myocyte, an elongated rod-shaped nucleus is localized. Common organelles are located along the poles of the nucleus: mitochondria (sarcosomes), elements of the granular endoplasmic reticulum, lamellar complex, free ribosomes, centrioles. The cytoplasm contains thin (7 nm) and thicker (17 nm) filaments. The thin filaments are made up of the protein actin, and the thick filaments are made up of myosin, and are mostly arranged parallel to the actin filaments. However, together actin and myosin filaments do not form typical myofibrils and sarcomeres, so there is no transverse striation in myocytes. In the sarcoplasm and on the inner surface of the sarcolemma, electron-microscopically, dense bodies are determined, in which actin filaments end and which are considered as analogues of Z-bands in the sarcomeres of skeletal muscle fiber myofibrils. Fixation of myosin components to specific structures has not been established. Myosin and actin filaments make up the contractile apparatus of the myocyte. Due to the interaction of actin and myosin filaments, actin filaments slide along myosin filaments, bring together their points of attachment on the dense bodies of the cytolemma, and shorten the length of the myocyte. It has been established that, in addition to actin and myosin filaments, myocytes also contain intermediate ones (up to 10 nm), which are attached to cytoplasmic dense bodies, and with other ends to the cytolemma and transmit the contraction forces of the centrally located contractile filaments to the sarcolemma. With the contraction of the myocyte, its contours become uneven, the shape is oval, and the nucleus twists in a corkscrew shape. For the interaction of actin and myosin filaments in the myocyte, as well as in the skeletal muscle fiber, energy is needed in the form of ATP, calcium ions and biopotentials. ATP is produced in mitochondria, calcium ions are contained in the sarcoplasmic reticulum, which is presented in a reduced form in the form of vesicles and thin tubules. Under the sarcolemma there are small cavities - caveolae, which are considered as analogues of T-tubules. All these elements ensure the transfer of biopotentials to the vesicles in the tubules, the release of calcium ions, the activation of ATP, and then the interaction of actin and myosin filaments. The basal plate of the myocyte consists of thin collagen, reticulin and elastic fibers, as well as an amorphous substance, which are the product of the synthesis and secretion of the myocytes themselves. Consequently, the myocyte has not only a contractile, but also a synthetic and secretory function, especially at the stage of differentiation. The fibrillar components of the basal plates of neighboring myocytes connect to each other and thereby unite individual myocytes into functional muscle fibers and functional syncytia. However, between myocytes, in addition to the mechanical connection, there is also a functional connection. It is provided with the help of slot-like contacts, which are located in places of close contact of myocytes. In these places, the basal plate is absent, the cytolemmas of neighboring myocytes approach each other and form slit-like contacts through which ion exchange is carried out. Thanks to mechanical and functional contacts, a friendly contraction of a large number of myocytes that are part of a functional muscle fiber, or syncytium, is ensured. Efferent innervation of smooth muscle tissue is carried out by the autonomic nervous system. At the same time, the terminal branches of the axons of efferent autonomic neurons, passing over the surface of several myocytes, form small varicose thickenings on them, which somewhat bend the plasmalemma and form myoneural synapses. When nerve impulses enter the synaptic cleft, mediators - acetylcholine and norepinephrine - are released. They cause depolarization of the plasmolemma of myocytes and their contraction. However, not all myocytes have nerve endings. Depolarization of myocytes that do not have autonomic innervation is carried out through slit-like contacts from neighboring myocytes that receive efferent innervation. In addition, excitation and contraction of myocytes can occur under the influence of various biologically active substances (histamine, serotonin, oxytocin), as well as mechanical stimulation of an organ containing smooth muscle tissue. There is an opinion that, despite the presence of efferent innervation, nerve impulses do not induce contraction, but only regulate its duration and strength. The contraction of smooth muscle tissue is usually prolonged, which ensures the maintenance of the tone of hollow internal organs and blood vessels. Smooth muscle tissue does not form muscles in the anatomical sense of the word. However, in the hollow internal organs and in the wall of the vessels between the bundles of myocytes, there are layers of loose fibrous connective tissue that form a kind of endomysium, and between layers of smooth muscle tissue - perimysium. Regeneration of smooth muscle tissue is carried out in several ways: 1) through intracellular regeneration (hypertrophy with increased functional load); 2) through mitotic division of myocytes (proliferation); 3) through differentiation from cambial elements (from adventitial cells and myofibroblasts). Special smooth muscle tissue Among special smooth muscle tissues, tissues of neural and epidermal origin can be distinguished. Tissues of neural origin develop from the neuroectoderm, from the edges of the optic cup, which is a protrusion of the diencephalon. From this source, myocytes develop, forming two muscles of the iris of the eye - the muscle that narrows the pupil, and the muscle that expands the pupil. In their morphology, these myocytes do not differ from mesenchymal ones, but differ in their innervation. Each myocyte has autonomic innervation: the muscle that expands the pupil is sympathetic, and the muscle that narrows is parasympathetic. Due to this, the muscles contract quickly and in a coordinated manner, depending on the power of the light beam. Tissues of epidermal origin develop from the skin ectoderm and are star-shaped cells located in the terminal sections of the salivary, mammary and sweat glands, outside the secretory cells. In its processes, the myoepithelial cell contains actin and myosin filaments, due to which the processes of the cells contract and contribute to the release of secretions from the terminal sections and small ducts into larger ones. These myocytes also receive efferent innervation from the autonomic nervous system. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Muscle tissues. Skeletal muscle tissue >> Forward: nervous tissue
The existence of an entropy rule for quantum entanglement has been proven
09.05.2024 Mini air conditioner Sony Reon Pocket 5
09.05.2024 Energy from space for Starship
08.05.2024
▪ Prosthetic hand sense of touch ▪ Magnetic spray creates robots ▪ Dell Venue 7 and Venue 8 Android Tablets ▪ Fires in the Amazonian forests have accelerated the melting of glaciers in the Andes
▪ section of the site Application of microcircuits. Article selection ▪ article Oh, it's not an easy job to drag a hippopotamus out of the swamp! Popular expression ▪ Do sleepwalkers sleepwalk on full moon nights? Detailed answer ▪ article Working with an electric drill. Standard instruction on labor protection ▪ article Doorbell Patriot. Encyclopedia of radio electronics and electrical engineering
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
We recommend interesting articles Section 
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page