Features of cardiac tissue. Cardiac muscle tissue structural features

cardiac muscle tissue forms the middle shell (myocardium) of the atria and ventricles of the heart and is represented by two varieties of working and conducting.

Working muscle tissue consists of cardiomyocyte cells, the most important feature of which is the presence of perfect contact zones. Connecting with each other, they form a structure similar to a muscle fiber with their end ends. On the lateral surfaces, cardiomyocytes have branches. Connecting ends with the branches of neighboring cardiomyocytes, they form anastomoses. The boundaries between the ends of neighboring cardiomyocytes are intercalated disks with straight or stepped contours. In a light microscope, they look like transverse dark stripes. With the help of intercalated discs and anastomoses, a single structural and functional contractile system was formed.

Electron microscopy revealed that in the area of ​​the intercalated discs, one cell protrudes into another with finger-like protrusions, on the side surfaces of which there are desmosomes, which ensures high adhesion strength. Slit-like contacts were found at the ends of the finger-like protrusions, through which nerve impulses quickly propagate from cell to cell without the participation of a mediator, synchronizing the contraction of cardiomyocytes.

Cardiac myocytes are mononuclear, sometimes binuclear cells. The nuclei are located in the center in contrast to skeletal muscle fibers. The perinuclear zone contains components of the Golgi apparatus, mitochondria, lysosomes, and glycogen granules.

The contractile apparatus of myocytes, as well as in skeletal muscle tissue, consists of myofibrils, which occupy the peripheral part of the cell. Their diameter is from 1 to 3 microns.

Myofibrils are similar to skeletal muscle myofibrils. They are also built from anisotropic and isotropic disks, which also causes transverse striation.

The plasmalemma of cardiomyocytes at the level of Z-bands invaginates into the depths of the cytoplasm, forming transverse tubules, which differ from skeletal muscle tissue in their large diameter and the presence of a basement membrane that covers them from the outside, like the sarcolemma. Depolarization waves traveling from the plasmolemma into cardiac myocytes cause sliding of actin myofilaments (protofibrils) in relation to myosin ones, causing contraction, as in skeletal muscle tissue.

T-tubules in cardiac working cardiomyocytes form dyads, that is, they are connected to the cisterns of the sarcoplasmic reticulum only on one side. Working cardiomyocytes have a length of 50-120 microns, a width of 15-20 microns. The number of myofibrils in them is less than in muscle fibers.

Cardiac muscle tissue contains a lot of myoglobin, which is why it is dark red in color. There are a lot of mitochondria and glycogen in myocytes, i.e.: the heart muscle tissue receives energy both from the breakdown of ATP and as a result of glycolysis. Thus, the heart muscle works continuously throughout life, due to the powerful energy equipment.

The intensity and frequency of contractions of the heart muscle are regulated by nerve impulses.

In embryogenesis, the working muscle tissue develops from special sections of the visceral sheet of non-segmented mesoderm (splanchnotome). In the formed working muscle tissue of the heart there are no cambial cells (myosatellites), therefore, if the myocardium is damaged in the injured area, cardiomyocytes die and fibrous connective tissue develops at the site of damage.

Conductive muscle tissue of the heart is part of a complex of formations of the sinoatrial node located at the mouth of the cranial vena cava, the atrioventricular node lying in the interatrial septum, the atrioventricular trunk (His bundle) and its branches, located under the endocardium of the interventricular septum and in the connective tissue layers myocardium.

All components of this system are formed by atypical cells, specialized either in generating an impulse that propagates throughout the heart and causes contraction of its departments in the required sequence (rhythm), or in conducting an impulse to working cardiomyocytes.

Atypical myocytes are characterized by a significant amount of cytoplasm, in which a few myofibrils occupy the peripheral part and do not have a parallel orientation, as a result of which these cells are not characterized by transverse striation. The nuclei are located in the center of the cells. The cytoplasm is rich in glycogen, but few in mitochondria, indicating intense glycolysis and low levels of aerobic oxidation. Therefore, cells of the conducting system are more resistant to oxygen starvation than contractile cardiomyocytes.

As part of the sinoatrial node, atypical cardiomyocytes are smaller, rounded. Nerve impulses are formed in them and they are among the main pacemakers. The myocytes of the atrioventricular node are somewhat larger, and the fibers of the His bundle (Purkinje fibers) consist of large rounded and oval myocytes with an eccentrically located nucleus. Their diameter is 2-3 times larger than the working cardiomyocytes. Electron-microscopically revealed that in atypical myocytes the sarcoplasmic reticulum is underdeveloped, there is no system of T-tubules. The cells are connected not only by the ends, but also by the side surfaces. Intercalated discs are simpler and do not contain finger-like junctions, desmosomes, or nexuses.

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.

Contractile cardiomyocyte It 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 a 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 transfer to type III cells, from which biopotentials are distributed to contractile cardiomyocytes.

Sources of development cardiomyocytes - myoepicardial plates, which are certain areas of visceral splanchiotomas.

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 a myocyte - a spindle-shaped cell, 30 - 100 microns long (up to 500 microns in a 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, while 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 efforts of contraction 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 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 the layers of smooth muscle tissue - perimysium.

Regeneration 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. Myocytes develop from this source, 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 a vegetative 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.

This tissue is localized in the muscular membrane of the heart (myocardium) and the mouths of the large vessels associated with it.

Functional Features

1) automatism,

2) rhythm,

3) involuntary,

4) low fatigue.

The activity of contractions is influenced by hormones and the nervous system (sympathetic and parasympathetic).

B.2.1. Histogenesis of cardiac muscle tissue

The source of development of cardiac muscle tissue is the myoepicardial plate of the visceral leaf of the splanchnotome. SCM (stem cells of myogenesis) are formed in it, differentiating into cardiomyoblasts, actively multiplying by mitosis. In their cytoplasm, myofilaments gradually form, forming myofibrils. With the advent of the latter, cells are called cardiomyocytes(or cardiac myocytes). The ability of human cardiomyocytes to complete mitotic division is lost by the time of birth or in the first months of life. Processes begin in these cells polyploidization. Cardiac myocytes line up in chains, but do not merge with each other, as happens during the development of a skeletal muscle fiber. Cells form complex intercellular connections - intercalated discs that bind cardiomyocytes in functional fibers(functional syncytium).

The structure of cardiac muscle tissue

As already noted, cardiac muscle tissue is formed by cells - cardiomyocytes, connected to each other in the region of the intercalated discs and forming a three-dimensional network of branching and anastomosing functional fibers.

Varieties of cardiomyocytes

1. contractile

1) ventricular (prismatic)

2) atrial (process)

2. cardiomyocytes of the conduction system of the heart

1) pacemakers (P-cells, pacemakers of the 1st order)

2) transient (pacers of the 2nd order)

3) conducting (pacemakers of the 3rd order)

3. secretory (endocrine)

Cardiomyocytes of the conduction system of the heart (PSS)

Irregular prismatic shape

Length 8-20 microns, width 2-5 microns

Weak development of all organelles (including myofibrils)

Intercalated discs have fewer desmosomes

Secretory (endocrine) cardiomyocytes

Process form

Length 15-20 microns, width 2-5 microns

General plan of the building (see above SKMC)

Export synthesis organelles developed

Many secretory granules

Myofibrils are poorly developed

Structural and functional apparatuses of cardiomyocytes

1. contractile apparatus(most developed in SKMC)

Introduced myofibrils , each of which consists of thousands of telophragms connected in series sarcomeres containing actinic e(thin) and myosin (thick) myofilaments. The end sections of myofibrils are attached from the side of the cytoplasm to the intercalated discs with the help of sticking strips(splitting and weaving of actin filaments into the submembrane regions of the myocyte plasmolemma

Provides a strong rhythmic energy-intensive calcium-dependent contraction ↔ relaxation ("sliding thread model")

2. transport apparatus(developed in SKMC) - similar to that in skeletal muscle fibers

3. support apparatus

Submission n sarcolemma, intercalated discs, adhesion strips, anastomoses, cytoskeleton, telophragms, mesophragms.

Provides shaping, frame, locomotor and integration functions.

4. Trophy-energy apparatus - presented sarcosomes and inclusions of glycogen, myoglobin and lipids.

5. Apparatus for synthesis, structuring and regeneration.

Introduced free ribosomes, EPS, kG, lysosomes, secretory granules(in secretory cardiomyocytes)

Provides resynthesis contractile and regulatory proteins of myofibrils, other endoreproductive processes, secretion basement membrane components and PNUF (secretory cardiomyocytes)

6. Nervous apparatus

Introduced nerve fibers, receptor and motor nerve endings autonomic nervous system.

Provides adaptive regulation of contractile and other functions of cardiomyocytes.

Regeneration of cardiac muscle tissue

A. Mechanisms

1. Endoreproduction

2. Synthesis of basement membrane components

3. Proliferation of cardiomyocytes possible in embryogenesis

B. Species

1. Physiological

It proceeds constantly, provides age-related (including in children) increase in myocardial mass (working hypertrophy of myocytes without hyperplasia)

Increases with increasing load on the myocardium → working hypertrophy myocytes without hyperplasia (in people of physical labor, in pregnant women)

2. Reparative

The defect of muscle tissue is not replenished by cardiomyocytes (a connective tissue scar is formed at the site of damage)

Regeneration of cardiomyocytes (both physiological and reparative) is carried out only by the mechanism of endoreproduction. Causes:

1) there are no undifferentiated cells,

2) cardiomyocytes are not capable of division,

3) they are not capable of dedifferentiation.

MUSCLE TISSUES.

Muscle tissues- these are tissues of different origin and structure, but similar in ability to contract.

Morphofunctional characteristics of muscle tissue:

1. The ability to reduce.

2. Muscle has contractility due to special organelles - myofibril formed by filaments of contractile protein, actin and myosin.

3. The sarcoplasm contains inclusions of glycogen, lipids and myoglobin that binds oxygen. Organelles of general purpose are poorly developed, only EPS and mitochondria are well developed, which are located in a chain between myofibrils.

Functions:

1. movement of the organism and its parts in space;

2. muscles give shape to the body;

Classification

1. Morphofunctional:

A) smooth

B) Cross-striped (skeletal, cardiac).

2. Genetic (according to Khlopin)

smooth muscle tissue develops from 3 sources:

BUT) from mesenchyme- muscle tissue that forms the membranes of internal organs and the walls of blood vessels.

B) from ectoderm- myoepitheliocytes - cells with the ability to contract, have a stellate shape, in the form of a basket cover the terminal sections and small excretory ducts of the ectodermal glands. With their reduction, they contribute to the secretion.

AT) neural origin- these are muscles that constrict and dilate the pupil (it is believed that they develop from neuroglia).

striated muscle tissue develops from 2 sources:

BUT) from myotome ov skeletal tissues are laid.

B) from the myoepicardial plate of the visceral leaf of the splanchnotome in the cervical region of the embryo, cardiac muscle tissue is laid.

smooth muscle tissue

Histogenesis. Mesenchymal cells differentiate into myoblasts, from which myocytes are formed.

The structural unit of smooth muscle tissue is myocyte, and the structural-functional unit - layer of smooth muscle cells.

myocyte - a spindle-shaped cell. The size is 2x8 microns, during pregnancy it increases to 500 microns and acquires a stellate shape. The nucleus is rod-shaped; when the cell contracts, the nucleus bends or spirals. Organelles of general importance are poorly developed (with the exception of mitochondria) and are located near the poles of the nucleus. In the cytoplasm - special organelles - myofibrils (represented by actin and myosin filaments). actin filaments form a three-dimensional network that is attached to the myocyte plasmolemma by special cross-linking proteins (vinculin, etc.), which are visible on micrographs as dense bodies(consist of alpha - actinin). Myosin filaments in a relaxed state, they are depolymerized, and when they contract, they polymerize, while they form an actinomyosin complex with actin filaments. Actin filaments associated with the plasma membrane pull it during contraction, as a result of which the cell shortens and thickens. The starting point during contraction is calcium ions, which are in caveoli formed by invagination of the cytolemma. The myocyte over the plasmolemma is covered with a basement membrane, into which fibers of loose connective tissue are woven with vessels and nerves, forming endomysium. The terminals of nerve fibers are also located here, ending not directly on the myocytes, but between them. The mediator released from them through nexuses (between cells) is transmitted to several cells at once, which leads to a reduction in their entire layer.

Regeneration of smooth muscle tissue can go 3 ways:

1. compensatory hypertrophy (increase in cell size),

2. mitotic division of myocytes,

3. increase in the number of myofibroblasts.

striated muscle tissue

Skeletal.

Histogenesis. It develops from mesoderm myotomes. In the development of the skeletal muscle stage, the following stages are distinguished:

1. myoblastic stage - cells of myotomes are loosened, while one part of the cells remains in place and participates in the formation of autochthonous muscle tissue, and the other part of the cells migrates to the places of future muscle laying. In this case, cells differentiate in 2 directions: 1) myoblasts , which divide mitotically and 2) myosatellites.

2. formation of muscle tubules (myotubes)- myoblasts merge and form symplast. Then, in the symplast, myofibrils are formed, located along the periphery, and nuclei in the center, as a result of which myotubes or muscle tubules.

3. myosymplast formation - As a result of long-range differentiation, myotubes become myosymplast, while the nuclei are displaced to the periphery, and the myofibrils are in the center and take an ordered arrangement, which corresponds to the formation of the muscle fiber. Myosatellites are located on the surface of myosymplasts and remain poorly differentiated. They form the kaibium of skeletal muscle tissue. Due to them, the regeneration of muscle fibers occurs.

The structural unit of skeletal muscle tissue is muscle fiber, and structural-functional - mion. muscle fiber - this is a myosymplast reaching up to several cm in size and containing up to several tens of thousands of nuclei located along the periphery. In the center of the muscle fiber there are up to two thousand bundles of myofibrils. Mion - This is a muscle fiber surrounded by connective tissue with blood vessels and nerves.

Five devices are distinguished in the fiber:

1. trophic apparatus;

2. contractile apparatus;

3. specific membrane apparatus;

4. support apparatus;

5. nervous apparatus.

1. Trophic apparatus represented by nuclei and organelles of general importance. The nuclei are located along the periphery of the fiber and have an elongated shape, the boundaries of the muscle fiber are not expressed. There are organelles of general (agranular EPS, sarcosomes (mitochondria) are well developed, granular EPS is less developed, lysosomes are poorly developed, usually they are located at the poles of the nuclei) and of special significance (myofibrils).

2. contractile apparatus – myofibrils (from 200 to 2500). They run parallel to each other longitudinally, optically inhomogeneous. Each myofibril has dark and light areas (disks). Dark discs are located opposite the dark, and light discs opposite the light discs, therefore, a pattern of transverse striation of the fibers is created.

Strands of contractile protein myosin thick and arranged one under the other, forming a disk A (anisotropic), which is stitched with an M-line (mesophragm), consisting of the protein myomysin. Thin threads actin are also located one under the other, forming a light disk I (isotropic). It does not have birefringence, unlike disk A. Actin filaments enter between the myosin filaments for some distance. The section of the A disk formed only by myosin filaments is called the H-band, and the section containing actin and myosin filaments is called the A-band. Disc I is stitched with a Z-line. Z - line (telophragm) is formed by the alpha-actin protein, which has a reticular arrangement. Proteins, nebulin, and tetin promote the positioning of actin and myosin filaments and their fixation in the Z-band. Telophragms of adjacent bundles are fixed to each other, as well as to the cortical layer of the sarcoplasm with the help of intermediate filaments. This contributes to a strong fixation of the discs and does not allow them to move relative to each other.

The structural functional unit of myofibrils is sarcomere , within it there is a contraction of the muscle fiber. It is represented by ½ I-disk + A-disk + ½ I-disk. During contraction, the actin filaments enter between the myosin filaments, inside the H stripes and disk I as such disappears.

Between the bundles of myofibrils there is a chain of sarcosomes, as well as cisterns of the sarcoplasmic reticulum at the level of T-tubules, forming transverse cisterns (L-systems).

3. Specific membrane apparatus - it is formed by a T-tubule (these are invaginations of the cytolemma), which in mammals is located at a level between dark and light discs. Next to the T-tubule are the terminal cisterns of the sarcoplasmic reticulum - an agranular ER, in which calcium ions accumulate. T-tubule and two L-cistern together form triad . The triads play an important role in the initiation of muscle contraction.

4. support apparatus - educated meso - and telophragms , performing a support function for the myofibril bundle, as well as sarcolemma . Sarcolemma(muscle fiber sheath) is represented by two sheets: the inner one is the plasmolemma, the outer one is the basement membrane. Collagen and reticular fibers are woven into the sarcolemma, forming a layer of connective tissue with vessels and nerves - endomysium surrounding each fiber. Cells are located between leaves. myosatellites or myosatellitocytes - this type of cell is also formed from myotomes, giving two populations (myoblasts and myosatellitocytes). These are oval-shaped cells with an oval nucleus and all organelles and even a cell center. They are undifferentiated and are involved in the regeneration of muscle fibers.

5. Nervous apparatus (see nervous system - motor plaque).

Regeneration of skeletal striated muscle tissue can go by:

1. compensatory hypertrophy,

2. either in the following way: when a muscle fiber is cut, its part next to the cut degenerates and is absorbed by macrophages. Then, in the differentiated cisterns of the EPS and the Golgi complex, elements of the sarcoplasm begin to form, while a thickening forms at the damaged ends - muscle buds growing towards each other. Myosatelites, released when the fiber is damaged, divide, merge with each other and promote regeneration, building up into the muscle fiber.

Histophysiology of muscle contraction.

Molecule actin has a globular shape and consists of two chains of globules that are spirally twisted relative to each other, while between these threads a groove is formed, which contains the protein tropomyosin. Troponin protein molecules are located at a certain distance between tropomyosin. In a quiescent state, these proteins close the active centers of the actin protein. During contraction, an excitation wave occurs, which is transmitted from the sarcolemma through the T-tubules deep into the muscle fiber and the L-cistern of the sarcoplasmic reticulum, calcium ions are ejected from them, which change the configuration of troponin. Following this, troponin displaces tropomyosin, as a result of which the active centers of the actin protein open. protein molecules myosin They look like golf clubs. It distinguishes between two heads and a handle, while the heads and part of the handle are movable. During the contraction of the myosin head, moving along the active centers of the actin protein, they pull the actin molecules into the H-band of disk A and disk I almost disappears.

Muscle as an organ.

The muscle fiber is surrounded by a thin layer of loose fibrous connective tissue, this layer is called endomysium It contains blood vessels and nerves. A bundle of muscle fibers is surrounded by a wider layer of connective tissue - peremizium , and the entire muscle is covered with dense fibrous connective tissue - epimysium .

There are three types of muscle fibers :

2. red,

3. intermediate.

White - (skeletal muscles), this is a strong-willed, rapidly contracting muscle, which quickly gets tired during contraction, is characterized by the presence of ATP - a fast-type phase, and low activity of succinate dehydrogenase, high - phosphorylase. The nuclei are located along the periphery, and the myofibrils are in the center, the telophragm is at the level of the dark and light discs. White muscle fibers contain more myofibrils, but less myoglobin, a large supply of glycogen.

Red - (heart, tongue) - this is a non-volitional muscle, the contraction of these fibers is protracted tonic, without fatigue. ATP-phase of the slow type, high activity of succinate dehydrogenase, low activity of phosphorylase, nuclei are located in the center, myofibrils along the periphery, telophragm at the level of the T-tubule, contains more myoglobin, which provides a red color to the fibers than myofibrils.

Intermediate (part of skeletal muscles) - occupy an intermediate position between the red and white types of muscle fibers.

Cardiac muscle tissue.

Formed by 5 types of cells:

1. typical(contractile) muscles

2. atypical- consists of R-cells(pacemaker cells) in the cytoplasm of which there is a lot of free calcium. They have the ability to excite and generate an impulse, they are part of the pacemaker, ensuring the automatism of the heart. The impulse from the R-cell is transmitted to

3. transitional cells and then

4. conductive cells, from them to a typical myocardium.

5. secretory, which produce natriuretic factor, while they control urination.

cardiac muscle tissue refers to the striated and has a similar structure as the skeletal one (i.e., it has the same apparatus), but differs from the skeletal one in the following ways:

1. If skeletal muscle tissue is a symplast, then cardiac tissue has a cellular structure (cardiomyocytes).

2. Cardiomyocytes are connected to each other and form functional fibers.

3. intercalated plates are the boundaries between cells that have a complex structure and contain interdigestions, nexuses and desmosomes, where actin filaments are woven.

4. cells have one or two nuclei located in the center. And the bundles of myofibrils lie along the periphery.

5. cardiomyocytes form cytoplasmic outgrowths or oblique anastomoses that connect functional fibers to each other (therefore, the heart works according to the “all or nothing” law).

6. the red type of muscles is characteristic of cardiac muscle tissue (see above)

7. there is no source of regeneration (there are no myosatelites), regeneration occurs due to the formation of a connective tissue scar at the site of injury or compensatory hypertrophy.

8. develops from the myoepicardial plate of the visceral leaf of the splanchnotome.