What tissue forms skeletal muscles? The structure of skeletal muscle tissue

Muscles are one of the main components of the body. They are based on tissue whose fibers contract under the influence of nerve impulses, allowing the body to move and be supported in its environment.

Muscles are located in every part of our body. And even if we don’t know about their existence, they still exist. It’s enough, for example, to go to the gym or do aerobics for the first time - the next day even those muscles that you didn’t even know you had will start to ache.

They are responsible not only for movement. At rest, muscles also require energy to maintain their tone. This is necessary so that at any moment a certain one can respond to a nerve impulse with the appropriate movement, and does not waste time on preparation.

To understand how muscles are structured, we suggest remembering the basics, repeating the classification and looking into the cellular. We will also learn about diseases that can worsen their function, and how to strengthen skeletal muscles.

General concepts

According to their filling and the reactions that occur, muscle fibers are divided into:

striated;
smooth.

Skeletal muscles are elongated tubular structures, the number of nuclei in one cell can reach several hundred. They consist of muscle tissue, which is attached to various parts of the bone skeleton. Contractions of striated muscles contribute to human movements.

Varieties of forms

How are muscles different? The photos presented in our article will help us figure this out.

Skeletal muscles are one of the main components of the musculoskeletal system. They allow you to move and maintain balance, and are also involved in the process of breathing, voice production and other functions.

There are more than 600 muscles in the human body. As a percentage, their total mass is 40% of the total body mass. Muscles are classified by shape and structure:

thick fusiform;
thin lamellar.

Classification makes learning easier

The division of skeletal muscles into groups is carried out depending on their location and significance in the activity of various organs of the body. Main groups:

Muscles of the head and neck:

facial expressions - are used when smiling, communicating and creating various grimaces, while ensuring the movement of the constituent parts of the face;
chewing - promote a change in the position of the maxillofacial region;
voluntary muscles of the internal organs of the head (soft palate, tongue, eyes, middle ear).

Skeletal muscle groups of the cervical spine:

superficial - promote inclined and rotational movements of the head;
middle ones - create the lower wall of the oral cavity and promote downward movement of the jaw and laryngeal cartilages;
deep ones tilt and turn the head, create elevation of the first and second ribs.

The muscles, photos of which you see here, are responsible for the torso and are divided into muscle bundles of the following sections:

thoracic - activates the upper torso and arms, and also helps to change the position of the ribs when breathing;
abdominal section - allows blood to move through the veins, changes the position of the chest during breathing, affects the functioning of the intestinal tract, promotes flexion of the torso;
dorsal - creates the motor system of the upper limbs.

Muscles of the limbs:

upper - consist of muscle tissue of the shoulder girdle and free upper limb, help move the arm in the shoulder joint capsule and create movements of the wrist and fingers;
lower - play the main role in a person’s movement in space, are divided into the muscles of the pelvic girdle and the free part.

Structure of skeletal muscle

In its structure, it has a huge number of oblong shapes with a diameter of 10 to 100 microns, their length ranges from 1 to 12 cm. Fibers (microfibrils) are thin - actin, and thick - myosin.

The former consist of a protein that has a fibrillar structure. It's called actin. Thick fibers are composed of different types of myosin. They differ in the time it takes to decompose the ATP molecule, which causes different contraction rates.

Myosin in smooth muscle cells is dispersed, although there is a large amount of protein, which, in turn, is significant in prolonged tonic contraction.

The structure of skeletal muscle is similar to a rope or stranded wire woven from fibers. It is surrounded on top by a thin sheath of connective tissue called the epimysium. From its inner surface, deeper into the muscle, thinner branches of connective tissue extend, creating septa. Individual bundles of muscle tissue are “wrapped” in them, each containing up to 100 fibrils. Narrower branches extend from them even deeper.

The circulatory and nervous systems penetrate through all layers into the skeletal muscles. The arterial vein runs along the perimysium - this is the connective tissue covering the bundles of muscle fibers. Arterial and venous capillaries are located nearby.

Development process

Skeletal muscles develop from the mesoderm. Somites are formed on the side of the neural groove. After time, myotomes are released into them. Their cells, taking on a spindle shape, evolve into myoblasts, which divide. Some of them progress, while others remain unchanged and form myosatellite cells.

A small part of myoblasts, due to the contact of the poles, creates contact with each other, then the plasma membranes disintegrate in the contact zone. Thanks to the fusion of cells, symplasts are created. Undifferentiated young muscle cells move to them, being in the same environment with the myosymplast of the basement membrane.

Functions of skeletal muscles

This muscle is the basis of the musculoskeletal system. If it is strong, it is easier to maintain the body in the desired position, and the likelihood of stooping or scoliosis is minimized. Everyone knows about the benefits of playing sports, so let’s look at the role that muscles play in this.

The contractile tissue of skeletal muscles performs many different functions in the human body that are necessary for the correct positioning of the body and the interaction of its individual parts with each other.

Muscles perform the following functions:

create body mobility;
protect the thermal energy created inside the body;
promote movement and vertical retention in space;
promote contraction of the airways and help with swallowing;
form facial expressions;
promote heat production.

Ongoing support

When muscle tissue is at rest, there is always a slight tension in it, called muscle tone. It is formed due to minor impulse frequencies that enter the muscles from the spinal cord. Their action is determined by signals penetrating from the head to the spinal motor neurons. Muscle tone also depends on their general condition:

sprains;
level of filling of muscle cases;
blood enrichment;
general water and salt balance.

A person has the ability to regulate the level of muscle load. As a result of prolonged physical exercise or severe emotional and nervous stress, muscle tone involuntarily increases.

Skeletal muscle contractions and their types

This function is the main one. But even it, despite its apparent simplicity, can be divided into several types.

Types of contractile muscles:

isotonic - the ability of muscle tissue to shorten without changes in muscle fibers;
isometric - during the reaction, the fiber contracts, but its length remains the same;
auxotonic - the process of contraction of muscle tissue, where the length and tension of the muscles are subject to changes.

Let's look at this process in more detail.

First, the brain sends an impulse through a system of neurons, which reaches the motor neuron adjacent to the muscle bundle. Next, the efferent neuron is innervated from the synoptic vesicle, and a neurotransmitter is released. It binds to receptors on the sarcolemma of the muscle fiber and opens a sodium channel, which leads to depolarization of the membrane causing, in sufficient quantities, the neurotransmitter to stimulate the production of calcium ions. It then binds to troponin and stimulates its contraction. This, in turn, pulls back tropomeasesin, allowing actin to combine with myosin.

Next, the process of actin filament sliding relative to the myosin filament begins, resulting in skeletal muscle contraction. A schematic diagram will help you understand the process of compression of striated muscle bundles.

How skeletal muscles work

The interaction of a large number of muscle bundles contributes to various movements of the body.

The work of skeletal muscles can occur in the following ways:

synergistic muscles work in one direction;
Antagonist muscles promote opposite movements to produce tension.

The antagonistic action of muscles is one of the main factors in the activity of the musculoskeletal system. When performing any action, not only the muscle fibers that perform it, but also their antagonists are included in the work. They promote counteraction and give the movement concreteness and grace.

When acting on a joint, striated skeletal muscle performs complex work. Its character is determined by the location of the joint axis and the relative position of the muscle.

Some functions of skeletal muscle are poorly understood and often not discussed. For example, some of the bundles act as a lever for the operation of the bones of the skeleton.

Muscle work at the cellular level

The action of skeletal muscles is carried out by two proteins: actin and myosin. These components have the ability to move relative to each other.

For muscle tissue to work, it is necessary to consume energy contained in the chemical bonds of organic compounds. The breakdown and oxidation of such substances occurs in the muscles. There is always air present here, and energy is released, 33% of all this is spent on the performance of muscle tissue, and 67% is transferred to other tissues and spent on maintaining a constant body temperature.

Diseases of the skeletal muscles

In most cases, deviations from the norm in muscle functioning are due to the pathological state of the responsible parts of the nervous system.

The most common pathologies of skeletal muscles:

Muscle cramps are an imbalance of electrolyte in the extracellular fluid surrounding muscle and nerve fibers, as well as changes in osmotic pressure in it, especially its increase.
Hypocalcemic tetany is an involuntary tetanic contraction of skeletal muscle observed when the extracellular Ca2+ concentration falls to approximately 40% of normal levels.
characterized by progressive degeneration of skeletal muscle fibers and myocardium, as well as muscle disability, which can lead to death due to respiratory or cardiac failure.
Myasthenia gravis is a chronic autoimmune disease in which antibodies to the nicotinic ACh receptor are formed in the body.

Relaxation and restoration of skeletal muscles

Proper nutrition, lifestyle and regular exercise will help you become the owner of healthy and beautiful skeletal muscles. It is not necessary to exercise and build muscle mass. Regular cardio training and yoga are enough.

Do not forget about the mandatory intake of essential vitamins and minerals, as well as regular visits to saunas and baths with brooms, which allow you to enrich muscle tissue and blood vessels with oxygen.

Systematic relaxing massages will increase the elasticity and reproduction of muscle bundles. Visiting a cryosauna also has a positive effect on the structure and functioning of skeletal muscles.

General characteristics of muscle tissue. Classification.

Morphofunctional characteristics. Regeneration of muscle tissue.

a) striated skeletal muscle tissue;

b) striated cardiac muscle tissue;

c) smooth muscle tissue.

1. General characteristics of muscle tissue. Classification.

Muscle tissue provides contractile processes in hollow internal organs and vessels, movement of body parts relative to each other, maintaining posture and moving the body in space. In addition to movement, contraction releases a large amount of heat, and thus muscle tissue participates in thermoregulation of the body.

Almost all types of cells have the property of contractility due to the presence in their cytoplasm of a contractile apparatus, represented by a network of thin microfilaments (5–7 nm), consisting of contractile proteins - actin, myosin, tropomyosin, etc. Due to the interaction of these microfilament proteins, contractile processes are carried out and ensured movement in the cytoplasm of hyaloplasm, organelles, vacuoles, formation of pseudopodia and invaginations of the plasmalemma, as well as the processes of phago- and pinocytosis, exocytosis, cell division and movement.

Any type 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 to muscle elements and transmit the contraction forces of muscle elements to the skeleton. However, the functionally leading elements of muscle tissue are muscle cells, or muscle fibers.

Muscle tissues are classified according to their structure, sources of origin and innervation, and functional characteristics.

The main groups of muscle tissue by structure:

smooth (unstriated) – mesenchymal; includes special:

neural origin;

epidermal origin;

striated (striated):

skeletal;

cardiac.

Each of the 2 groups, in turn, is divided into varieties both according to their sources of origin and according to their structure and functional characteristics.

Smooth muscle tissue, which is part of the internal organs and blood vessels, develops from mesenchyme.

Special muscle tissues of neural origin include smooth muscle cells of the iris, and of epidermal origin - myoepithelial cells of the salivary, lacrimal, sweat and mammary glands.

Striated muscle tissue is divided into skeletal and cardiac.

Both of these varieties develop from different parts of the mesoderm:

skeletal - from myotomes of somites;
cardiac - from the visceral layer of the splanchnotome.

Each type of muscle tissue has its own structural and functional unit.

Smooth muscle tissue of internal organs and the iris - smooth muscle cell - myocyte;

special epidermal origin - basket myoepitheliocyte-

cardiac – cardiomyocyte;

skeletal – muscle fiber.

2. Morphofunctional characteristics

a) striated skeletal muscle tissue

The structural and functional unit of striated muscle tissue is the muscle fiber.

It is an elongated cylindrical formation with pointed ends, from 1 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 sheath - the sarcolemma, in which two layers are clearly visible under an electron microscope: the inner one is a typical plasmalemma, and the outer one is a thin connective tissue plate - the basal lamina.

In the narrow gap between the plasmalemma and the basal lamina there are small cells - myosatellites.

Thus, 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 transmitting contraction forces to the connective tissue elements of the muscle.

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

Myosymplast is the main structural component of muscle fiber, both in volume and in function. It is formed through the fusion of independent undifferentiated muscle cells - myoblasts.

Myosymplast can be considered as an elongated giant multinucleated cell, consisting of a large number of 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 defined granular endoplasmic reticulum, a lamellar complex, and a small number of mitochondria are localized near the nuclei. There are no centrioles in the symplast. Sarcoplasm contains inclusions of glycogen and myoglobin, an analogue of red blood cell hemoglobin.

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

myofibrils;

sarcoplasmic reticulum;

T-system tubules.

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

Myofibrils are heterogeneous in length and are divided into:

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

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

The dark and light discs of all myofibrils are located at the same level and determine 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-disc, a dark stripe runs transversely to the actin myofilaments - the telophragm, or Z-line; in the middle of the A-disk there is a less pronounced M-line, or mesophragm.

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

With partial contraction of the myofibril, the actin myofilaments are drawn into the A-disc, and a light zone, or H-stripe, is formed in it, limited by the free ends of the actin myofilaments. The width of the H-band depends on the degree of myofibril contraction.

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-disc and the 2 halves of the 1-disc 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 plasmalemma of the myosymplast by actin myofilaments.

The structural elements of the 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 actin-myosin bridges between them, through which actin myofilaments are retracted into A-disks - shortening the sarcomere. For the development of this process, 3 conditions are necessary.

Availability of energy in the form of ATP;

presence of calcium ions; presence of biopotential.

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

The last 2 conditions are fulfilled 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 myofibrils. It is divided into fragments surrounding individual sarcomeres. Each fragment consists of 2 terminal cisterns connected by hollow anastomosing tubules - L-tubules. In this case, the terminal cisterns cover the sarcomere in the area of I-disks, and the tubules – in the area of A-disks.

The terminal cisterns and tubules contain calcium ions, which, upon receipt of a nerve impulse and reaching a wave of depolarization of the membranes of the sarcoplasmic reticulum, leave the cisterns and tubules and are distributed between actin and myosin myofilaments, initiating their interaction. After the depolarization wave ceases, calcium ions rush back into the terminal cisterns and tubules.

Thus, the sarcoplasmic reticulum not only serves as 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 plasma membrane and then through the T-tubules. They are not independent structural elements and represent 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-stripe or slightly more medially - in the area of junction of actin and myosin myofilaments. Consequently, 2 T-tubules approach and surround each sarcomere.

On the sides of each T-tubule there are 2 terminal cisterns of the sarcoplasmic reticulum of neighboring sarcomeres, which together with the T-tubules form a triad. There are contacts between the wall of the T-tubule and the walls of the terminal cisterns, through which a 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 biopotential from the plasmalemma to the sarcoplasmic reticulum.

Regeneration of skeletal muscle tissue, like 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, as well as an increase in the number of nuclei, which ultimately manifests itself in an increase in the functional capacity of the muscle fiber. The radioisotope method has established 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 sarcomeric filaments. As a result of this, myofibrils first thicken, 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, thereby achieving their elongation.

The sarcoplasmic reticulum and T-tubules in the hypertrophying fiber are formed due to the proliferation of previous elements.

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

Age-related hypertrophy of muscle fibers intensively manifests itself with the onset of physical 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 along the muscle fiber, myosatellites in the area of damage and in adjacent areas are disinhibited, intensively proliferate, and then migrate to the area of the muscle fiber defect, where they line up in chains, forming a myotube. Subsequent differentiation of the myotube leads to completion of the defect and restoration of the integrity of the muscle fiber;

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

buds 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 only be carried out in the following cases.

firstly, with preserved motor innervation of muscle fibers;

secondly, if connective tissue elements (fibroblasts) do not reach the area of damage, otherwise a connective tissue scar develops at the site of the muscle fiber defect.

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

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

placing the crushed tissue in the fascial bed;

suturing the motor nerve fiber to the crushed graft;

the presence of contractile movements of antagonist and synergist 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 motor neurons of the anterior horns of the spinal cord, and the muscles of the face and head - from motor neurons of certain cranial nerves.

Each muscle fiber is approached either by a branch from the axon of the motor neuron, or by the entire axon. 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 primarily provide posture maintenance, there are 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 wave of depolarization from the nerve ending is transmitted to the plasmalemma of the myosymplast, spreads further along the T-tubules and in the region of the triads is transmitted to the terminal tanks of the sarcoplasmic reticulum, causing the release of calcium ions and the beginning of the process of muscle fiber contraction.

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

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

muscle spindle;

Golgi tendon organ;

nonspecific receptor endings of a bushy or tree-like shape, distributed in loose connective tissue:

endomysium;

perimysium;

epimysium.

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 contracts.

Tendon organs are specialized encapsulated receptors that include several tendon fibers surrounded by a capsule, among which the terminal branches of the dendrite of a pseudounipolar neuron are distributed. When a muscle contracts, the tendon fibers come together and compress the nerve endings. The 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 movement is ensured (for example, when walking).

Trophic (vegetative) innervation is provided by the autonomic nervous system (ANS) (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 large numbers of arteries and veins, arterioles, venules and arteriole-venular anastomoses. The endomysium contains only capillaries, mostly narrow (4.5–7 µm), which provide trophism to the muscle fiber. The muscle fiber, together with the capillaries surrounding it and the motor ending, makes up the myon.

Muscles contain a large number of arteriolovenular anastomoses, which provide adequate blood supply during various muscle activities.

b) striated cardiac muscle tissue

Structural and functional unit of cardiac striated muscle tissueiscell – cardiomyocyte.

According to the structure and functions, cardiomyoitis is divided into 2 main groups:

typical, or contractile, cardiomyocytes, which together form the myocardium;

atypical cardiomyocytes that make up the conduction system of the heart and are, in turn, divided into 3 types.

Contractile cardiomyocyte It is an almost rectangular cell 50–120 µm in length, 15–20 µm wide, covered on the outside with a basal lamina. Usually 1 nucleus is localized in the center. In the sarcoplasm of the cardiomyocyte, myofibrils are located on the periphery of the nucleus, and between them and near the nucleus, mitochondria are localized in large numbers.

Unlike skeletal muscle tissue, myofibrils of cardiomyocytes are not separate cylindrical formations, but essentially a network consisting of anastomosing myofibrils, since some myofilaments seem to split off from one myofibril and continue obliquely into another. In addition, the dark and light disks of neighboring myofibrils are not always located at the same level, and therefore the transverse striation in cardiomyocytes is not as clearly expressed as in skeletal muscle fibers.

The sarcoplasmic reticulum, covering the myofibrils, is represented by dilated anastomosing tubules. Terminal tanks and triads are absent. T-tubules are present, but they are short, wide and formed by recesses not only of the plasmalemma, but also of the basal lamina. The mechanism of contraction in cardiomyocytes is practically no different from that in skeletal muscle fibers.

Contractile cardiomyocytes, connecting end to end with each other, form functional muscle fibers, between which there are numerous anastomoses. Thanks to this, a network is formed from individual cardiomyocytes - a functional syncytium. The presence of gap-like junctions between cardiomyocytes ensures their simultaneous and friendly contraction, first in the atria and then in the ventricles.

The contact areas of neighboring cardiomyocytes are called intercalary discs, although in fact there are no additional structures (discs) between cardiomyocytes: intercalary discs are the sites of V contacts of the cytolemma of neighboring cardiomyocytes, including simple, desmosomal and gap-like contacts.

Typically, intercalated discs are divided into transverse and longitudinal fragments.

In the region of the transverse fragments there are expanded desmosomal junctions. In these same places, actin filaments of sarcomeres are attached to the inner side of the plasma membranes.

In the area of longitudinal fragments, gap-like contacts are localized.

Through intercalary disks, both mechanical and metabolic (primarily ionic) communication of cardiomyocytes is ensured.

Contractile cardiomyocytes of the atria and ventricles differ somewhat in morphology and function.

Atrial cardiomyocytes in the sarcoplasm contain fewer myofibrils and mitochondria, T-tubules are almost not expressed in them, and instead of them, under the plasmalemma, a large number of vesicles and caveolae are detected - analogues of T-tubules. In addition, in the sarcoplasm of atrial cardiomyocytes at the poles of the nuclei, specific atrial granules are localized, consisting of glycoprotein complexes, L. Released from cardiomyocytes into the blood of the atria, these substances affect the level of blood pressure in the heart and blood vessels, and also prevent the formation of blood clots in the atria. Consequently, atrial cardiomyocytes, in addition to contractile, also have a secretory function.

In ventricular cardiomyocytes, contractile elements are more pronounced, and secretory granules are absent.

The second type of cardiomyocytes is atypical cardiomyocytes.

They form the conduction system of the heart, which includes:

sinoatrial node;

atrioventricular node;

atrioventricular bundle (bundle of His),

barrel, right and left

the terminal branches of the legs are Purkinje fibers.

Atypical cardiomyocytes ensure the generation of biopotentials, their conduction and transmission to contractile cardiomyocytes. In their morphology, atypical cardiomyocytes differ from the typical ones in a number of features: they are larger (length 100 µm, thickness 50 µm);

the cytoplasm contains few myofibrils, which are arranged in a disorderly manner, and therefore atypical cardiomyocytes do not have cross-striations; the plasmalemma does not form T-tubules;

in the intercalated discs between these cells there are no desmosomes or gap junctions.

Atypical cardiomyoitis of various parts of the otovascular conduction system

main varieties:

P-cells (pacemakers) - pacemakers (type I);

transitional cells (type II);

His bundle cells and Purkinje fibers (type III).

Type I cells (P cells) form the basis of the sinoatrial node and are also found in small quantities in the atrioventricular node. These cells are capable of independently generating biopotentials at a certain frequency and transmitting them to transitional cells (type II), and the latter transmit impulses to type III cells, from which the biopotentials are transmitted to contractile cardiomyocytes.

The sources of development of cardiomyocytes are myoepithelial plates, which are certain areas of the visceral layers of the splanchnotome, and more specifically, from the coelomic epithelium of these areas.

Biopotentials contractile cardiomyoitis are obtained from 2 sources:

conduction system of the heart (primarily from the sinus-atrial node);

ANS (from its sympathetic and parasympathetic parts).

Regeneration of cardiac muscle tissue differs in that cardiomyocytes regenerate only by the intracellular type. No proliferation of cardiomyocytes was observed. Cambial elements are absent in cardiac muscle tissue. When large areas of the myocardium are damaged (in particular, during myocardial infarction), restoration of the defect occurs due to the proliferation of connective tissue and the formation of scars (plastic regeneration). Naturally, there is no contractile function in these areas.

Damage to the conduction system is accompanied by disturbances in heart rhythm.

c) smooth muscle tissue

The vast majority smooth muscle tissue of the body (internal organs and blood vessels) is of mesenchymal origin. The structural and functional unit of smooth muscle tissue of internal organs and blood vessels is the myocyte.

Most often it is a spindle-shaped cell (20–500 µm long, 5-8 µm in diameter), covered on the outside with a basal lamina, but process myocytes are also found. In the center there is an elongated nucleus, at the poles of which common organelles are localized: granular endoplasmic reticulum, lamellar complex, mitochondria, cytocenter.

The cytoplasm contains thick (17 nm) myosin and thin (7 nm) actin myofilaments, which are located mainly parallel to each other along the myocyte axis and do not form A- and I-disks, which explains the lack of transverse striation of myocytes. In the cytoplasm of myocytes and on the inner surface of the plasma membrane there are numerous dense bodies to which actin, myosin, and intermediate filaments are attached. The plasmalemma forms small depressions - caveolae, which are considered as analogues of T-tubules. Numerous vesicles are localized under the plasmalemma, which, together with thin tubules of the cytoplasm, are elements of the sarcoplasmic reticulum.

The mechanism of contraction in myocytes is in principle similar to the contraction of sarcomeres in myofibrils in skeletal muscle fibers. It is carried out due to the interaction and sliding of actin myofilaments along myosin ones.

This interaction requires energy in the form of ATP, calcium ions and the presence of biopotential. Biopotentials come from the efferent endings of autonomic nerve fibers directly to myocytes or indirectly from neighboring cells through slit-like contacts and are transmitted through caveolae to the elements of the sarcoplasmic reticulum, causing the release of calcium ions from them into the sarcoplasm. Under the influence of calcium ions, mechanisms of interaction between actin and myosin filaments develop, similar to those that occur in the sarcomeres of skeletal muscle fibers, resulting in the sliding of these myofilaments and the movement of dense bodies in the cytoplasm. In myocytes, in addition to actin and myosin filaments, there are also intermediate filaments, which at one end are attached to the cytoplasmic dense bodies, and at the other to the attachment bodies on the plasmalemma and thus transfer the forces of interaction between actin and myosin filaments to the sarcolemma of the myocyte, which is how its shortening is achieved.

Myocytes are surrounded on the outside by loose fibrous connective tissue - endomysium and are connected to each other by lateral surfaces.

In the area of close contact of neighboring myocytes, the basal plates are interrupted. Myocytes are in direct contact with plasmalemmas and in these places there are gap-like contacts through which ionic communication and transfer of biopotential from one myocyte to another occurs, which leads to their simultaneous and friendly contraction.

A chain of myocytes, united by mechanical and metabolic connections, constitutes a functional muscle fiber. In the endomysium there are blood capillaries that provide trophism to myocytes, and in the layers of connective tissue between the bundles and layers of myocytes in the perimysium there are larger vessels and nerves, as well as vascular and nerve plexuses.

Efferent innervation of smooth muscle tissue is carried out by the ANS. In this case, the terminal branches of the axons of efferent autonomic neurons, passing along the surface of several myocytes, form small varicose thickenings on them, which slightly bend the plasmalemma and form myoneural synapses. When nerve impulses enter the synaptic cleft, mediators (acetylcholine or norepinephrine) are released and cause depolarization of the myocyte membranes and their subsequent contraction. Through gap-like junctions, biopotentials pass from one myocyte to another, which is accompanied by excitation and contraction of those smooth muscle cells that do not contain nerve endings. Excitation and contraction of myocytes are usually prolonged and provide tonic contraction of the smooth muscle tissue of blood vessels and hollow internal organs, including smooth muscle sphincters. These organs also contain numerous receptor endings in the form of bushes, trees or diffuse fields.

Regeneration of smooth muscle tissue is carried out in several ways:

through intracellular regeneration of hypertrophy with increased functional load;

through mitotic division of myocytes when they are damaged (reparative regeneration);

through differentiation from cambial elements - from adventitial cells and myofibroblasts.

Special smooth muscle tissues of neural origin develop from the neuroectoderm, from the edges of the wall of the optic cup, which is a protrusion of the diencephalon. From this source, myocytes develop, which form 2 muscles of the iris - the muscle that constricts the pupil and the muscle that dilates the pupil. In their morphology, myocytes of the iris do not differ from mesenchymal myocytes, however, each myocyte receives vegetative efferent innervation (the muscle that dilates the pupil is sympathetic, the muscle that constricts the pupil is parasympathetic). Thanks to this, the named muscles contract quickly and coordinated, depending on the power of the light beam. Myocytes of epidermal origin develop from the skin ectoderm and are not typical spindle-shaped, but stellate-shaped cells - myoepithelial cells located on the terminal sections of the salivary, mammary, lacrimal and sweat glands outside the secretory cells.

In their processes, myoepithelial cells contain actin and myosin filaments, thanks to the interaction of which the cell processes contract and promote the secretion of secretions from the terminal sections and small ducts of these glands into larger ducts. Efferent innervation is also obtained from the autonomic part of the nervous system.

Development. Human skeletal muscle tissue develops from the myotomes of mesodermal somites, and is therefore called somatic. Myotome cells differentiate in 2 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.

Muscle fiber(myofibra). Consists of 2 components: 1) myosatellitocytes and 2) myosymplast. The muscle fiber is approximately the same length as the muscle itself, with a diameter of 20-50 microns. On the outside, the fiber is covered with a sheath - sarcolemma, consisting of 2 membranes. The outer membrane is called basement membrane, and internal - 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. Cytoplasm muscle fiber is called sarcoplasm. 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, consisting 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, body phragm, mesophragm, and sarcolemma. Thanks to the cytosklet, 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-band, 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 at 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, limited 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.

Sarcomere- this is a structural and functional unit of myofibrils, which is a section located between two telophragms.

Sarcomere formula: 0.5 disk I + disk A + 0.5 disk I.

Myofibrils are surrounded by well-developed mitochondria and well-developed smooth ER.

Smooth XPS 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 Ca 2+ ions.

T-channels- these are invaginations of the plasmalemma. At the border of the disks from the plasmalemma 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) two lateral cisterns of smooth EPS. The function of triads is that in the relaxed state of myofibrils, Ca 2+ 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, Ca 2+ ions come out of the lateral cisterns. Without the latter, contraction of myofibrils is impossible, because in actin filaments the centers of interaction with myosin filaments are blocked by tropomyosin. Ca 2+ ions unblock these centers, after which the interaction of actin filaments with myosin filaments and contraction begin.

The mechanism of myofibril contraction. When actin filaments interact with myosin filaments, Ca 2+ ions unblock the adhesion centers of actin filaments with the heads of myosin molecules, after which these protrusions attach to the adhesion centers on actin filaments and, like an oar, 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, and since the ends of the actin filaments also approach the mesophragm and each other, the H-band narrows.

Thus, during myofibril contraction, disc I and the H-stripe narrow.

After the termination of the action potential, Ca 2+ 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 the myofibrils, their relaxation occurs, i.e., the actin filaments return to their original position, and the width of disk I and the H-stripe is restored.

Myosatellite cells 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. The function of myosatellitocytes is cambial cells involved in the regeneration of muscle fibers when they are damaged.

The 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 the 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 epimysium.

Connection of muscle fibers with collagen fibers of tendons. 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, myoglobin content, glycogen, and enzyme activity.

Type I(red fibers) is characterized by a high myoglobin content (therefore the fibers are red), high succinate dehydrogenase activity, a slow type ATPase, not very rich in glycogen content, duration of contraction and low fatigue.

II type(white fibers) is 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 types I and II muscle fibers, there are also intermediate ones that have the properties of both.

Every muscle contains all types of muscle fibers. Their number may vary depending on physical activity.

Regeneration of striated muscle tissue. When muscle fibers are damaged, their ends at the site of injury undergo necrosis. After the fibers break, macrophages arrive at their fragments, which phagocytose the necrotic areas, clearing them of dead tissue. Then 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.

1st way regeneration lies in the fact that at the ends of broken fibers the granular EPS hypertrophies, 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 dangling end to the other and eventually 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.

2nd way regeneration consists in the fact 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 terminal and pierce the basement membrane of the sarcolemma and then plunge into the depths of the muscle fiber, dragging the plasmalemma with it. As a result, a neuromuscular ending is formed - a motor plaque.

The structure of the neuromuscular ending. The neuromuscular ending has 2 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 Na + ions move from the outer surface of the plasma membrane to the inner surface, and K + 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.

Sensory nerve endings(neuromuscular spindles - fusi neuromuscularis) 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 bag (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 encircle 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.

Professor Suvorova G.N.

Muscle tissue.

They are a group of tissues that perform 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) maintaining posture

4) movement of the organism in space.

Muscle tissues have the following morphofunctional characteristics:

1) Their structural elements have an elongated shape.

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

3) Muscle contraction requires a large amount of energy, therefore they contain:

Contains a large number of mitochondria

There are trophic inclusions

The 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 distinguished, these include

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

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

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

Skeletal – from myotomes of somites

Cardiac - from the visceral layer of the splanchnotome.

Skeletal muscle tissue

Makes up about 35-40% of the human body weight. As a main component, it is part of skeletal muscles; in addition, it forms the muscular basis of the tongue, is part of the muscular lining 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

Muscular tubules

The definitive form of myogenesis is muscle fiber.

The structure of skeletal muscle tissue.

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

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

1. myosymplast

2. myosatellite cells.

On the outside, the muscle fiber is covered with a basement membrane, which, together with the myosymplast plasmalemma, forms the so-called sarcolemma.

Myosimplast is the main component of muscle fiber both in volume and function. 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, and 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

Excitation transmission apparatus from the sarcolemma

to the contractile apparatus.

Energy

Support

Contractile apparatus muscle fiber is represented by myofibrils.

Myofibrils have the form of threads (muscle fiber length) with a diameter of 1-2 microns. They have transverse striations due to the alternation of sections (disks) that refract polarized light differently - 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 coincide. This determines the striation of the entire fiber.

The dark and light discs are in turn made up of thick and thin filaments called myofilaments.

In the middle of the light disk, transverse to the thin myofilaments, there is a dark stripe - the telophragm, or Z-line.

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

Sarcomere is considered the structural and functional unit of the myofibril - it includes the A-disc and the two halves of the I-disc located on either side of it.

Fat 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.

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 filaments do not touch. Thick and thin filaments slide past each other without hindrance, resulting in muscle fibers that do not resist passive stretching. This condition is characteristic of the extensor muscle when the corresponding flexor contracts.

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

The Ca++ ions bind to the troponin molecule, which is displaced, exposing the myosin binding sites on the thin filaments.

The myosin head attaches to the myosin-binding regions of the thin filament.

The myosin head changes conformation and makes a rowing motion that moves the thin filament towards the center of the sarcomere.

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

Sarcotubular system– ensures the accumulation of calcium ions and is an excitation transmission apparatus. For this, a wave of depolarization passing through the plasmalemma leads to 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 coupling. 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 depolarization wave propagated along the plasma membrane first reaches the T-tubules. There are specialized contacts between the wall of the T-tubule and the terminal cisternae, through which the depolarization wave reaches the membrane of the terminal cisterns, after which calcium ions are released.

Support apparatus muscle fiber is represented by cytoskeletal elements that provide an ordered arrangement of myofilaments and myofibrils. These include:

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

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

In addition, 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 the sarcolemma.

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

In addition to myosymplast, muscle fibers include myosatellite cells. These are small cells that are located between the plasmalemma and the basement membrane and represent 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, high activity of oxidative enzymes (SDH), and a predominance of aerobic processes. These fibers have the ability of slow but prolonged tonic contraction and low fatigue.

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

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 trophism to the fiber. 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.

Skeletal muscle tissue

Sectional diagram of a skeletal muscle.

Structure of skeletal muscle

Skeletal (striated) muscle tissue- elastic, elastic tissue capable of contracting under the influence of nerve impulses: one of the types of muscle tissue. Forms the skeletal muscles of humans and animals, designed to perform various actions: body movement, contraction of the vocal cords, breathing. Muscles consist of 70-75% water.

Histogenesis

The source of development of skeletal muscles are myotome cells - myoblasts. Some of them differentiate in places where so-called autochthonous muscles are formed. Others migrate from myotomes to mesenchyme; at the same time, they are already determined, although outwardly they do not differ from other mesenchymal cells. Their differentiation continues in places where other muscles of the body are formed. During differentiation, 2 cell lines arise. The cells of the first merge, forming symplasts - muscle tubes (myotubes). The cells of the second group remain independent and differentiate into myosatellites (myosatellite cells).

In the first group, differentiation of specific organelles of myofibrils occurs; gradually they occupy most of the lumen of the myotube, pushing the cell nuclei to the periphery.

The cells of the second group remain independent and are located on the surface of the myotubes.

Structure

The structural unit of muscle tissue is the muscle fiber. It consists of myosimplast and myosatellitocytes (companion cells), covered with a common basement membrane.

The length of the muscle fiber can reach several centimeters with a thickness of 50-100 micrometers.

The structure of myosymplast

The structure of myosatellites

Myosatellites are mononuclear cells adjacent to the surface of the myosymplast. These cells are poorly differentiated and serve as adult stem cells of muscle tissue. In case of fiber damage or prolonged increase in load, cells begin to divide, ensuring the growth of myosymplast.

Mechanism of action

The functional unit of skeletal muscle is the motor unit (MU). ME includes a group of muscle fibers and the motor neuron that innervates them. The number of muscle fibers that make up one IU varies in different muscles. For example, where fine control of movements is required (in the fingers or in the muscles of the eye), the motor units are small, they contain no more than 30 fibers. And in the calf muscle, where fine control is not needed, there are more than 1000 muscle fibers in the ME.

Motor units of the same muscle can be different. Depending on the speed of contraction, motor units are divided into slow (S-ME) and fast (F-ME). And F-ME, in turn, is divided according to its resistance to fatigue into fatigue-resistant (FR-ME) and fast-fatigable (FF-ME).

The motor neurons innervating these MEs are divided accordingly. There are S-motoneurons (S-MN), FF-motoneurons (F-MN) and FR-motoneurons (FR-MN). S-ME are characterized by a high content of myoglobin protein, which is capable of binding oxygen (O2). Muscles predominantly composed of this type of ME are called red muscles due to their dark red color. Red muscles perform the function of maintaining human posture. Extreme fatigue of such muscles occurs very slowly, and restoration of functions occurs, on the contrary, very quickly.

This ability is determined by the presence of myoglobin and a large number of mitochondria. Red muscle MEs typically contain a large number of muscle fibers. FR-ME constitute muscles that are capable of performing rapid contractions without noticeable fatigue. FR-ME fibers contain a large number of mitochondria and are capable of generating ATP through oxidative phosphorylation.

Typically, the number of fibers in FR-ME is less than in S-ME. FF-ME fibers are characterized by a lower mitochondrial content than FR-ME, as well as the fact that ATP is produced in them through glycolysis. They lack myoglobin, so muscles consisting of this type of ME are called white. The white muscles develop a strong and rapid contraction, but tire quite quickly.

Function

This type of muscle tissue provides the ability to perform voluntary movements. The contracting muscle acts on the bones or skin to which it is attached. In this case, one of the attachment points remains motionless - the so-called fixation point(lat. punctum fixum), which in most cases is considered as the initial section of the muscle. A moving muscle fragment is called moving point, (lat. punctum mobile), which is the place of its attachment. However, depending on the function performed, punctum fixum can act as punctum mobile, and vice versa.