Muscle tissue (lat. textus muscularis) - tissues that are different in structure and origin, but similar in their ability to undergo pronounced contractions. They consist of elongated cells that receive irritation from the nervous system and respond to it with contraction. They ensure movement in space of the body as a whole, its movement of organs within the body (heart, tongue, intestines, etc.) and consist of muscle fibers. Cells of many tissues have the ability to change shape, but in muscle tissue this ability becomes the main function.
The main morphological characteristics of muscle tissue elements: elongated shape, the presence of longitudinally located myofibrils and myofilaments - special organelles that ensure contractility, the location of mitochondria next to the contractile elements, the presence of inclusions of glycogen, lipids and myoglobin.
Special contractile organelles - myofilaments or myofibrils - provide contraction, which occurs when two main fibrillar proteins interact in them - actin and myosin - with the obligatory participation of calcium ions. Mitochondria provide these processes with energy. The supply of energy sources is formed by glycogen and lipids. Myoglobin is a protein that ensures the binding of oxygen and the creation of its reserve at the time of muscle contraction, when the blood vessels are compressed (the oxygen supply drops sharply).
Consists of mononuclear cells - spindle-shaped myocytes with a length of 20-500 microns. Their cytoplasm in a light microscope looks uniform, without transverse striations. This tissue has special properties: it slowly contracts and relaxes, is automatic, and is involuntary (that is, its activity is not controlled by the will of a person). It is part of the walls of internal organs: blood and lymphatic vessels, urinary tract, digestive tract (contraction of the walls of the stomach and intestines).
Consists of myocytes that are long (up to several centimeters) and have a diameter of 50-100 microns; these cells are multinucleated, containing up to 100 or more nuclei; in a light microscope, the cytoplasm appears as alternating dark and light stripes. The properties of this muscle tissue are high speed of contraction, relaxation and volition (that is, its activity is controlled by the will of the person). This muscle tissue is part of the skeletal muscles, as well as the wall of the pharynx, the upper part of the esophagus, it forms the tongue, and the extraocular muscles. The fibers are 10 to 12 cm long.
Consists of 1 or 2 nuclear cardiomyocytes with transverse striations of the cytoplasm (along the periphery of the cytolemma). Cardiomyocytes are branched and form connections with each other - intercalated discs, in which their cytoplasm is united. There is also another intercellular contact - anostamoses (invagination of the cytolemma of one cell into the cytolemma of another) This type of muscle tissue forms the myocardium of the heart. Develops from the myoepicardial plate (visceral layer of the splanchnotome of the fetal neck). A special property of this tissue is automaticity - the ability to rhythmically contract and relax under the influence of excitation that occurs in the cells themselves (typical cardiomyocytes). This tissue is involuntary (atypical cardiomyocytes). There is a 3rd type of cardiomyocytes - secretory cardiomyocytes (they do not have fibrils). They synthesize the hormone troponin, which lowers blood pressure and dilates the walls of blood vessels.
Muscle tissues are tissues that differ both in their structure and origin. However, what they have in common is that they are capable of pronounced contractions. Muscle tissue is based on elongated cells, which receive impulses from the central nervous system, and the reaction to this is their contraction. Thanks to muscle tissue, the body and internal organs and systems (heart, lungs, intestines, etc.) of which it consists are able to move, changing their position in space. Cells of other tissues also have the ability to change shape and contract. However, in muscle tissue this function is basic.
Features of the structure of muscle tissue
The most important features of the main components of muscle tissue are their elongated shape, the presence of elongated and appropriately arranged myofilaments and myofibrils (which ensure muscle contractility), as well as the presence of mitochondria, lipids, glycogen and myoglobin. Inside the contractile organelles, myosin and actin interact (with the simultaneous participation of Ca ions in the reaction), resulting in muscle contraction. The source of energy for contractile processes is mitochondria, lipids and glycogen. Oxygen is bound and stored through a protein called myoglobin, which occurs when muscle contraction and simultaneous compression of blood vessels.
Classification of muscle fibers
Taking into account the nature of the contraction, tonic and phasic muscle fibers are distinguished. In particular, the first type of fibers is designed to provide tone (or static muscle tension), which is especially important for maintaining a particular body position relative to spatial coordinates. Phasic fibers are designed to ensure the ability to perform rapid contractions, but are not able to maintain the shortening of the muscle fiber at a certain level for a long time. Taking into account the biochemical characteristics, as well as color, white and red fibers are distinguished. The color of muscle tissue is determined by the concentration of myoglobin in it (the so-called degree of vascularization). One of the features of red muscle fiber is the presence in its composition of chains of mitochondria surrounded by myofibrils. Slightly lower number of mitochondria in white muscle fiber. They are usually evenly distributed in the sarcoplasm.
Depending on the characteristics of oxidative metabolism, muscle fibers can be glycolytic, oxidative and intermediate. Fibers are distinguished based on information about the degree of activity of the SDH enzyme, which is a marker for the so-called Krebs cycle and mitochondria. The intensity of energy metabolism can be determined by the degree of activity of this enzyme. Glycolytic fibers (or A-type fibers) are characterized by low activity of the above enzyme, while oxidative (or C-type fibers), on the contrary, have increased succinate dehydrogenase activity. B-type fibers are fibers that occupy an intermediate position. The process of transition from type A fibers to type C fibers is a transition to oxygen-dependent metabolism from anaerobic glycolysis. An example is a situation where sports training in combination with nutrition is aimed at the rapid development and formation of glycolytic muscle fibers, which contain large quantities of glycogen, and energy production is carried out anaerobically. This type of training is usually reserved for bodybuilders or sprinters. At the same time, for those sports that require endurance, it is necessary to develop oxidative muscle fibers, which have more blood vessels and mitochondria that provide aerobic glycolysis.
Muscle tissue can be of several types, if we consider their sources of development. That is, depending on the type of embryonic rudiments, they can be mesenchymal (desmal rudiment), epidermal (prechordal plate or cutaneous ectoderm), coelomic (myoepicardial plate of the so-called visceral section of the splanchnotome), neural (neural tube) or somatic/myotome.
Types of muscle tissue
There are smooth and striated (skeletal and cardiac) muscle tissue. The smooth tissue contains predominantly myocytes (mononuclear cells) having the shape of a spindle. The cytoplasm of such myocytes is homogeneous and does not have transverse stripes. Smooth muscle tissue has special properties. First of all, it relaxes and contracts extremely slowly. In addition, she is uncontrollable by humans and usually all her reactions are involuntary. The walls of the vessels of the lymphatic and circulatory systems, urinary tract, stomach and intestines are composed of smooth muscle tissue. Striated skeletal tissue contains very long multinucleated (one hundred or more nuclei) myocytes. If you examine the cytoplasm under a microscope, it will look like alternating light and dark stripes. Striated skeletal muscle tissue is characterized by a fairly high rate of contraction and relaxation. The activity of this type of tissue can be controlled by a person, and it itself is present in the skeletal muscles, in the upper esophagus, in the tongue, as well as in the muscles responsible for the movements of the eyeball.
The composition of striated cardiac muscle tissue includes cardiomyocytes with one or two nuclei, as well as cytoplasm, striated along the periphery of the cytolemma with transverse stripes. Cardiomyocytes are quite highly branched and form intercalated discs with cytoplasm integrated into them at the junctions. Cells also contact through cytolemmas, resulting in the formation of anastomoses. Striated cardiac muscle tissue is found in the myocardium. The most important feature of this tissue is its ability, in the case of cellular excitation, to rhythmic contractions and subsequent relaxations. Striated cardiac muscle tissue belongs to involuntary tissues (so-called atypical cardiomycytes). There is also a third type of cardiomycytes - these are secretory cardiomycytes, which lack fibrils.
The most important functions of muscle tissue
The main functional features of muscle tissue include its abilities such as conductivity, excitability, and contractility. Muscle tissue provides the functions of heat exchange, movement and protection. In addition to the above, one more functional feature of muscle tissue can be identified - facial (or, as it is also called, social). In particular, a person’s facial muscles control his facial expressions, thereby transmitting a certain information message to other people around him.
Blood supply to muscle tissue
Blood enters muscle tissue due to its work. This provides the muscle with the necessary amount of oxygen. If a muscle is at rest, then it, as a rule, requires much less oxygen (usually this figure is five hundred times less than the figure reflecting the oxygen requirement of an actively working muscle). Thus, during active muscle contractions, the volume of blood entering the muscle increases many times over. This is approximately 300 to 500 capillaries per cubic millimeter, or approximately twenty times more than the amount of blood required by a muscle at rest.
Muscle tissue combines the ability to contract.
Structural features: contractile apparatus, which occupies a significant part of the cytoplasm of the structural elements of muscle tissue and consists of actin and myosin filaments, which form organelles for special purposes - myofibrils .
Classification of muscle tissue
1. Morphofunctional classification:
1) Striated or striated muscle tissue: skeletal and cardiac;
2) Unstriated muscle tissue: smooth.
2. Histogenetic classification (depending on the sources of development):
1) Somatic type(from myotomes of somites) – skeletal muscle tissue (striated);
2) Coelomic type(from the myoepicardial plate of the visceral layer of the splanchnotome) – cardiac muscle tissue (striated);
3) Mesenchymal type(develops from mesenchyme) – smooth muscle tissue;
4) From cutaneous ectoderm And prechordal plate– myoepithelial cells of glands (smooth myocytes);
5) Neural origin (from the neural tube) - myoneural cells (smooth muscles that constrict and dilate the pupil).
Functions of muscle tissue: movement of a body or its parts in space.
SKELETAL MUSCLE TISSUE
Striated (cross-striped) muscle tissue makes up up to 40% of the mass of an adult, is part of the skeletal muscles, muscles of the tongue, larynx, etc. They are classified as voluntary muscles, since their contractions are subject to the will of the person. These are the muscles that are used when playing sports.
Histogenesis. Skeletal muscle tissue develops from myotome cells, myoblasts. There are head, cervical, thoracic, lumbar, and sacral myotomes. They grow in the dorsal and ventral directions. The branches of the spinal nerves grow into them early. Some myoblasts differentiate in place (form autochthonous muscles), while others, from the 3rd week of intrauterine development, migrate into the mesenchyme and, merging with each other, form muscular tubes (myotubes)) with large centrally oriented nuclei. In myotubes, differentiation of special organelles of myofibrils occurs. Initially they are located under the plasmalemma, and then fill most of the myotube. The nuclei are shifted to the periphery. Cell centers and microtubules disappear, grEPS is significantly reduced. This multi-core structure is called simplast , and for muscle tissue – myosimplast . Some myoblasts differentiate into myosatellitocytes, which are located on the surface of myosymplasts and subsequently take part in the regeneration of muscle tissue.
The structure of skeletal muscle tissue
Let us consider the structure of muscle tissue at several levels of living organization: at the organ level (muscle as an organ), at the tissue level (muscle tissue itself), at the cellular level (the structure of muscle fiber), at the subcellular level (the structure of myofibril) and at the molecular level (the structure of actin and myosin threads).
On the map:
1 - gastrocnemius muscle (organ level), 2 - cross-section of the muscle (tissue level) - muscle fibers, between which the RVST: 3 - endomysium, 4 - nerve fiber, 5 - blood vessel; 6 - cross section of muscle fiber (cellular level): 7 - nuclei of muscle fiber - symplast, 8 - mitochondria between myofibrils, blue - sarcoplasmic reticulum; 9 — cross section of myofibril (subcellular level): 10 — thin actin filaments, 11 — thick myosin filaments, 12 — heads of thick myosin filaments.
1) Organ level: structure muscles as an organ.
Skeletal muscle consists of bundles of muscle fibers linked together by a system of connective tissue components. Endomysium– PBCT layers between muscle fibers where blood vessels and nerve endings pass . Perimysium– surrounds 10-100 bundles of muscle fibers. Epimysium– the outer shell of the muscle, represented by dense fibrous tissue.
2) Tissue level: structure muscle tissue.
The structural and functional unit of skeletal striated (striated) muscle tissue is muscle fiber– a cylindrical formation with a diameter of 50 microns and a length from 1 to 10-20 cm. Muscle fiber consists of 1) myosymplast(see its formation above, structure - below), 2) small cambial cells - myosatellite cells, adjacent to the surface of the myosymplast and located in the recesses of its plasmalemma, 3) the basement membrane, which covers the plasmalemma. The complex of plasmalemma and basement membrane is called sarcolemma. The muscle fiber is characterized by transverse striations, the nuclei are shifted to the periphery. Between the muscle fibers there are layers of PBST (endomysium).
3) Cellular level: structure muscle fiber (myosymplast).
The term “muscle fiber” implies “myosymplast”, since myosymplast provides the contraction function, myosatellite cells are involved only in regeneration.
Myosimplast, like a cell, consists of 3 components: a nucleus (more precisely, many nuclei), cytoplasm (sarcoplasm) and plasmolemma (which is covered with a basement membrane and is called sarcolemma). Almost the entire volume of the cytoplasm is filled with myofibrils - special-purpose organelles; general-purpose organelles: grEPS, aEPS, mitochondria, Golgi complex, lysosomes, and also nuclei are shifted to the periphery of the fiber.
In the muscle fiber (myosymplast), functional devices are distinguished: membrane, fibrillar(contractive) and trophic.
Trophic apparatus includes nuclei, sarcoplasm and cytoplasmic organelles: mitochondria (energy synthesis), grEPS and Golgi complex (synthesis of proteins - structural components of myofibrils), lysosomes (phagocytosis of worn-out structural components of the fiber).
Membrane apparatus: each muscle fiber is covered with a sarcolemma, where an outer basement membrane and a plasmalemma (under the basement membrane) are distinguished, which forms invaginations ( T-tubes). To each T- the tube is adjacent to two tanks triad: two L-tubes (aEPS tanks) and one T-tubule (invagination of the plasmalemma). AEPS are concentrated in tanks Ca 2+ required for reduction. Myosatellite cells are adjacent to the plasmalemma on the outside. When the basement membrane is damaged, the mitotic cycle of myosatellite cells starts.
Fibrillar apparatus.Most of the cytoplasm of the striated fibers is occupied by special-purpose organelles - myofibrils, oriented longitudinally, providing the contractile function of the tissue.
4) Subcellular level: structure myofibrils.
When examining muscle fibers and myofibrils under a light microscope, there is an alternation of dark and light areas in them - discs. Dark disks are birefringent and are called anisotropic disks, or A- disks. Light-colored disks are not birefringent and are called isotropic, or I-disks.
In the middle of the disk A there is a lighter area - N- a zone where only thick filaments of the myosin protein are contained. In the middle N-zones (which means A-disk) the darker one stands out M-line consisting of myomesin (necessary for the assembly of thick filaments and their fixation during contraction). In the middle of the disk I there is a dense line Z, which is built from protein fibrillar molecules. Z-line is connected to neighboring myofibrils using the protein desmin, and therefore all the named lines and disks of neighboring myofibrils coincide and a picture of striated muscle fiber is created.
The structural unit of the myofibril is sarcomere (S) — it is a bundle of myofilaments enclosed between two Z-lines. The myofibril consists of many sarcomeres. Formula describing the structure of the sarcomere:
S = Z 1 + 1/2 I 1 + A + 1/2 I 2 + Z 2
5) Molecular level: structure actin And myosin filaments .
Under an electron microscope, myofibrils appear as aggregates of thick, or myosin, and thin, or actin, filaments. Between the thick filaments there are thin filaments (diameter 7-8 nm).
Thick filaments, or myosin filaments,(diameter 14 nm, length 1500 nm, distance between them 20-30 nm) consist of myosin protein molecules, which is the most important contractile protein of muscle, 300-400 myosin molecules in each strand. The myosin molecule is a hexamer consisting of two heavy and four light chains. Heavy chains are two helically twisted polypeptide strands. They bear spherical heads at their ends. Between the head and the heavy chain there is a hinge section with which the head can change its configuration. In the area of the heads there are light chains (two on each). Myosin molecules are arranged in a thick filament in such a way that their heads face outward, protruding above the surface of the thick filament, and the heavy chains form the core of the thick filament.
Myosin has ATPase activity: the released energy is used for muscle contraction.
Thin filaments, or actin filaments,(diameter 7-8 nm), formed by three proteins: actin, troponin and tropomyosin. The main protein by mass is actin, which forms a helix. Tropomyosin molecules are located in the groove of this helix, troponin molecules are located along the helix.
Thick filaments occupy the central part of the sarcomere - A-disc, thin occupy I- discs and partially insert between thick myofilaments. N-zone consists only of thick threads.
At rest interaction of thin and thick filaments (myofilaments) impossible, because The myosin-binding sites of actin are blocked by troponin and tropomyosin. At a high concentration of calcium ions, conformational changes in tropomyosin lead to the unblocking of the myosin-binding regions of actin molecules.
Motor innervation of muscle fiber. Each muscle fiber has its own innervation apparatus (motor plaque) and is surrounded by a network of hemocapillaries located in the adjacent RVST. This complex is called mion. A group of muscle fibers innervated by a single motor neuron is called neuromuscular unit. In this case, the muscle fibers may not be located nearby (one nerve ending can control from one to dozens of muscle fibers).
When nerve impulses arrive along the axons of motor neurons, muscle fiber contraction.
Muscle contraction
During contraction, the muscle fibers shorten, but the length of the actin and myosin filaments in the myofibrils does not change, but they move relative to each other: myosin filaments move into the spaces between actin filaments, actin filaments - between myosin filaments. As a result, the width is reduced I-disk, H-stripes and the length of the sarcomere decreases; width A-disk does not change.
Sarcomere formula at full contraction: S = Z 1 + A+ Z 2
Molecular mechanism of muscle contraction
1. The passage of a nerve impulse through the neuromuscular synapse and depolarization of the plasmalemma of the muscle fiber;
2. The depolarization wave travels along T-tubules (invaginations of the plasmalemma) to L-tubules (cisterns of the sarcoplasmic reticulum);
3. Opening of calcium channels in the sarcoplasmic reticulum and release of ions Ca 2+ into sarcoplasm;
4. Calcium diffuses to the thin filaments of the sarcomere, binds to troponin C, leading to conformational changes in tropomyosin and freeing active centers for binding myosin and actin;
5. Interaction of myosin heads with active centers on the actin molecule with the formation of actin-myosin “bridges”;
6. Myosin heads “walk” along actin, forming new connections between actin and myosin during movement, while the actin filaments are pulled into the space between the myosin filaments towards M-lines, bringing two together Z-lines;
7. Relaxation: Ca 2+ -ATPase of the sarcoplasmic reticulum pumps Ca 2+ from sarcoplasm into cisterns. In the sarcoplasm the concentration Ca 2+ becomes low. Troponin bonds are broken WITH with calcium, tropomyosin closes the myosin-binding sites of thin filaments and prevents their interaction with myosin.
Each movement of the myosin head (attachment to actin and detachment) is accompanied by the expenditure of ATP energy.
Sensory innervation(neuromuscular spindles). Intrafusal muscle fibers, together with sensory nerve endings, form neuromuscular spindles, which are receptors for skeletal muscle. A spindle capsule is formed on the outside. When striated (striated) muscle fibers contract, the tension of the connective tissue capsule of the spindle changes and the tone of the intrafusal (located under the capsule) muscle fibers changes accordingly. A nerve impulse is formed. When a muscle is overstretched, a feeling of pain occurs.
Classification and types of muscle fibers
1. By the nature of the contraction: phasic and tonic muscle fibers. Phasic are capable of performing rapid contractions, but cannot maintain the achieved level of shortening for a long time. Tonic muscle fibers (slow) ensure the maintenance of static tension or tone, which plays a role in maintaining a certain position of the body in space.
2. By biochemical characteristics and color allocate red and white muscle fibers. The color of the muscle is determined by the degree of vascularization and myoglobin content. A characteristic feature of red muscle fibers is the presence of numerous mitochondria, the chains of which are located between the myofibrils. In white muscle fibers there are fewer mitochondria and they are located evenly in the sarcoplasm of the muscle fiber.
3. By type of oxidative metabolism : oxidative, glycolytic and intermediate. Identification of muscle fibers is based on the activity of the enzyme succinate dehydrogenase (SDH), which is a marker for mitochondria and the Krebs cycle. The activity of this enzyme indicates the intensity of energy metabolism. Release muscle fibers A-type (glycolytic) with low SDH activity, WITH-type (oxidative) with high SDH activity. Muscle fibers IN-types occupy an intermediate position. Transition of muscle fibers from A-type in WITH-type marks changes from anaerobic glycolysis to oxygen-dependent metabolism.
For sprinters (athletes, when a quick short contraction is needed, bodybuilders), training and nutrition are aimed at the development of glycolytic, fast, white muscle fibers: they have a lot of glycogen reserves and energy is produced primarily through the anaeolbic pathway (white meat in chicken). Stayers (athletes - marathon runners, in those sports where endurance is required) have a predominance of oxidative, slow, red fibers in the muscles - they have a lot of mitochondria for aerobic glycolysis, blood vessels (they need oxygen).
4. In striated muscles, two types of muscle fibers are distinguished: extrafusal, which predominate and determine the actual contractile function of the muscle and intrafusal, which are part of proprioceptors - neuromuscular spindles.
Factors that determine the structure and function of skeletal muscle are the influence of nervous tissue, hormonal influence, location of the muscle, level of vascularization and motor activity.
CARDIAC MUSCLE TISSUE
Cardiac muscle tissue is located in the muscular layer of the heart (myocardium) and in the mouths of the large vessels associated with it. It has a cellular type of structure and the main functional property is the ability to spontaneous rhythmic contractions (involuntary contractions).
It develops from the myoepicardial plate (visceral layer of the splanchnotome of the mesoderm in the cervical region), the cells of which multiply by mitosis and then differentiate. Myofilaments appear in the cells, which further form myofibrils.
Structure. The structural unit of cardiac muscle tissue is a cell cardiomyocyte. Between the cells there are layers of PBCT with blood vessels and nerves.
Types of cardiomyocytes : 1) typical ( workers, contractile), 2) atypical(conductive), 3) secretory.
Typical cardiomyocytes
Typical (working, contractile) cardiomyocytes– cylindrical cells, up to 100-150 microns long and 10-20 microns in diameter. Cardiomyocytes form the main part of the myocardium, connected to each other in chains by the bases of the cylinders. These zones are called insert discs, in which desmosomal contacts and nexuses (slit-like contacts) are distinguished. Desmosomes provide mechanical cohesion that prevents cardiomyocytes from separating. Gap junctions facilitate the transmission of contraction from one cardiomyocyte to another.
Each cardiomyocyte contains one or two nuclei, sarcoplasm and plasmalemma, surrounded by a basement membrane. There are functional apparatuses, the same as in muscle fiber: membrane, fibrillar(contractile), trophic, and energetic.
Trophic apparatus includes the nucleus, sarcoplasm and cytoplasmic organelles: grEPS and Golgi complex (synthesis of proteins - structural components of myofibrils), lysosomes (phagocytosis of structural components of the cell). Cardiomyocytes, like fibers of skeletal muscle tissue, are characterized by the presence in their sarcoplasm of the iron-containing oxygen-binding pigment myoglobin, which gives them a red color and is similar in structure and function to erythrocyte hemoglobin.
Energy apparatus represented by mitochondria and inclusions, the breakdown of which provides energy. Mitochondria are numerous, lying in rows between fibrils, at the poles of the nucleus and under the sarcolemma. The energy required by cardiomyocytes is obtained by splitting: 1) the main energy substrate of these cells - fatty acids, which are deposited in the form of triglycerides in lipid droplets; 2) glycogen, located in granules located between fibrils.
Membrane apparatus : Each cell is covered with a membrane consisting of a plasmalemma complex and a basement membrane. The shell forms invaginations ( T-tubes). To each T-the tubule is adjacent to one tank (unlike the muscle fiber - there are 2 tanks) sarcoplasmic reticulum(modified aEPS), forming dyad: one L-tube (aEPS tank) and one T-tubule (invagination of the plasmalemma). In AEPS tanks ions Ca 2+ do not accumulate as actively as in muscle fibers.
Fibrillar (contractile) apparatus .Most of the cytoplasm of the cardiomyocyte is occupied by special-purpose organelles - myofibrils, oriented longitudinally and located along the periphery of the cell. The contractile apparatus of working cardiomyocytes is similar to skeletal muscle fibers. When relaxed, calcium ions are released into the sarcoplasm at a low rate, which ensures automaticity and frequent contractions of cardiomyocytes. T-tubules are wide and form dyads (one T-tube and one tank network), which converge in the area Z-lines.
Cardiomyocytes, connecting with the help of intercalary discs, form contractile complexes that contribute to the synchronization of contraction; lateral anastomoses are formed between cardiomyocytes of neighboring contractile complexes.
Function of typical cardiomyocytes: providing the force of contraction of the heart muscle.
Conducting (atypical) cardiomyocytes have the ability to generate and quickly conduct electrical impulses. They form nodes and bundles of the conduction system of the heart and are divided into several subtypes: pacemakers (in the sinoatrial node), transitional cells (in the atrioventricular node) and cells of the His bundle and Purkinje fibers. Conducting cardiomyocytes are characterized by weak development of the contractile apparatus, light cytoplasm and large nuclei. The cells do not have T-tubules or cross-striations because the myofibrils are arranged in a disorderly manner.
Function of atypical cardiomyocytes– generation of impulses and transmission to working cardiomyocytes, ensuring automaticity of myocardial contraction.
Secretory cardiomyocytes
Secretory cardiomyocytes are located in the atria, mainly in the right; characterized by a process form and weak development of the contractile apparatus. In the cytoplasm, near the poles of the nucleus, there are secretory granules containing natriuretic factor, or atriopeptin(a hormone that regulates blood pressure). The hormone causes loss of sodium and water in the urine, dilation of blood vessels, decreased blood pressure, and inhibition of the secretion of aldosterone, cortisol, and vasopressin.
Function of secretory cardiomyocytes: endocrine.
Regeneration of cardiomyocytes. Cardiomyocytes are characterized only by intracellular regeneration. Cardiomyocytes are not capable of division; they lack cambial cells.
SMOOTH MUSCLE TISSUE
Smooth muscle tissue forms the walls of internal hollow organs and blood vessels; characterized by a lack of striations and involuntary contractions. Innervation is carried out by the autonomic nervous system.
Structural and functional unit of non-striated smooth muscle tissue - smooth muscle cell (SMC), or smooth myocyte. The cells are spindle-shaped, 20-1000 µm long and 2 to 20 µm thick. In the uterus, the cells have an elongated process shape.
Smooth myocyte
A smooth myocyte consists of a rod-shaped nucleus located in the center, cytoplasm with organelles and sarcolemma (plasmolemma and basement membrane complex). In the cytoplasm at the poles there is a Golgi complex, many mitochondria, ribosomes, and a developed sarcoplasmic reticulum. Myofilaments are located obliquely or along the longitudinal axis. In SMCs, actin and myosin filaments do not form myofibrils. There are more actin filaments and they are attached to dense bodies, which are formed by special cross-linking proteins. Myosin monomers (micromyosin) are located near the actin filaments. Having different lengths, they are much shorter than thin threads.
Contraction of smooth muscle cells occurs through the interaction of actin filaments and myosin. The signal traveling along the nerve fibers causes the release of a mediator, which changes the state of the plasmalemma. It forms flask-shaped invaginations (caveolae), where calcium ions are concentrated. Contraction of SMCs is induced by the influx of calcium ions into the cytoplasm: caveolae are detached and, together with calcium ions, enter the cell. This leads to the polymerization of myosin and its interaction with actin. Actin filaments and dense bodies come closer together, the force is transferred to the sarcolemma and the SMC is shortened. Myosin in smooth myocytes is able to interact with actin only after phosphorylation of its light chains by a special enzyme, light chain kinase. After the signal stops, calcium ions leave the caveolae; myosin depolarizes and loses its affinity for actin. As a result, the myofilament complexes disintegrate; the contraction stops.
Special types of muscle cells
Myoepithelial cells are derivatives of ectoderm and do not have striations. They surround the secretory sections and excretory ducts of the glands (salivary, mammary, lacrimal). They are connected to glandular cells by desmosomes. By contracting, they promote secretion. In the terminal (secretory) sections, the shape of the cells is branched and stellate. The nucleus is in the center, in the cytoplasm, mainly in the processes, myofilaments are localized, which form the contractile apparatus. These cells also contain cytokeratin intermediate filaments, which emphasizes their similarity to epithelial cells.
Myoneural cells develop from the cells of the outer layer of the optic cup and form the muscle that constricts the pupil and the muscle that dilates the pupil. The structure of the first muscle is similar to SMCs of mesenchymal origin. The muscle that dilates the pupil is formed by cell processes located radially, and the nuclear-containing part of the cell is located between the pigment epithelium and the stroma of the iris.
Myofibroblasts belong to loose connective tissue and are modified fibroblasts. They exhibit the properties of fibroblasts (synthesize intercellular substance) and smooth myocytes (have pronounced contractile properties). As a variant of these cells we can consider myoid cells as part of the wall of the convoluted seminiferous tubule of the testicle and the outer layer of the theca of the ovarian follicle. During wound healing, some fibroblasts synthesize smooth muscle actins and myosins. Myofibroblasts provide contraction of the wound edges.
Endocrine smooth myocytes are modified SMCs that represent the main component of the juxtaglomerular apparatus of the kidneys. They are located in the wall of the arterioles of the renal corpuscle, have a well-developed synthetic apparatus and a reduced contractile apparatus. They produce the enzyme renin, which is located in granules and enters the blood through the mechanism of exocytosis.
Regeneration of smooth muscle tissue. Smooth myocytes are characterized by intracellular regeneration. With an increase in functional load, myocyte hypertrophy and hyperplasia (cellular regeneration) occur in some organs. Thus, during pregnancy, the smooth muscle cells of the uterus can increase 300 times.
Muscle tissues are classified into smooth and striated or striated. Striated is divided into skeletal and cardiac. Depending on their origin, muscle tissue is divided into 5 types:
mesenchymal (smooth muscle tissue);
epidermal (smooth muscle tissue);
neural (smooth muscle tissue);
coelomic (cardiac);
somatic or myotome (skeletal striated).
SMOOTH MUSCLE TISSUE DEVELOPING FROM SPLANCHNOTOMIC MESENCHYME
localized in the walls of hollow organs (stomach, blood vessels, respiratory tract, etc.) and non-hollow organs (in the muscle of the ciliary body of the eye of mammals). Smooth muscle cells develop from mesenchymocytes that lose their processes. They develop the Golgi complex, mitochondria, granular ER and myofilaments. At this time, type V collagen is actively synthesized on the granular EPS, due to which a basement membrane is formed around the cell. With further differentiation, organelles of general importance atrophy, the synthesis of collagen molecules in the cell decreases, but the synthesis of contractile myofilament proteins increases.
STRUCTURE OF SMOOTH MUSCLE TISSUE. It consists of smooth myocytes, spindle-shaped, with a length of 20 to 500 microns. with a diameter of 6-8 microns. Externally, myocytes are covered with plasmalemma and basement membrane.
Myocytes are closely adjacent to each other. There are contacts between them - nexuses. In the place where there are nexuses, there are holes in the basement membrane of the myocyte membrane. At this point, the plasmalemma of one myocyte approaches the plasmalemma of another myocyte at a distance of 2-3 nm. Through the nexuses, ions are exchanged, water molecules are transported, and the contractile impulse is transmitted.
On the outside, myocytes are covered with type V collagen, which forms the exocytoskeleton of the cell. The cytoplasm of myocytes is stained oxyphilic. It contains poorly developed organelles of general importance: granular ER, Golgi complex, smooth ER, cell center, lysosomes. These organelles are located at the poles of the nucleus. Well-developed organelles are mitochondria. Cores have a rod-shaped form.
Myocytes have well-developed myofilaments, which are the contractile apparatus of the cells. Among the myofilaments there are
thin, actin, consisting of actin protein;
thick myosin, consisting of the contractile protein myosin, which appear only after an impulse arrives to the cell;
intermediate filaments consisting of connectin and nebulin.
There is no striation in myocytes because all of the above filaments are arranged in a disorderly manner.
ACTIN Filaments connect to each other and to the plasmalemma using dense bodies. In those places where they connect to each other, the bodies contain alpha-actinin; in those places where the filaments connect to the plasmalemma, the bodies contain vinculin. The arrangement of actin filaments is predominantly longitudinal, but they can be located at an angle relative to the longitudinal axis. Myosin filaments are also located predominantly longitudinally. The filaments are arranged so that the ends of the actin filaments are located between the ends of the myosin filaments.
FUNCTION OF FILAMENTS- contractile. The contraction process is carried out as follows: after the arrival of the contractile impulse, pinocytosis vesicles containing calcium ions approach the filaments; Calcium ions trigger the contractile process, which involves the ends of actin filaments moving deeper between the ends of myosin filaments. The traction force is applied to the plasmalemma, to which actin filaments are connected using dense bodies, as a result of which the myocyte contracts.
FUNCTIONS OF MYOCYTES: 1) contractile (ability for long-term contraction); 2) secretory (they secrete type V collagen, elastin, proteoglycans, since they have granular EPS).
REGENERATION smooth muscle tissue is carried out in 2 ways: 1) mitotic division of myocytes; 2) transformation of myofibroblasts into smooth myocytes.
STRUCTURE OF SMOOTH MUSCLE TISSUE AS AN ORGAN. In the wall of hollow organs, smooth myocytes form bundles. These bundles are surrounded by layers of loose connective tissue called perimysium. The layer of connective tissue around the entire layer of muscle tissue is called epimysium. The perimysium and epimysium contain blood and lymphatic vessels and nerve fibers.
INNERVATION OF SMOOTH MUSCLE TISSUE carried out by the autonomic nervous system, therefore contractions of smooth muscles do not obey the will of the person (involuntary). Sensory (afferent) and motor (efferent) nerve fibers approach smooth muscle tissue. Efferent nerve fibers end in motor nerve endings in the connective tissue layer. When an impulse arrives, mediators are released from the endings, which, spreading diffusely, reach the myocytes, causing them to contract.
SMOOTH MUSCLE TISSUE OF EPIDERMAL ORIGIN located in the terminal sections and small ducts of the glands that develop from the skin ectoderm (salivary, sweat, mammary and lacrimal glands). Smooth myocytes (myoepitheliocytes) are located between the basal surface of glandular cells and the basement membrane, covering the basal part of the glandulocytes with their processes. When these processes contract, the basal part of the glandulocytes is compressed, causing secretion to be released from the glandular cells.
SMOOTH MUSCLE TISSUE OF NEURAL ORIGIN develops from optic cups growing from the neural tube. This muscle tissue forms only 2 muscles located in the iris of the eye: the constrictor pupillary muscle and the dilator pupillary muscle. It is believed that the muscles of the iris develop from neuroglia.
STRIPED SKELETAL MUSCLE TISSUE develops from the myotomes of mesodermal somites, and is therefore called somatic. Myotome cells differentiate in two directions: 1) from some, myosatellite cells are formed; 2) myosymplasts are formed from others.
FORMATION OF MYOSYMPLASTS. Myotome cells differentiate into myoblasts, which fuse together to form myotubes. During the process of maturation, myotubes transform into myosymplasts. In this case, the nuclei are shifted to the periphery, and the myofibrils - to the center.
STRUCTURE OF MUSCLE FIBER. Muscle fiber (miofibra) consists of 2 components: 1) myosatellite cells and 2) myosymplast. The muscle fiber is approximately the same length as the muscle itself, with a diameter of 20-50 microns. The fiber is covered on the outside with a sheath - sarcolemma, consisting of 2 membranes. The outer membrane is called the basement membrane, and the inner membrane is called the plasmalemma. Between these two membranes are myosatellite cells.
MUSCLE FIBER NUCLEI are located under the plasmalemma, their number can reach several tens of thousands. They have an elongated shape and do not have the ability for further mitotic division. The CYTOPLASM of a muscle fiber is called SARCOPLASMA. The sarcoplasm contains a large amount of myoglobin, glycogen inclusions and lipids; There are organelles of general importance, some of which are well developed, others less well developed. Organelles such as the Golgi complex, granular ER, and lysosomes are poorly developed and are located at the poles of the nuclei. Mitochondria and smooth ER are well developed.
In muscle fibers, myofibrils are well developed, which are the contractile apparatus of the fiber. Myofibrils have striations because the myofilaments in them are arranged in a strictly defined order (unlike smooth muscle). There are 2 types of myofilaments in myofibrils: 1) thin actin, consisting of actin protein, troponin and tropomyosin; 2) thick myosin consists of the protein myosin. Actin filaments are arranged longitudinally, their ends are at the same level and extend somewhat between the ends of the myosin filaments. Around each myosin filament there are 6 actin filament ends. The muscle fiber has a cytoskeleton, including intermediate filaments, telophragm, mesophragm, and sarcolemma. Thanks to the cytoskeleton, identical myofibril structures (actin, myosin filaments, etc.) are arranged in an orderly manner.
That part of the myofibril in which only actin filaments are located is called disk I (isotropic or light disk). A Z-stripe, or telophragm, about 100 nm thick and consisting of alpha-actinin, passes through the center of disk I. Actin filaments are attached to the telophragm (the zone of attachment of thin filaments).
Myosin filaments are also arranged in a strictly defined order. Their ends are also on the same level. Myosin filaments, together with the ends of actin filaments extending between them, form disk A (an anisotropic disk with birefringence). Disc A is also divided by the mesophragm, which is similar to the telophragm and consists of M protein (myomysin).
In the middle part of disk A there is an H-stripe, bounded by the ends of actin filaments that extend between the ends of the myosin filaments. Therefore, the closer the ends of the actin filaments are located to each other, the narrower the H-band.
SARCOMER is a structural and functional unit of myofibrils, which is a section located between two telophragms. Sarcomere formula: 1.5 disks I + disk A + 1.5 disks I. Myofibrils are surrounded by well-developed mitochondria and well-developed smooth ER.
SMOOTH EPS forms a system of L-tubules that form complex structures in each disc. These structures consist of L-tubules located along the myofibrils and connecting to transversely directed L-tubules (lateral cisterns). FUNCTIONS of smooth ER (L-tubule system): 1) transport; 2) synthesis of lipids and glycogen; 3) deposition of calcium ions.
T-CHANNELS- these are invaginations of the plasmalemma. At the border of the disks from the plasma membrane deep into the fiber, an invagination occurs in the form of a tube located between two lateral cisterns.
TRIAD includes: 1) T-canal and 2) 2 lateral cisterns of smooth EPS. THE FUNCTION OF TRIADS is that in the relaxed state of myofibrils, calcium ions accumulate in the lateral cisterns; at the moment when an impulse (action potential) moves along the plasmalemma, it passes to the T-channels. When an impulse moves along the T-channel, calcium ions come out of the lateral cisterns. Without calcium ions, contraction of myofibrils is impossible, because in actin filaments the centers of interaction with myosin filaments are blocked by tropomyosin. Calcium ions unblock these centers, after which the interaction of actin filaments with myosin filaments begins and contraction begins.
MECHANISM OF MYOFIBRILL CONTRACTION. When actin filaments interact with myosin filaments, Ca ions unblock the adhesion centers of actin filaments with the heads of myosin molecules, after which these outgrowths attach to the adhesion centers on the actin filaments and, like a paddle, carry out the movement of actin filaments between the ends of the myosin filaments. At this time, the telophragm approaches the ends of the myosin filaments, since the ends of the actin filaments also approach the mesophragm and each other, and the H-stripe narrows. Thus, during myofibril contraction, disc I and the H-stripe narrow. After the termination of the action potential, calcium ions return to the L-tubules of the smooth ER, and tropomyosin again blocks the centers of interaction with myosin filaments in actin filaments. This leads to the cessation of contraction of myofibrils, their relaxation occurs, i.e. actin filaments return to their original position, the width of disk I and the H-band is restored.
MYOSATELLITOCYTES muscle fibers are located between the basement membrane and the plasmalemma of the sarcolemma. These cells are oval in shape, their oval nucleus is surrounded by a thin layer of organelle-poor and weakly stained cytoplasm. FUNCTION of myosatellite cells- these are cambial cells involved in the regeneration of muscle fibers when they are damaged.
STRUCTURE OF MUSCLE AS AN ORGAN . Each muscle of the human body is a unique organ with its own structure. Each muscle is made up of muscle fibers. Each fiber is surrounded by a thin layer of loose connective tissue - endomysium. Blood and lymphatic vessels and nerve fibers pass through the endomysium. The muscle fiber together with blood vessels and nerve fibers is called "myon". Several muscle fibers form a bundle surrounded by a layer of loose connective tissue called perimysium. The entire muscle is surrounded by a layer of connective tissue called the epimysium.
CONNECTION OF MUSCLE FIBERS WITH COLLAGEN FIBERS OF TENDON.
At the ends of the muscle fibers there are invaginations of the sarcolemma. These invaginations include collagen and reticular fibers of the tendons. Reticular fibers pierce the basement membrane and, using molecular linkages, connect to the plasmalemma. Then these fibers return to the lumen of the invagination and braid the collagen fibers of the tendon, as if tying them to the muscle fiber. Collagen fibers form tendons that attach to the bone skeleton.
TYPES OF MUSCLE FIBERS. There are 2 main types of muscle fibers:
Type I (red fibers) and type II (white fibers). They differ mainly in the speed of contraction, the content of myoglobin, glycogen and enzyme activity.
TYPE 1 (red fibers) are characterized by a high myoglobin content (that's why they are red), high succinate dehydrogenase activity, slow type ATPase, not so rich in glycogen content, duration of contraction and low fatigue.
TYPE 2 (white fibers) are characterized by low myoglobin content, low succinate dehydrogenase activity, fast-type ATPase, rich glycogen content, rapid contraction and high fatigue.
The slow (red) and fast (white) types of muscle fibers are innervated by different types of motor neurons: slow and fast. In addition to the 1st and 2nd types of muscle fibers, there are intermediate ones that have the properties of both.
Each muscle contains all types of muscle fibers. Their number may vary and depends on physical activity.
REGENERATION OF STRIPED SKELETAL MUSCLE TISSUE . When muscle fibers are damaged (ruptured), their ends at the site of injury undergo necrosis. After rupture, macrophages arrive at the fragments of fibers, which phagocytose the necrotic areas, clearing them of dead tissue. After this, the regeneration process is carried out in 2 ways: 1) due to increased reactivity in muscle fibers and the formation of muscle buds at the sites of rupture; 2) due to myosatellite cells.
The 1st PATH is characterized by the fact that at the ends of broken fibers the granular ER is hypertrophied, on the surface of which the proteins of myofibrils, membrane structures inside the fiber and sarcolemma are synthesized. As a result, the ends of the muscle fibers thicken and transform into muscle buds. These buds, as they grow, move closer to each other from one torn end to the other, and finally the buds connect and grow together. Meanwhile, due to the endomysium cells, new formation of connective tissue occurs between the muscle buds growing towards each other. Therefore, by the time the muscle buds join, a connective tissue layer is formed, which will become part of the muscle fiber. Consequently, a connective tissue scar is formed.
The 2nd WAY of regeneration is that myosatellite cells leave their habitats and undergo differentiation, as a result of which they turn into myoblasts. Some myoblasts join the muscle buds, some join into myotubes, which differentiate into new muscle fibers.
Thus, during reparative muscle regeneration, old muscle fibers are restored and new ones are formed.
INNERVATION OF SKELETAL MUSCLE TISSUE carried out by motor and sensory nerve fibers ending in nerve endings. MOTOR (motor) nerve endings are the terminal devices of the axons of motor nerve cells of the anterior horns of the spinal cord. The end of the axon, approaching the muscle fiber, is divided into several branches (terminals). The terminals pierce the basement membrane of the sarcolemma and then plunge deep into the muscle fiber, dragging the plasmalemma with them. As a result, a neuromuscular ending (motor plaque) is formed.
STRUCTURE OF THE NEUROMUSCULAR endings The neuromuscular ending has two parts (poles): nervous and muscular. There is a synaptic gap between the nerve and muscle parts. The nerve part (axon terminals of the motor neuron) contains mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine. In the muscular part of the neuromuscular ending there are mitochondria, an accumulation of nuclei, and there are no myofibrils. The synaptic cleft, 50 nm wide, is bounded by a presynaptic membrane (axon plasmalemma) and a postsynaptic membrane (muscle fiber plasmalemma). The postsynaptic membrane forms folds (secondary synaptic clefts), it contains receptors for acetylcholine and the enzyme acetylcholinesterase.
FUNCTION of neuromuscular endings. The impulse moves along the axon plasmalemma (presynaptic membrane). At this time, synaptic vesicles with acetylcholine approach the plasmalemma, from the vesicles acetylcholine flows into the synaptic cleft and is captured by receptors of the postsynaptic membrane. This increases the permeability of this membrane (muscle fiber plasma membrane), as a result of which sodium ions move from the outer surface of the plasma membrane to the inner surface, and potassium ions move to the outer surface - this is a depolarization wave or a nerve impulse (action potential). After the occurrence of an action potential, acetylcholinesterase of the postsynaptic membrane destroys acetylcholine and the transmission of the impulse through the synaptic cleft stops.
SENSITIVE NERVE ENDINGS(neuromuscular spindles - fusi neuro-muscularis) end the dendrites of the sensory neurons of the spinal ganglia. Neuromuscular spindles are covered with a connective tissue capsule, inside which there are 2 types of intrafusal (intraspindle) muscle fibers: 1) with a nuclear bursa (in the center of the fiber there is a thickening in which there is an accumulation of nuclei), they are longer and thicker; 2) with a nuclear chain (the nuclei in the form of a chain are located in the center of the fiber), they are thinner and shorter.
Thick nerve fibers penetrate into the endings, which entwine both types of intrafusal muscle fibers in a ring and thin nerve fibers ending in grape-shaped endings on muscle fibers with a nuclear chain. At the ends of the intrafusal fibers there are myofibrils and motor nerve endings approach them. Contractions of intrafusal fibers do not have great strength and do not add up to the rest (extrafusal) muscle fibers.
FUNCTION of neuromuscular spindles consists in the perception of the speed and force of muscle stretching. If the tensile force is such that it threatens to rupture the muscle, then the contracting antagonist muscles from these endings reflexively receive inhibitory impulses.
CARDIAC MUSCLE TISSUE develops from the anterior section of the visceral layers of the splanchnotome. From these sheets, 2 myoepicardial plates stand out: right and left. The cells of the myoepicardial plates differentiate in two directions: from some the mesothelium covering the epicardium develops, from others - cardiomyocytes of five varieties;
contractile
pacemaker
conductive
intermediate
secretory or endocrine
STRUCTURE OF CARDIOMYOCYTES . Cardiomyocytes have a cylindrical shape, 50-120 µm long, 10-20 µm in diameter. Cardiomyocytes connect their ends to each other and form functional cardiac muscle fibers. The junction of cardiomyocytes is called intercalated discs (discus intercalatus). The discs contain interdigitations, desmosomes, attachment sites for actin filaments, and nexuses. Metabolism between cardiomyocytes occurs through nexuses.
On the outside, cardiomyocytes are covered with a sarcolemma, consisting of an outer (basal) membrane and a plasmalemma. Processes extend from the lateral surfaces of the cardiomyocytes and intertwine into the lateral surfaces of the cardiomyocytes of the adjacent fiber. These are muscle anastomoses.
CORE cardiomyocytes (one or two), oval in shape, usually polyploid, located in the center of the cell. MYOFIBRILLS are localized along the periphery. ORGANELLES - some are poorly developed (granular ER, Golgi complex, lysosomes), others are well developed (mitochondria, smooth ER, myofibrils). The oxyphilic CYTOPLASMA contains inclusions of myoglobin, glycogen and lipids.
STRUCTURE OF MYOFIBRILLS the same as in skeletal muscle tissue. Actin filaments form a light disk (I), separated by a telophragm; due to myosin filaments and actin ends, disk A (anisotropic) is formed, separated by a mesophragm. In the middle part of disk A there is an H-stripe bounded by the ends of actin filaments.
Cardiac muscle fibers differ from skeletal muscle fibers in that they consist of individual cells - cardiomyocytes, the presence of muscle anastomoses, the central location of the nuclei (in the skeletal muscle fiber - under the sarcolemma), the increased thickness of the diameter of T-channels, since they include plasmalemma and basement membrane (in skeletal muscle fibers - only plasmalemma).
REDUCTION PROCESS in the fibers of the heart muscle is carried out according to the same principle as in the fibers of skeletal muscle tissue.
CONDUCTING CARDIOMYOCYTES characterized by a thicker diameter (up to 50 μm), lighter cytoplasm, central or eccentric arrangement of nuclei, low content of myofibrils, and a simpler arrangement of intercalary discs. The discs have fewer desmosomes, interdigitations, nexuses, and actin filament attachment sites.
Conducting cardiomyocytes lack T channels. Conducting cardiomyocytes can connect to each other not only with their ends, but also with their lateral surfaces. The FUNCTION of conducting cardiomyocytes is to produce and transmit a contractile impulse to contractile cardiomyocytes.
ENDOCRINE CARDIOMYOCYTES are located only in the atria, have a more process-shaped shape, poorly developed myofibrils, intercalary discs, and T-channels. They have well-developed granular ER, Golgi complex and mitochondria, and their cytoplasm contains secretion granules.
FUNCTION OF endocrine cardiomyocytes- secretion of atrial natriuretic factor (ANF), which regulates the contractility of the heart muscle, the volume of circulating fluid, blood pressure, and diuresis.
REGENERATION of cardiac muscle tissue is only physiological, intracellular. When cardiac muscle fibers are damaged, they are not restored, but are replaced by connective tissue (histotypic regeneration).
Muscle tissue: types, structural features, location in the body
Muscle tissue (textus musculares)– these are specialized tissues that provide movement (movement in space) of the body as a whole, as well as its parts and internal organs. Contraction of muscle cells or fibers is carried out with the help of myofilaments and special organelles - myofibrils and is the result of the interaction of contractile protein molecules.
According to the morphological classification, muscle tissue is divided into two groups:
I - striated (striated) muscle tissue - constantly contains complexes of actin and myosin myofilaments - myofibrils and has transverse striations;
II - smooth (unstriated) muscle tissue - consists of cells that constantly contain only actin myofilaments and do not have transverse striations.
Striated muscle tissue
Striated muscle tissue is divided into skeletal and cardiac. Both of these varieties develop from mesoderm.
Striated skeletal muscle tissue. This tissue forms skeletal muscles, muscles of the mouth, pharynx, partly the esophagus, muscles of the perineum, etc. It has its own characteristics in different sections. Has a high contraction speed and fatigue. This type of contractile activity is called tetanic. Striated skeletal muscle tissue cuts arbitrarily in response to impulses coming from the cerebral cortex. However, some muscles (intercostal muscles, diaphragm, etc.) not only contract voluntarily, but also contract without the participation of consciousness under the influence of impulses from the respiratory center, and the muscles of the pharynx and esophagus contract involuntarily.
The structural unit is the striated muscle fiber- simplast, cylindrical in shape with rounded or pointed ends, with which the fibers are adjacent to each other or woven into the connective tissue of tendons and fascia.
Their contractile apparatus is striated myofibrils., which form a bundle of fibers. These are protein threads located along the fiber. Their length coincides with the length of the muscle fiber. Myofibrils consist of dark and light areas - disks. Since the dark and light discs of all myofibrils of one muscle fiber are located at the same level, transverse striations are formed; therefore, the muscle fiber is called striated. Dark discs in polarized light are birefringent and are called anisotropic, or A-discs; light discs are not birefringent and are called isotropic, or I-discs.
The different light refractive ability of the disks is due to their different structure. Light (I) wheels homogeneous in composition: formed only by parallel thin threads – actin myofilaments consisting predominantly of protein actin, and troponin And tropomyosin. Dark (A) wheels heterogeneous: formed as thick myosin myofilaments consisting of protein myosin, and partially penetrating between them with thin actin myofilaments.
In the middle of each I-disc there is a dark line called Z-line, or telophragm. One end of the actin filaments is attached to it. The area of myofibril between two telophragms is called sarcomere. Sarcomere is a structural and functional unit of myofibril. In the center of the A-disk you can identify a light stripe, or zone H, containing only thick threads. In the middle there is a thin dark line M, or mesophragm. Thus, each sarcomere contains one A-band and two halves of an I-band.
Striated cardiac muscle tissue. Forms the myocardium of the heart. Contains, like the skeletal one, myofibrils, consisting of dark and light disks. Consists of cells - cardiomyocytes, interconnected by insertion disks. In this case, chains of cardiomyocytes are formed - functional muscle fibers, which anastomose with each other (transition into one another), forming a network. This system of connections ensures contraction of the myocardium as a whole. Reduction heart muscle involuntary, is regulated by the autonomic nervous system.
Among cardiomyocytes there are:
· contractile (working) cardiomyocytes - contain fewer myofibrils than skeletal muscle fibers, but a lot of mitochondria, therefore they contract with less force, but do not get tired for a long time; with the help of intercalary disks, mechanical and electrical communication of cardiomyocytes is carried out;
· atypical (conductive) cardiomyocytes – form the conduction system of the heart for the formation and conduction of impulses to contractile cardiomyocytes;
· secretory cardiomyocytes – located in the atria, capable of producing a hormone-like peptide – sodium uretic factor, lowering blood pressure.
Smooth muscle tissue
It develops from mesenchyme and is located in the wall of tubular organs (intestine, ureter, bladder, blood vessels), as well as the iris and ciliary body of the eye and the muscles that raise hair in the skin.
Smooth muscle tissue has cellular structure (smooth myocyte) and has contractile apparatus in the form of smooth myofibrils. It contracts slowly and is able to remain in a state of contraction for a long time, consuming a relatively small amount of energy and without getting tired. This type of contractile activity is called tonic. Autonomic nerves approach smooth muscle tissue, and unlike skeletal muscle tissue, it is not subject to consciousness, although it is under the control of the cerebral cortex.
The smooth muscle cell has a spindle-shaped shape and pointed ends. It has a nucleus, cytoplasm (sarcoplasm), organelles and a membrane (sarcolemma). Contractile myofibrils are located along the periphery of cells along its axis. These cells are closely adjacent to each other. The supporting apparatus in smooth muscle tissue is thin collagen and elastic fibers located around the cells and connecting them to each other.
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