Structural unit of striated muscle tissue. Histological structure of muscle tissue. How to get rid of back, muscle and joint pain

It is an elongated cylindrical formation with pointed ends from 1 to 40 mm long (and according to some sources, up to 120 mm) and 0.1 mm in diameter.

The muscle fiber is surrounded by a shell - a sarcolemma, in which 2 sheets are clearly distinguished under an electron microscope: the inner one is a typical plasmalemma, and the outer one is a thin connective tissue plate - the basal plate.

In a narrow gap between the plasmalemma and the basal plate are small cells - myosatellites.

Thus, the muscle fiber is a complex formation and consists of the following main structural components:

Myosymplast;

Myosatellite cells;

Basal plate.

The basal plate is formed by thin collagen and reticular fibers, belongs to the supporting apparatus and performs an auxiliary function of transferring contraction forces to the connective tissue elements of the muscle.

Myosatellite cells are cambial (growth) elements muscle fibers and play a role in the processes of their physiological and reparative regeneration.

The myosymplast is the main structural component of the muscle fiber, both in terms of volume and functions. It is formed by the fusion of independent undifferentiated muscle cells - myoblasts.

Myosymplast can be considered as an elongated giant multinucleated cell, consisting of a large number nuclei, cytoplasm (sarcoplasm), plasmalemma, inclusions, general and special organelles. The myosymplast contains several thousand (up to 10 thousand) longitudinally elongated light nuclei located on the periphery under the plasmalemma. Fragments of a weakly expressed granular endoplasmic reticulum, a lamellar complex, and a small number of mitochondria are localized near the nuclei. There are no centrioles in the symplast. The sarcoplasm contains inclusions of glycogen and myoglobin, an analogue of erythrocyte hemoglobin.

A distinctive feature of the myosymplast is also the presence of specialized organelles in it, which include:

Myofibrils;

Sarcoplasmic reticulum;

Tubules of the T-system.

Myofibrils - the contractile elements of the myosymplast - in large numbers (up to 1-2 thousand) are localized in the central part of the sarcoplasm of the myosymplast. They are combined into bundles, between which are layers of sarcoplasm. A large number of mitochondria (sarcosomes) are localized between myofibrils. Each myofibril extends longitudinally throughout the entire myosymplast and, with its free ends, is attached to its plasmolemma at the conical ends. The diameter of the myofibril is 0.2–0.5 µm.

Myofibrils are heterogeneous in length and are subdivided:

On dark (anisotropic), or A-disks, which are formed by thicker myofilaments (10-12 nm), consisting of the protein myosin;

Light (isotropic), or I-discs, which are formed by thin myofilaments (5-7 nm), consisting of actin protein.

Dark and light discs of all myofibrils are located at the same level and cause the transverse striation of the entire muscle fiber.

Dark and light discs consist of even thinner fibers - protofibrils, or myofilaments.

In the middle of the I-disk, a dark strip passes across the actin myofilaments - a telophragm, or Z-line, in the middle of the A-disk there is a less pronounced M-line, or mesophragm.

Actin myofilaments in the middle of the I-disk are held together by proteins that make up the Z-line, with their free ends partially entering the A-disk between thick myofilaments. At the same time, around 1 myosin filament are located in actin filaments.

With a partial contraction of the myofibril, the actin myofilaments seem to be drawn into the A-disk, and a light zone, or H-strip, is formed in it, bounded by the free ends of the actin myofilaments. The width of the H-band depends on the degree of contraction of the myofibril.

The section of the myofibril located between the 2 Z-lines is called the sarcomere and is the structural and functional unit of the myofibril.

The sarcomere includes the A-disk and 2 halves of the 1-disk located on either side of it.

Therefore, each myofibril is a collection of sarcomeres.

It is in the sarcomere that the contraction process takes place.

The terminal sarcomeres of each myofibril are attached to the plasmolemma of the myosymplast by actin myofilaments.

Structural elements of a sarcomere in a relaxed state can be expressed by the formula

Z + 1/21 + 1/2A + M + 1/2A + 1/21 + Z.

The contraction process is carried out through the interaction of actin and myosin filaments and the formation of actinmyosin bridges between them, through which the actin myofilaments are drawn into the A-disks - shortening of the sarcomere. For the development of this process, 3 conditions are necessary.

The presence of energy in the form of ATP;

The presence of calcium ions; presence of biopotential.

ATP is formed in sarcosomes (mitochondria), in a large number localized between myofibrils.

The fulfillment of the last 2 conditions is carried out with the help of 2 more specialized organelles - the sarcoplasmic reticulum and T-tubules.

The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum and consists of dilated cavities and anastomosing tubules surrounding the myofibrils. It is subdivided into fragments surrounding individual sarcomeres. Each fragment consists of 2 terminal cisterns connected by hollow anastomosing tubules - L-tubules. At the same time, the terminal cisterns cover the sarcomere in the region of the I-disks, and the tubules - in the region of the A-disks.

The terminal cisterns and tubules contain calcium ions, which, when a nerve impulse arrives and a wave of depolarization of the membranes of the sarcoplasmic reticulum is reached, leave the cisterns and tubules and are distributed between actin and myosin myofilaments, initiating their interaction. After the termination of the depolarization wave, calcium ions rush back to the terminal cisterns and tubules.

Thus, the sarcoplasmic reticulum is not only a reservoir for calcium ions, but also plays the role of a calcium pump.

The wave of depolarization is transmitted to the sarcoplasmic reticulum from the nerve ending, first through the plasmalemma, and then through the T-tubules. They are not independent structural elements and are tubular protrusions of the plasmalemma into the sarcoplasm.

Penetrating deep, T-tubules branch and cover each myofibril within 1 bundle strictly at the same level, usually at the level of the Z-strip or somewhat more medially - in the area of ​​\u200b\u200bjunction of actin and myosin myofilaments. Therefore, each sarcomere is approached and surrounded by 2 T-tubules.

On the sides of each T-tubule are 2 terminal cisterns of the sarcoplasmic reticulum of neighboring sarcomeres, which, together with the T-tubules, form a triad. Between the wall of the T-tubule and the walls of the terminal cisterns there are contacts through which the depolarization wave is transmitted to the membranes of the cisterns and causes the release of calcium ions from them and the onset of contraction. Thus, the functional role of T-tubules is to transfer the biopotential from the plasmolemma to the sarcoplasmic reticulum.

Skeletal regeneration muscle tissue, as in other tissues, is divided into 2 types - physiological and reparative.

Physiological regeneration manifests itself in the form of hypertrophy of muscle fibers, which is expressed in an increase in their thickness and even length, an increase in the number of organelles, mainly myofibrils, and an increase in the number of nuclei, which ultimately manifests itself in an increase in the functional ability of the muscle fiber. It has been established by radioisotope method that an increase in the number of nuclei in muscle fibers under conditions of hypertrophy is achieved due to the division of myosatellite cells and the subsequent entry of daughter cells into the myosymplast.

The increase in the number of myofibrils is carried out through the synthesis of actin and myosin proteins by free ribosomes and the subsequent assembly of these proteins into actin and myosin myofilaments in parallel with the corresponding sarcomere filaments. As a result of this, myofibrils thicken first, and then they split and form daughter myofibrils. In addition, the formation of new actin and myosin myofilaments is possible not in parallel, but end to end with the previous myofibrils, which results in their elongation.

The sarcoplasmic reticulum and T-tubules in a hypertrophied fiber are formed due to the growth of the previous elements.

With certain types muscle training predominantly red type of muscle fibers (in stayers) or white type of muscle fibers (in sprinters) can be formed.

Age-related hypertrophy of muscle fibers is intensely manifested with the onset of motor activity of the body (1–2 years), which is primarily due to increased nervous stimulation.

In old age, as well as in conditions of low muscle load

atrophy of special and general organelles, thinning of muscle fibers and a decrease in their functional ability occur.

Reparative regeneration develops after damage to muscle fibers.

The method of regeneration depends on the size of the defect:

With significant damage throughout the muscle fiber, myosatellites in the area of ​​damage and in adjacent areas are disinhibited, proliferate intensively, and then migrate to the area of ​​the defect in the muscle fiber, where they line up in chains, forming a myotube. The subsequent differentiation of the myotube leads to the completion of the defect and the restoration of the integrity of the muscle fiber;

Under conditions of a small defect in the muscle fiber, muscle fibers are formed at its ends due to the regeneration of intracellular organelles.

kidneys that grow towards each other and then merge, leading to the closure of the defect.

Reparative regeneration and restoration of the integrity of muscle fibers can be carried out only in the following cases.

Firstly, with preserved motor innervation of muscle fibers;

Secondly, if elements of connective tissue (fibroblasts) do not fall into the area of ​​damage, otherwise a connective tissue scar develops at the site of the defect in the muscle fiber.

Soviet scientist A.N. Studitsky proved the possibility of amtotransplantation of skeletal muscle tissue and even whole muscles under certain conditions:

mechanical grinding of the muscle tissue of the graft in order to disinhibit satellite cells and their subsequent proliferation;

Placement of the crushed tissue in the fascial bed;

suturing of the motor nerve fiber to the crushed graft;

The presence of contractile movements of antagonist and synergistic muscles.

2. Skeletal muscles receive the following innervation:

Motor (efferent)

sensitive (afferent);

trophic (vegetative).

The skeletal muscles of the trunk and limbs receive motor (efferent) innervation from the motor neurons of the anterior horns. spinal cord, and the muscles of the face and head - from the motor neurons of certain cranial nerves.

Either a branch from the axon of a motor neuron, or the entire axon, approaches each muscle fiber. In muscles that provide fine coordinated movements (muscles of the hands, forearms, neck), each muscle fiber is innervated by 1 motor neuron. In the muscles that provide mainly the maintenance of the posture, dozens and even

hundreds of muscle fibers receive motor innervation from 1 motor neuron through the branching of its axon.

The motor nerve fiber, approaching the muscle fiber, penetrates under the endomysium and the basal plate and breaks up into terminals, which, together with the adjacent specific area of ​​the myosymplast, form an axo-muscular synapse or motor plaque. Under the influence of a nerve impulse, a depolarization wave from the nerve ending is transmitted to the plasmolemma of the myosymplast, propagates further along the T-tubules and, in the area of ​​the triads, is transmitted to the terminal cisterns of the sarcoplasmic reticulum, causing the release of calcium ions and the beginning of the process of contraction of the muscle fiber.

Sensitive (afferent) innervation of skeletal muscles is carried out by pseudounipolar neurons of the spinal ganglia, through various receptor endings in the dendrites of these cells.

The receptor endings of skeletal mice can be divided into 2 groups: specific receptor devices that are characteristic only for skeletal muscles:

muscle spindle;

Golgi tendon organ;

non-specific receptor endings of a bushy or tree-like form, distributed in loose connective tissue:

Endomysia;

Perymysia;

Epimysia.

Muscle spindles are rather complex encapsulated devices. Each muscle contains from several units to several tens and even hundreds of muscle spindles. Each muscle spindle contains not only nerve elements, but also 10-12 specific muscle fibers - intrafusal, surrounded by a capsule. These fibers are located parallel to the contractile muscle fibers (extrafusal) and receive not only sensitive, but also special motor innervation. Muscle spindles perceive irritations both when a given muscle is stretched, caused by contraction of antagonist muscles, and when it is contracted.

Tendon organs are specialized encapsulated receptors, including several tendon fibers surrounded by a capsule, among which the terminal branches of the pseudounipolar neuron dendrite are distributed. When the muscle contracts, the tendon fibers come together and compress the nerve endings. Tendon organs perceive only the degree of contraction of a given muscle. Through muscle spindles and tendon organs, with the participation of spinal centers, automatic movements are ensured (for example, when walking).

Trophic (vegetative) innervation is provided by autonomic nervous system(VNS) (its sympathetic part) and is carried out mainly indirectly, through the innervation of blood vessels.

Skeletal muscles are richly supplied with blood. The loose connective tissue of the perimysium contains a large number of arteries and veins, arterioles, venules, and arteriolo-venular anastomoses. In the endomysium, only capillaries are located, mostly narrow (4.5–7 microns), which provide the trophism of the muscle fiber. The muscle fiber, together with the capillaries surrounding it and the motor ending, makes up the mion.

The muscles contain a large number of arteriolo-venular anastomoses that provide adequate blood supply during various muscle activities.

1. Types of muscle tissue Almost all types of cells have the property of contractility, due to the presence in their cytoplasm of the contractile apparatus, represented by a network of thin microfilaments (5-7 nm), consisting of contractile proteins- actin, myosin, tropomyosin and others. Due to the interaction of these microfilament proteins, contractile processes are carried out and the movement of hyaloplasm, organelles, vacuoles in the cytoplasm, the formation of pseudopodia and plasmolemma invaginations, as well as the processes of phago- and pinocytosis, exocytosis, cell division and movement are ensured. The content of contractile elements, and, consequently, contractile processes are not equally expressed in different types of cells. Contractile structures are most pronounced in cells whose main function is contraction. Such cells or their derivatives form muscle tissues , which provide contractile processes in hollow internal organs and vessels, moving parts of the body relative to each other, maintaining posture and moving the body in space. In addition to movement during contraction, a large amount of heat is released, and, therefore, muscle tissues are involved in the thermoregulation of the body.
Muscle tissues are not the same by structure, sources of origin and innervation, according to functional features. Finally, it should be noted that any kind of muscle tissue, in addition to contractile elements (muscle cells and muscle fibers), includes cellular elements and fibers of loose fibrous connective tissue and vessels that provide trophism of muscle elements, transfer the forces of contraction of muscle elements to the skeleton. However, functionally leading elements of muscle tissue are muscle cells or muscle fibers.
Classification of muscle tissue:

• smooth (non-striated) - mesenchymal;
• special - neural origin and epidermal origin;
• striated (striated ):
• skeletal;
• cardiac.

• skeletal - from somite myotomes;
• cardiac - from the visceral leaf of the splanchnotome.

2. Organization of striated skeletal muscle tissue Structural and functional unitstriated muscle tissue is muscle fiber . It is an elongated cylindrical formation with pointed ends from 1 mm to 40 mm long (and according to some sources up to 120 mm), with a diameter of 0.1 mm. The muscle fiber is surrounded by a shell - a sarcolemma, in which two sheets are clearly distinguished under an electron microscope: the inner one is a typical plasmalemma, and the outer one is a thin connective tissue plate - the basal plate. In a narrow gap between the plasmalemma and the basal plate are small cells - myosatellites. Thus, the muscle fiber is a complex formation and consists of the following main structural components:

• myosymplast;
• myosatellite cells;
• basal plate.

• myofibrils;
• sarcoplasmic reticulum;
• tubules of the T-system.

• dark (anisotropic) or A-discs, which are formed by thicker myofilaments (10-12 nm), consisting of the protein myosin;
• and light (isotropic) or I-discs, which are formed by thin myofilaments (5-7 nm), consisting of actin protein.

3. Muscle contractions reduction process is carried out through the interaction of actin and myosin filaments and the formation between them actin-myosin bridges, through which the actin myofilaments are drawn into the A-disks and the sarcomere is shortened. For this process to develop, three conditions:

• the presence of energy in the form of ATP ;
• the presence of calcium ions;
• presence of biopotential .

4. Types of muscle fibers In muscle tissue, there are two main types of muscle fibers windows, between which there are intermediate, differing from each other, primarily in features metabolic processes and functional properties and, to a lesser extent, structural features.

• Type I fibers - red muscle fibers- are characterized primarily by a high content of myoglobin in the sarcoplasm (which gives them a red color), a large number of sarcosomes, a high activity of succinate dehydrogenase (SDH) in them, and a high activity of slow-type ATPase. These fibers have the ability of slow but prolonged tonic contraction and low fatigue;
• Type II fibers - white muscle fibers- characterized by a low content of myoglobin, but a high content of glycogen, high activity of phosphorylase and fast-type ATP base. Functionally characterized by the ability of fast, strong, but short contraction. Between the two extreme types of muscle fibers are intermediate, characterized by various combinations of these inclusions and different activities of the listed enzymes.

• endomysium;
• perimysium;
• epimysium;
• as well as tendons.

Professor Suvorova G.N.

Muscle tissues.

They are a group of tissues that carry out the motor functions of the body:

1) contractile processes in hollow internal organs and vessels

2) movement of body parts relative to each other

3) posture maintenance

4) movement of the organism in space.

Muscle tissue has the following morphofunctional characteristics:

1) Their structural elements have an elongated shape.

2) Contractile structures (myofilaments and myofibrils) are arranged longitudinally.

3) For muscle contraction, a large amount of energy is needed, therefore, in them:

Contains a large number of mitochondria

There are trophic inclusions

Iron-containing protein myoglobin may be present

Structures in which Ca ++ ions are deposited are well developed.

Muscle tissue is divided into two main groups

1) smooth (non-striated)

2) Cross-striped (striated)

Smooth muscle tissue: is of mesenchymal origin.

In addition, a group of myoid cells is isolated, these include

Myoid cells of neural origin (forms the muscles of the iris)

Myoid cells of epidermal origin (myoepithelial cells of sweat, salivary, lacrimal, and mammary glands)

striated muscle tissue subdivided into skeletal and cardiac. Both of these varieties develop from the mesoderm, but from different parts of it:

Skeletal - from somite myotomes

Cardiac - from the visceral leaf of the splanchnotome.

Skeletal muscle tissue

It makes up about 35-40% of the human body weight. As the main component, it is part of the skeletal muscles, in addition, it forms the muscular basis of the tongue, is part of the muscular membrane of the esophagus, etc.

Skeletal muscle development. The source of development is the cells of the myotomes of the somites of the mesoderm, determined in the direction of myogenesis. Stages:

Myoblasts

muscle tubules

The definitive form of myogenesis is the muscle fiber.

The structure of skeletal muscle tissue.

The structural and functional unit of skeletal muscle tissue is muscle fibre. It is an elongated cylindrical formation with pointed ends, with a diameter of 10 to 100 microns, variable length (up to 10-30 cm).

muscle fiber is a complex (cellular-symplastic) formation, which consists of two main components

1. myosymplast

2. myosatellitocytes.

Outside, the muscle fiber is covered with a basement membrane, which, together with the plasmolemma of the myosymplast, forms the so-called sarcolemma.

Myosymplast is the main component of the muscle fiber, both in terms of volume and function. The myosymplast is a giant supracellular structure that is formed by the fusion of a huge number of myoblasts during embryogenesis. On the periphery of the myosymplast, there are from several hundred to several thousand nuclei. Fragments of the lamellar complex, EPS, single mitochondria are localized near the nuclei.

The central part of the myosymplast is filled with sarcoplasm. Sarcoplasm contains all organelles of general importance, as well as specialized apparatus. These include:

Contractile

Device for transmitting excitation from the sarcolemma

to the contractile apparatus.

Energy

reference

contractile apparatus muscle fiber is represented by myofibrils.

myofibrils have the form of threads (the length of the muscle fiber) with a diameter of 1-2 microns. They have a transverse striation due to the alternation of differently refracting polarized light areas (disks) - isotropic (light) and anisotropic (dark). Moreover, myofibrils are located in the muscle fiber with such a degree of order that the light and dark disks of neighboring myofibrils exactly match. This causes the striation of the entire fiber.

Dark and light discs, in turn, consist of thick and thin filaments called myofilaments.

In the middle of the light disk, a dark strip passes transversely to the thin myofilaments - the telophragm, or Z-line.

The section of myofibril between two telophragms is called a sarcomere.

Sarcomere It is considered the structural and functional unit of the myofibril - it includes the A-disk and two halves of the I-disk located on both sides of it.

thick filaments (myofilaments) are formed by orderly packed molecules of the fibrillar protein myosin. Each thick filament consists of 300-400 myosin molecules.

Thin The filaments contain the contractile protein actin and two regulatory proteins: troponin and tropomyosin.

The mechanism of muscle contraction described by the theory of sliding threads, which was proposed by Hugh Huxley.

At rest, at a very low concentration of Ca ++ ions in the myofibril of a relaxed fiber, thick and thin threads do not touch. Thick and thin filaments slide freely relative to each other, as a result, muscle fibers do not resist passive stretching. This condition is characteristic of the extensor muscle when the corresponding flexor is contracted.

muscle contraction caused by a sharp increase in the concentration of Ca ++ ions and consists of several stages:

Ca ++ ions bind to the troponin molecule, which shifts, opening myosin binding sites on thin filaments.

The myosin head is attached to the myosin-binding sites of a thin filament.

The myosin head changes conformation and makes a stroking motion that propels the thin filament to the center of the sarcomere.

The myosin head binds to the ATP molecule, which leads to the separation of myosin from actin.

Sarcotubular system- provides the accumulation of calcium ions and is an apparatus for transmitting excitation. For this, a wave of depolarization passing through the plasmalemma led to an effective contraction of myofibrils. It consists of the sarcoplasmic reticulum and T-tubules.

The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum and consists of a system of cavities and tubules that surrounds each myofibril in the form of a sleeve. At the border of the A- and I-discs, the tubules merge, forming pairs of flat terminal cisterns. The sarcoplasmic reticulum performs the functions of depositing and releasing calcium ions.

The wave of depolarization propagating along the plasma membrane first reaches the T-tubules. There are specialized contacts between the wall of the T-tubule and the terminal cistern, through which the depolarization wave reaches the membrane of the terminal cistern, after which calcium ions are released.

support apparatus muscle fiber is represented by elements of the cytoskeleton, which provide an ordered arrangement of myofilaments and myofibrils. These include:

Telophragm (Z-line) - the area of ​​​​attachment of thin myofilaments of two adjacent sarcomeres.

Mesophragm (M-line) - a dense line located in the center of the A-disk, thick filaments are attached to it.

In addition, the muscle fiber contains proteins that stabilize its structure, for example:

Dystrophin - at one end is attached to actin filaments, and at the other - to a complex of glycoproteins that penetrate into the sarcolemma.

Titin is an elastic protein that stretches from the M- to the Z-line, prevents overstretching of the muscle.

In addition to the myosymplast, muscle fibers include myosatellocytes. These are small cells that are located between the plasma membrane and the basement membrane, they are the cambial elements of skeletal muscle tissue. They are activated when muscle fibers are damaged and provide their reparative regeneration.

There are three main types of fibers:

Type I (red)

Type IIB (white)

Type IIA (intermediate)

Type I fibers are red muscle fibers, characterized by a high content of myoglobin in the cytoplasm, which gives them a red color, a large number of sarcosomes, a high activity of oxidative enzymes (SDH), a predominance of aerobic processes. These fibers have the ability of a slow but long tonic contraction and low fatigue.

Type IIB fibers - white - glycolytic, are characterized by a relatively low content of myoglobin, but a high content of glycogen. They have a larger diameter, fast, tetanic, with great force of contraction, quickly get tired.

Type IIA fibers are intermediate, fast, fatigue-resistant, oxidative-glycolytic.

Muscle as an organ- consists of muscle fibers connected together by a system of connective tissue, blood vessels and nerves.

Each fiber is surrounded by a layer of loose connective tissue, which contains blood and lymphatic capillaries that provide fiber trophism. The collagen and reticular fibers of the endomysium are woven into the basement membrane of the fibers.

Perimysium - surrounds bundles of muscle fibers. It contains larger vessels

Epimysium - fascia. A thin connective tissue sheath of dense connective tissue that surrounds the entire muscle.

Structural and functional unit striated skeletal muscle tissue is a muscle fiber. The fiber can reach 12 cm in length, contains a large volume of sarcoplasm and hundreds of nuclei. Each fiber is covered with a sarcolemma, which consists of two layers: the inner one - the plasmolemma 8-10 nm thick and the outer one - the basement membrane 30-40 nm thick. Between the plasmalemma and the basement membrane there is a space 15-25 nm wide. In addition, reticular fibers are woven into the basement membrane.

Significant volume sarcoplasms occupy contractile organelles - myofibrils. Each myofibril consists of a large number of regularly alternating dark and light bands (disks). In polarized light, dark disks exhibit birefringence and are therefore called anisotropic (A-disks). Light disks do not have this property and are called isotropic (I-disks). Each myofibril is formed by a bundle of parallel myofilaments. A-discs are made up of thick and thin myofilaments, while I-discs are made up of only thin ones. Thin filaments (5-8 nm) are formed by proteins actin, tropomyosin, troponin, and thick filaments (10-12 nm) are formed by myosin, M- and H-band proteins, and others. Thin filaments are located between thick ones, forming a hexagonal arrangement.

The structural and functional unit of the myofibril is sarcomere. The conditional formula of the sarcomere is 1/2 1-disk + A-disk + 1/2 I-disk. The cross-linking line of neighboring sarcomeres corresponds to the Z-line (telophragm), which consists of alpha-actinin, desmin, and vimentin proteins. In vertebrates, the length of the sarcomere is 2–3 μm. The middle part of the myosin disc, where actin myofilaments do not reach, is lighter and is called the H-band. It is crossed by the M-line (mesophragm), which fastens the myosin filaments in the middle of the sarcomere. In the submembrane layer of the symplast, the proteins vinculin and spectrin, which are part of the symplast skeleton, were found.

Components of the metabolic environment symplasta well expressed. In histogenesis, with an increase in the degree of maturity of symplasts, an increase in the number of mitochondria is observed, which are oriented along the sides of the Z-line between myofibrils and under the sarcolemma. Glycogen granules, lipid drops form clusters between myofibrils and under the sarcolemma. The content of myoglobin (oxygen-binding pigment) varies depending on the lifestyle of the animal. Ribosomes are presented as polysomes. A small number of lysosomes take part in the processes of intrasymplastic regeneration. There is no cell center in the symplast.

sarcoplasmic reticulum and T-tubules develop in parallel. The latter are invaginations of the plasmalemma that encircle each sarcomere. In the longitudinal direction around each myofibril there are tubules of the sarcoplasmic reticulum. This is how the longitudinal and transverse systems are formed, which are visible on sections as triads. The triad is a complex consisting of a transverse tubule and profiles of two cisterns of the sarcoplasmic reticulum located symmetrically on either side of the T-tubule. In the cisterns of the sarcoplasmic reticulum, calcium ions accumulate, which are necessary for the contraction of myofibrils.

In late ontogeny a number of ultrastructural changes occur in cells and symplasts. The most significant are the thickening of the basement membrane, the disorganization of myofibrils and the Z-line, the occurrence of clusters of mitochondria under the sarcolemma, the separation of myosatellitocytes from the symplast and their transition into the interstitial space. The innervation of muscle fibers is carried out by motor neurons of the anterior horns of the spinal cord, which form neuromuscular synapses approximately in the central part of the fiber.

Regeneration. For successful regeneration of muscle tissue, it is necessary to maintain muscle tension, restore blood supply and nerve connection. The main source of regeneration are myosatellitocytes. After activation of the latter, their mitotic division occurs, myoblasts arise, which undergo differentiation, merge with each other and form symplasts. The development of symplasts continues with the participation of multiplying myosatellitocytes, some of which merge with growing symplasts. This is how new cellular-symplastic systems are formed - muscle fibers.