Structural unit of smooth muscle tissue. Skeletal muscle tissue
1. Types of muscle tissue 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 and others. Due to the interaction of the named microfilament proteins, contractile processes are carried out and the movement of hyaloplasm, organelles, vacuoles in the cytoplasm, the formation of pseudopodia and invaginations of the plasmalemma, 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 unequally 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 tissue
, which provide 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, therefore, muscle tissue participates in thermoregulation of the body.
Muscle tissues are not the same by structure, sources of origin And innervation, by functional features. Finally, it should be noted that 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 the muscle elements and transmit the contraction forces of the muscle elements to the skeleton. However, functionally leading elements of muscle tissue are muscle cells or muscle fibers.
Classification of muscle tissue:
- smooth (unstriated) - mesenchymal;
- special - neural origin and epidermal origin;
- striated (striated ):
- skeletal;
- cardiac.
Smooth muscle tissue, which is part of the internal organs and blood vessels, develops from mesenchyme.
TO special muscle tissues of neural origin include smooth muscle cells of the iris, epidermal origin - myoepithelial cells of the salivary, lacrimal, sweat and mammary glands.
Cross-striped muscle tissue is divided into skeletal and cardiac. Both of these varieties develop not only from the mesoderm, but from its different parts:
- skeletal - from myotomes of somites;
- cardiac - from the visceral layer 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, length from 1 mm to 40 mm (and according to some data 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.
Myosatellite cells are cambial (germ) elements of muscle fibers and play a role in the processes of their physiological and reparative regeneration.
Myosimplast is the main structural component of muscle fiber, both in volume and in functions performed. 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 10,000) 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 erythrocyte hemoglobin.
A distinctive feature of myosimplast is also the presence in it specialized organelles, which include :
- myofibrils;
- sarcoplasmic reticulum;
- T-system tubules.
By its structure myofibrils are heterogeneous in length and are divided into:
- 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.
The section of myofibril located between two Z-lines is called sarcomere and is the structural and functional unit of the myofibril. The sarcomere includes the A-disc and the two halves of the I-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. It should be noted that the terminal sarcomeres of each myofibril are attached to the plasma membrane of the myosymplast by actin myofilaments. The structural elements of the sarcomere in a relaxed state can be expressed formula:
Z+1/2I+1/2A+M+1/2A+1/2I+Z.
3. Muscle contractions Reduction process carried out through the interaction of actin and myosin filaments and the formation between them actin-myosin bridges, through which actin myofilaments are retracted into A-discs and the sarcomere is shortened. To develop this process it is necessary three conditions:
- availability of energy in the form of ATP ;
- presence of calcium ions;
- presence of biopotential .
Sarcoplasmic reticulum is a modified smooth endoplasmic reticulum and consists of dilated cavities and anastomosing tubules surrounding myofibrils. In this case, the sarcoplasmic reticulum is divided into fragments surrounding individual sarcomeres. Each fragment consists of two terminal tanks, connected by hollow anastomosing tubules - L-tubules. In this case, the terminal cisterns cover the sarcomere in the region of the I-discs, and the tubules - in the region of the A-disc. 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 is not only a reservoir for calcium ions, but also plays the role of a calcium pump.
Depolarization wave transmitted to the sarcoplasmic reticulum from the nerve ending first along the plasmalemma, and then along T-tubules , which are not independent structural elements.
They are tubular protrusions of the plasmalemma into the sarcoplasm. Penetrating deeper, T-tubules branch and cover each myofibril within one bundle strictly at the same level, usually at the level of the Z-stripe or somewhat more medially - in the area of junction of actin and myosin myofilaments. Consequently, two T-tubules approach and surround each sarcomere. On the sides of each T-tubule there are two 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.
For the interaction of actin and myosin myofilaments and subsequent contraction, in addition to calcium ions, energy is also required in the form of ATP, which is produced in sarcosomes, located in large quantities between the myofibrils.
The process of interaction between actin and myosin filaments can be simplified as follows. Under the influence of calcium ions, the ATPase activity of myosin is stimulated, which leads to the breakdown of ATP, with the formation of ADP and energy. Thanks to the released energy, bridges are established between actin and myosin (more specifically, bridges are formed between the heads of the myosin protein and certain points on the actin filament) and due to the shortening of these bridges, the actin filaments are pulled between the myosin filaments. Then these bonds break down (again using energy) and the myosin heads form new contacts with other points on the actin filament, but located more distal to the previous ones. This results in a gradual retraction of actin filaments between myosin filaments and shortening of the sarcomere. The degree of this contraction depends on the concentration of calcium ions near the myofilaments and on the ATP content. After the death of the organism, ATP is not formed in sarcosomes, its remains are spent on the formation of actin-myosin bridges, and there is no longer enough for decay, resulting in post-mortem muscle rigor, which stops after autolysis (disintegration) of tissue elements.
When the sarcomere contracts completely, actin filaments reach the M-strip of the sarcomere. In this case, H-stripes and I-disks disappear, and the sarcomere formula can be expressed as follows:
Z+1/2IA+M+1/2AI+Z.
With partial contraction, the sarcomere formula can be presented as follows:
Z+1/nI+1/nIA+1/2H+M+1/2H+1/nAJ+1/nI+Z.
The simultaneous concomitant contraction of all sarcomeres of each myofibril leads to a contraction of the entire muscle fiber. The extreme sarcomeres of each myofibril are attached by actin myofilaments to the plasmalemma of the myosymplast, which is folded at the ends of the muscle fiber. At the same time, at the ends of the muscle fiber the basal plate does not enter the folds of the plasmalemma. It is pierced by thin collagen and reticular fibers, penetrates into the recesses of the folds of the plasmalemma and attaches to those places to which actin filaments of distal sarcomeres are attached from the inside. This creates a strong connection between the myosymplast and the fibrous structures of the endomysium. . The collagen and reticular fibers of the terminal muscle fibers, together with the fibrous structures of the endomysium and perimysium, collectively form muscle tendons, which are attached to certain points of the skeleton or are woven into the reticular layer of the dermis in the facial area. Due to muscle contraction, parts or the entire body move, as well as a change in the relief of the face.
4. Types of muscle fibers In muscle tissue there are two main types of muscle fibers windows, between which there are intermediate ones, differing from each other, primarily in the characteristics of metabolic processes and functional properties and, to a lesser extent, in 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, high activity of succinate dehydrogenase (SDH) in them, and 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 myoglobin content, but a high glycogen content, high phosphorylase activity and a fast-type ATP base. Functionally characterized by the ability of fast, strong, but short-term contraction. Between the two extreme types of muscle fibers are intermediate, characterized by different combinations of these inclusions and different activities of the listed enzymes.
Fibrous connective tissue forms layers in the muscle:
- endomysium;
- perimysium;
- epimysium;
- as well as tendons.
Perimysium surrounds several muscle fibers collected in bundles. It contains larger vessels (arteries and veins, as well as arteriole-venular anastomoses).
Epimysium or fascia surrounds the entire muscle, promotes the functioning of the muscle as an organ. Any muscle contains all types of muscle fibers in varying proportions. Red fibers predominate in the muscles that maintain posture. In the muscles that provide movement of the fingers and hands, white or transitional fibers predominate. The character of muscle fiber can change depending on functional load and training. It has been established that the biochemical, structural and functional characteristics of muscle fiber depend on innervation. Cross transplantation of efferent nerve fibers and their endings from red fiber to white and vice versa leads to a change in metabolism, as well as structural and functional features in these fibers to the opposite type.
Chapter 9. MUSCLE TISSUE
Chapter 9. MUSCLE TISSUE
Muscle tissue (textus muscularis) call tissues that are different in structure and origin, but similar in their ability to contract. They provide movement in space of the body as a whole, its parts and the movement of organs within the body (heart, tongue, intestines, etc.).
Cells of many tissues have the property of contracting and changing shape, but in muscle tissue this ability becomes the main function.
9.1. GENERAL MORPHOFUNCTIONAL CHARACTERISTICS AND CLASSIFICATION
The main morphological characteristics of muscle tissue elements are an elongated shape, the presence of longitudinally located myofibrils and myofilaments - special organelles that ensure contractility, the location of mitochondria next to the contractile elements, the presence of inclusions of glycogen, lipids and myoglobin.
Special contractile organelles - myofilaments or myofibrils, provide contraction, which occurs when two main fibrillar proteins interact in them - actin and myosin, with the obligatory participation of calcium ions. Mitochondria provide energy for these processes. The reserve of energy sources is formed by glycogen and lipids. Myoglobin is a protein that ensures the binding of oxygen and the creation of its reserve at the time of muscle contraction, when the blood vessels are compressed (the supply of oxygen is sharply reduced).
Classification. The classification of muscle tissue is based on two principles - morphofunctional and histogenetic. In accordance with the morphofunctional principle, depending on the structure of the organelles of contraction, muscle tissue is divided into two subgroups.
First subgroup- striated (striated) muscle tissue (textus muscularis striatus). In the cytoplasm of their elements, myosin filaments
You are constantly polymerized and form permanent myofibrils with actin filaments. The latter are organized into characteristic complexes - sarcomeres. In neighboring myofibrils, the structural subunits of sarcomeres are located at the same level and create transverse striation.
Second subgroup- smooth (unstriated) muscle tissue (textus muscularis nonstriatus). These tissues are characterized by the fact that, outside of contraction, the myosin filaments are depolymerized. In the presence of calcium ions, they polymerize and interact with actin filaments. The myofibrils formed in this case do not have transverse striations: with special stains they are represented by uniformly colored (smooth) threads along the entire length.
In accordance with the histogenetic principle, depending on the sources of development (embryonic rudiments), muscle tissue and muscle elements are divided into: somatic (myotome), coelomic (from the myoepicardial plate of the visceral layer of the splanchnotome), mesenchymal (from the desmal rudiment as part of the mesenchyme), neural (from neural tube), epidermal (from the cutaneous ectoderm and from the prechordal plate).
9.2. STRIPED MUSCLE TISSUE
There are two main types of striated (striated) tissues - skeletal (myotome) and cardiac (coelomic).
9.2.1. Skeletal muscle tissue
Histogenesis. Source of development of elements of skeletal (somatic) striated muscle tissue (textus muscularis striatus sceletalis) are the stem cells of myotomes - promyoblasts. Some of them differentiate in situ and participate in the formation of so-called autochthonous muscles. Other cells migrate from the myotomes to the mesenchyme. 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, two cell lineages arise. Cells of one of the lines merge, forming elongated symplasts - muscular tubes (myotubes). In them, differentiation of special organelles - myofibrils - occurs (Fig. 9.1). At this time, a well-developed granular endoplasmic reticulum is noted in the myotubes. Myofibrils are first located under the plasma membrane and then fill most of the myotube. The nuclei, on the contrary, move from the central sections to the periphery. Cell centers and microtubules completely disappear. Granular endo-
Rice. 9.1. Histogenesis of skeletal muscle tissue (according to A. A. Klishov):
A- promyoblasts; b- myosimplast; V- muscular tube; G- mature muscle
fiber. 1 - myosatellite cell; 2 - myosimplast core; 3 - myofibrils
the plasmatic reticulum is significantly reduced. Such definitive structures are called myosymplasts.
Cells of the other line remain independent and differentiate into myosatellite cells. These cells are located on the surface of myosymplasts. Myosatellite cells, multiplying, merge with myosymplasts, thus participating in the creation of optimal nuclear
Rice. 9.2. Structure of striated muscle tissue (micrograph):
1 - muscle fibers; 2 - sarcolemma; 3 - sarcoplasm and myofibrils; 4 - cores
myosymplast. Staining - iron hematoxylin
sarcoplasmic ratio necessary for the synthesis of specific symplast proteins.
Structure. The main structural unit of skeletal muscle tissue is the muscle fiber, consisting of myosymplast and myosatellite cells covered with a common basement membrane (Fig. 9.2-9.4). The length of the entire fiber can be measured in centimeters with a thickness of 50-100 microns. The complex consisting of the myosymplast plasma membrane and the basement membrane is called the sarcolemma.
The structure of myosymplast. The myosymplast has many elongated nuclei located directly under the plasmalemma. Their number in one symplast can reach several tens of thousands (see Fig. 9.2). At the poles of the nuclei there are organelles of general importance - the Golgi complex and small fragments of the agranular endoplasmic reticulum. Myofibrils fill the main part of the myosymplast and are located longitudinally (see Fig. 9.3).
Sarcomere- structural unit of myofibril. Each myofibril has transverse dark and light disks with unequal refraction (anisotropic A-disks and isotropic I-disks). Each myofibril is surrounded by longitudinally located and anastomosing loops of the agranular endoplasmic reticulum - the sarcoplasmic reticulum. Adjacent sarcomeres have a common boundary structure - the Z-line (Fig. 9.5). It is built in the form of a network of protein fibrillar molecules, among which alpha-actinin plays a significant role. The ends of actin filaments are associated with this network. From neighboring Z-lines, actin new filaments are directed towards the center of the sarcomere, but do not reach its middle. Actin filaments are combined with the Z-line and myosin filaments
Rice. 9.3. Scheme of the ultramicroscopic structure of myosymplast (according to R.V. Krstic, with modifications) (a): 1 - sarcomere; 2 - anisotropic disk (band A); 2a - isotropic disk (band I); 3 - line M (mesophragm) in the middle of the anisotropic disk; 4 - line Z (telophragm) in the middle of the isotropic disk; 5 - mitochondria; 6 - sarcoplasmic reticulum; 6a - final tank; 7 - transverse tubule (T-tubule); 8 - triad; 9 - sarcolemma; b- diagram of the spatial arrangement of mitochondria in the symplast. The upper and lower planes of the figure limit the anisotropic dissarcomere (according to L. E. Bakeeva, V. P. Skulachev, Yu. S. Chentsov); V- endomysium. Scanning electron micrograph, magnification 2600 (preparation by Yu. A. Khoroshkov): 1 - muscle fibers; 2 - collagen fibrils
fibrillar inextensible nebulin molecules. In the middle of the dark disk of the sarcomere there is a network built of myomyosin. It forms an M-line in cross-section. The ends of myosin filaments are attached to the nodes of this M-line. Their other ends are directed towards the Z-lines and
Rice. 9.4. Superficial portion of myosymplast and myosatellite cell. Electron micrograph, magnification 10,000 (preparation by V. L. Goryachkina and S. L. Kuznetsov): 1 - basement membrane; 2 - plasmalemma; 3 - myosimplast core; 4 - myos-tellitocyte nucleus; 5 - myofibrils; 6 - tubules of the agranular endoplasmic (sarcoplasmic) reticulum; 7 - mitochondria; 8 - glycogen
Rice. 9.5. Sarcomere (diagram):
1 - line Z; 2 - line M; 3 - actin filaments; 4 - myosin filaments; 5 - fibrillar titin molecules (according to B. Alberts, D. Bray, J. Lewis and others, with modifications)
Rice. 9.6. Conformational changes entailing mutual displacement of actin and myosin filaments:
a-c- consistent changes in spatial relationships. 1 - actin; 2 - head of the myosin molecule (according to B. Alberts, D. Bray, J. Lewis et al., with modifications)
located between actin filaments, but also do not reach the Z-lines themselves. At the same time, these ends are fixed in relation to the Z-lines by stretchable giant titin protein molecules.
Myosin molecules have a long tail and two heads at one end. With an increase in the concentration of calcium ions in the area where the heads are attached (the hinge region), the molecule changes its configuration (Fig. 9.6). In this case (since actin filaments are located between the myosin filaments), the myosin heads bind to actin (with the participation of auxiliary proteins - tropomyosin and troponin). The myosin head then bends and pulls the actin molecule toward the M line. The Z-lines come closer together, the sarcomere shortens.
Alpha-actinin networks of Z-lines of neighboring myofibrils are connected to each other by intermediate filaments. They approach the inner surface of the plasmalemma and are fixed in its cortical layer, so that the sarcomeres of all myofibrils are located at the same level. When observed through a microscope, this creates the impression of transverse striations throughout the entire fiber.
The source of calcium ions is the cisterns of the agranular endoplasmic reticulum. They are extended along the myofibrils near each sarcomere and form the sarcoplasmic reticulum. It is in it that calcium ions accumulate when the myosymplast is in a relaxed state. At the level of the Z-lines (in amphibians) or at the border of the A- and I-disks (in mammals), the tubules of the network change direction and are located transversely, forming expanded terminal or lateral (L) cisterns.
From the surface into the depth of the myosymplast, the plasmalemma forms long tubes running transversely (T-tubules) at the level of the boundaries between the dark and light disks. When the myosymplast receives a signal to begin contraction, it moves along the plasmalemma in the form of an action potential and spreads to the membrane of the T-tubules. Since this membrane is close to the membranes of the sarcoplasmic reticulum, the state of the latter changes, calcium is released from the cisterns of the network and interacts with actin-myosin complexes (they contract). When the action potential disappears, calcium again accumulates in the tubules of the network and myofibril contraction stops. Energy is needed to develop the contraction force. It is released due to the conversion of ATP to ADP. The role of ATPase is performed by myosin. The source of ATP is mainly mitochondria, which is why they are located directly between myofibrils.
Inclusions of myoglobin and glycogen play a major role in the activity of myosymplasts. Glycogen serves as a source of energy necessary not only to perform muscle work, but also to maintain the thermal balance of the entire body. Myoglobin binds oxygen when the muscle is relaxed and blood flows freely through small blood vessels. During muscle contraction, the vessels are compressed, and stored oxygen is released and participates in biochemical reactions.
Myosatellite cells. These poorly differentiated cells are the source of muscle tissue regeneration. They are adjacent to the surface of the myosymplast, so that their plasmalemmas are in contact (see Fig. 9.1, 9.4). Myosatellitocytes are mononuclear, their dark nuclei are oval in shape and smaller than in symplasts. They possess all organelles of general importance (including the cell center).
Types of muscle fibers. Different muscles (like organs) function under different biomechanical conditions. Therefore, muscle fibers within different muscles have different strength, speed and duration of contraction, as well as fatigue. The activity of enzymes in them is different, and they are presented in different isomeric forms. The content of respiratory enzymes - glycolytic and oxidative - is also different in them.
Rice. 9.7. Activity of succinate dehydrogenase in muscle fibers of different types (preparation by V.F. Chetvergov, treatment according to Nakhlas et al.): 1 - high; 2 - low; 3 - average
Based on the ratio of myofibrils, mitochondria and myoglobin, white, red and intermediate fibers are distinguished. According to their functional characteristics, muscle fibers are divided into fast, slow and intermediate, which is determined by the molecular organization of myosin. Among its isoforms, there are two main ones - “fast” and “slow”. When performing histochemical reactions, they are identified by ATPase activity. The activity of respiratory enzymes also correlates with these properties. Typically, glycolytic processes predominate in fast fibers; they are rich in glycogen and contain less myoglobin, which is why they are called white. In slow fibers, on the contrary, the activity of oxidative enzymes is higher, they are richer in myoglobin, and look more red.
Along with white and red, there are also intermediate fibers. In most skeletal muscles, fibers of different histochemical types are arranged in a mosaic pattern (Fig. 9.7).
The properties of muscle fibers change with changes in loads - sports, professional, and also in extreme conditions (weightlessness). Such changes are reversible when returning to normal activities. In some diseases (muscular atrophy, dystrophy, consequences of denervation), muscle fibers with different initial properties change differently. This allows you to clarify the diagnosis, for which skeletal muscle biopsies are examined.
Regeneration. The nuclei of myosimplasts cannot divide, since there are no cellular centers in the sarcoplasm. The cambial elements are myosatellite cells. While the body grows, they divide, and the daughter cells merge with myosymplasts. At the end of growth, the proliferation of myosatellite cells dies out. After damage to the muscle fiber at some distance from the site of injury, it is destroyed and its fragments
you are phagocytosed by macrophages. Tissue restoration is carried out through two mechanisms: compensatory hypertrophy of the symplast itself and proliferation of myosatellite cells. In the symplast, the granular endoplasmic reticulum and the Golgi complex are activated. The synthesis of substances necessary for the restoration of sarcoplasm and myofibrils occurs, as well as the assembly of membranes, so that the integrity of the plasmalemma is restored. The damaged end of the myosymplast thickens, forming a muscle bud. Myosatellite cells preserved near the damage divide. Some of them migrate to the muscle bud and integrate into it, others merge (just like myoblasts during histogenesis) and form new myotubes, which develop into muscle fibers.
9.2.2. Skeletal muscle as an organ
The transmission of contraction forces to the skeleton is carried out through tendons or muscle attachments directly to the periosteum. At the end of each muscle fiber, the plasmalemma forms deep narrow invaginations. Thin collagen fibers penetrate into them from the side of the tendon or periosteum. The latter are spirally braided with reticular fibers. The ends of the fibers are directed to the basement membrane, enter it, turn back and, upon exiting, again braid the collagen fibers of the connective tissue.
Between the muscle fibers there are thin layers of loose fibrous connective tissue - endomysium. The collagen fibers of the outer layer of the basement membrane are woven into it (see Fig. 9.3, c), which contributes to the pooling of forces during contraction of the myosymplasts. Thicker layers of loose connective tissue surround several muscle fibers, forming perimysium and dividing the muscle into bundles. Several bundles are combined into larger groups, separated by thicker connective tissue layers. The connective tissue surrounding the surface of a muscle is called epimysium.
Vascularization. The arteries enter the muscle and spread through the layers of connective tissue, gradually thinning. Branches of the fifth and sixth order form arterioles in the perimysium. Capillaries are located in the endomysium. They run along the muscle fibers, anastomosing with each other. Venules, veins and lymphatic vessels pass next to the afferent vessels. As usual, there are many mast cells near the vessels, which take part in regulating the permeability of the vascular wall.
Innervation. Myelinated efferent (motor), afferent (sensory), as well as unmyelinated autonomic nerve fibers were identified in the muscles. The process of a nerve cell, bringing an impulse from a motor neuron of the spinal cord, branches in the perimysium. Each of its branches penetrates the basement membrane and forms terminals at the surface of the symplast on the plasmalemma, participating in the organization of the so-called motor plaque (see Chapter 10, Fig. 10.18). On admission
Rice. 9.8. A fragment of a muscle spindle containing muscle fibers with a nuclear chain (a) and a nuclear bag (b) (diagram according to G. S. Katinas): 1 - nuclei; 2 - myofibrils (organelles of general importance are not shown)
of a nerve impulse, acetylcholine is released from the terminals, a mediator that causes excitation (action potential), spreading from here along the plasmalemma of the myosymplast.
So, each muscle fiber is innervated independently and surrounded by a network of hemocapillaries, forming a complex called mion.
A group of muscle fibers innervated by one motor neuron is called neuromuscular unit. Muscle fibers belonging to one neuromuscular unit do not lie side by side, but are located mosaically among fibers belonging to other units.
Sensitive nerve endings are not located on working (extrafusal) muscle fibers, but are associated with specialized muscle fibers in the so-called muscle fibers.
tenah (with intrafusal muscle fibers), which are located in the perimysium.
Intrafusal muscle fibers. The intrafusal muscle fibers of the spindles are much thinner than the workers. There are two types of them - fibers with a nuclear bag and fibers with a nuclear chain (Fig. 9.8). The nuclei in both are rounded and located in the thickness of the symplast, and not at its surface. In fibers with a nuclear bursa, the symplast nuclei form clusters in its thickened middle part. In fibers with a nuclear chain, no thickening is formed in the middle part of the symplast; the nuclei lie here longitudinally, one after the other. Organelles of general importance are located near the clusters of nuclei.
Myofibrils are located at the ends of symplasts. The fiber sarcolemma connects to the neuromuscular spindle capsule, which is composed of dense fibrous connective tissue. Each spindle muscle fiber is spirally wrapped around a sensory nerve fiber terminal. As a result of contraction or relaxation of working muscle fibers, the tension of the connective tissue capsule of the spindle changes, and the tone of the intrafusal muscle fibers changes accordingly. As a result, the sensitive nerve endings that wrap around them are excited, and afferent nerve impulses arise in the area of the terminals. Each myosymplast also has its own motor plaque. That is why intrafusal muscle fibers are constantly in tension, adapting to the length of the muscle belly as a whole.
9.2.3. Cardiac muscle tissue
Histogenesis and cell types. Sources of development of cardiac striated muscle tissue (textus muscularis striatus cardiacus)- symmetrical areas of the visceral layer of the splanchnotome in the cervical part of the embryo - myoepicardial plates. Epicardial mesothelial cells also differentiate from them. The original cells of cardiac muscle tissue - cardiomyoblasts- characterized by a number of characteristics: the cells are flattened, contain a large nucleus, light cytoplasm, poor in ribosomes and mitochondria. Subsequently, the development of the Golgi complex, a granular endoplasmic reticulum, occurs. Cardiomyoblasts exhibit fibrillar structures, but no myofibrils. The cells have a high proliferative potential.
After a series of mitotic cycles, cardiomyoblasts differentiate into cardiomyocytes, in which sarcomerogenesis begins (Fig. 9.9). In the cytoplasm of cardiomyocytes, the number of polysomes and tubules of the granular endoplasmic reticulum increases, glycogen granules accumulate, and the volume of the actomyosin complex increases. Cardiomyocytes contract, but do not lose the ability to further proliferation and differentiation. The development of the contractile apparatus in the late embryonic and postnatal periods occurs through the addition of new sarcomeres and the layering of newly synthesized myofilaments.
Differentiation of cardiomyocytes is accompanied by an increase in the number of mitochondria, their distribution at the poles of the nuclei and between myofibrils and proceeds in parallel with the specialization of the contacting surfaces of cells. Cardiomyocytes form cardiac muscle fibers through end-to-end and end-to-side contacts, and in general the tissue is a network-like structure. Some cardiomyocytes in the early stages of cardiomyogenesis are contractile-secretory. Subsequently, as a result of divergent differentiation, “dark” (contractile) and “light” (conductive) myocytes arise, in which secretory granules disappear, while in atrial myocytes they are preserved. This is how the differential of endocrine cardiomyocytes is formed. These cells contain a centrally located nucleus with dispersed chromatin and one or two nucleoli. In the cytoplasm, the granular endoplasmic reticulum and dictyosomes of the Golgi complex are well developed, in close connection with the elements of which there are numerous secretory granules with a diameter of about 2 microns, containing electron-dense material. Subsequently, secretory granules are found under the sarcolemma and are released into the intercellular space by exocytosis.
In general, during histogenesis, five types of cardiomyocytes arise - working (contractile), sinus (pacemaker), transitional, conducting, and secretory. Working (contractile) cardiomyocytes form their own chains (Fig. 9.10). It is they, when shortened, that provide the force of contraction of the entire heart muscle. Working cardiomyocytes are capable of
Rice. 9.9. Histogenesis of cardiac muscle tissue (scheme according to P. P. Rumyantsev): A- cardiomyocytes in the wall of the heart tube; b - cardiomyocytes in late embryogenesis; V- cardiomyocytes in the postnatal period. 1 - cardiomyocyte; 2 - mitotically dividing cardiomyocyte; 3 - myofilaments and myofibrils
transmit control signals to each other. Sinus (pacemaker) cardiomyocytes capable of automatically changing a state of contraction to a state of relaxation in a certain rhythm. The cells perceive control signals from nerve fibers, in response to which they change the rhythm of contractile activity. Sinus (pacemaker) cardiomyocytes transmit control signals transitional cardiomyocytes, and the latter - to conducting and working cardiomyocytes. Conducting cardiomyocytes form chains of cells connected at their ends and are located under the endo-
Rice. 9.10. The structure of cardiac muscle tissue (micrograph). Staining - iron hematoxylin:
1 - cardiomyocyte nucleus; 2 - chain of cardiomyocytes; 3 - insert discs
carded The first cell in the chain receives control signals from sinus cardiomyocytes and transmits them further to other conducting cardiomyocytes. The cells that close the chain transmit the signal through the transitional cardiomyocytes to the workers. Secretory cardiomyocytes perform a special function. They produce the peptide hormone cardiodilatin, which circulates in the blood in the form of cardionatrine, causes contraction of smooth muscle cells of arterioles, increases renal blood flow, accelerates glomerular filtration and sodium excretion. All cardiomyocytes are covered with a basement membrane.
The structure of contractile (working) cardiomyocytes. The cells have an elongated (100-150 µm) shape, close to cylindrical. Their ends are connected to each other, so that the chains of cells form the so-called functional fibers (up to 20 microns thick). In the area of cell contacts, so-called intercalary discs are formed (Fig. 9.10). Cardiomyocytes can branch and form a spatial network. Their surfaces are covered with a basement membrane, into which reticular and collagen fibers are woven from the outside. The nucleus of the cardiomyocyte (sometimes there are two of them) is oval and lies in the central part of the cell. A few organelles of general importance are concentrated at the poles of the nucleus, with the exception of the agranular endoplasmic reticulum and mitochondria.
Special organelles that provide contraction are called myofibrils. They are weakly separated from each other and can split. Their structure is similar to the structure of myofibrils of myosymplast of skeletal muscle fiber. Each mitochondria is located throughout the entire sarcomere. T-tubules located at the level of the Z-line are directed from the surface of the plasmalemma into the depths of the cardiomyocyte. Their membranes are close together
contact with the membranes of the smooth endoplasmic (sarcoplasmic) reticulum. The loops of the latter are elongated along the surface of the myofibrils and have lateral extensions (L-systems), which together with the T-tubules form triads or dyads (Fig. 9.11, a). The cytoplasm contains inclusions of glycogen and lipids, especially many inclusions of myoglobin. The mechanism of contraction of cardiomyocytes is the same as that of myosymplast.
Organization of cardiomyocytes into tissue. Cardiomyocytes connect to each other in an end-to-end manner. Intercalated discs are formed here: these areas look like thin plates at medium magnification of a light microscope. In fact, the ends of cardiomyocytes have an uneven surface, so the protrusions of one cell fit into the depressions of another. The transverse sections of the protrusions of neighboring cells are connected to each other by interdigitations and desmosomes (Fig. 9.11, b).
Rice. 9.11. Structure of a cardiomyocyte: A- diagram (according to Yu. I. Afanasyev and V. L. Goryachkina); b- electron micrograph of the intercalary disk. Magnification 20,000. 1 - myofibrils; 2 - mitochondria; 3 - sarcotubular network; 4 - T-tubules; 5 - basement membrane; 6 - lysosome; 7 - insert disk; 8 - desmosome; 9 - zone of myofibril attachment; 10 - slot contacts; 11 - glycogen
Each desmosome is approached from the cytoplasm by a myofibril, which is fixed at its end in the desmo-plakin complex. Thus, during contraction, the thrust of one cardiomyocyte is transferred to another. The lateral surfaces of the cardiomyocyte protrusions are united by nexuses (gap junctions). This creates metabolic connections between them and ensures synchronized contractions.
Regeneration. In the histogenesis of cardiac muscle tissue, the cambium does not appear. Therefore, tissue regeneration proceeds on the basis of intracellular hyperplastic processes. At the same time, the cardiomyocytes of mammals, primates and humans are characterized by the process of polyploidy.
tions. For example, in monkeys, the nuclei of up to 50% of terminally differentiated cardiomyocytes become tetra- and octoploid. Polyploid cardiomyocytes arise due to acytokinetic mitosis, which leads to multinucleation. In conditions of pathology of the human cardiovascular system (rheumatism, congenital heart defects, myocardial infarction, etc.), intracellular regeneration, polyploidization of nuclei, and the emergence of multinuclear cardiomyocytes play an important role in compensating for damage to cardiomyocytes.
9.3. SMOOTH MUSCLE TISSUE
There are three groups of smooth (non-striated) muscle tissues (textus muscularis nonstriatus) and cells: mesenchymal, neural and myoepithelial cells.
9.3.1. Muscle tissue of mesenchymal origin
Histogenesis. This tissue is divided into two types: visceral and vascular. In embryonic histogenesis, even electron microscopically it is difficult to distinguish mesenchymal precursors of fibroblasts from smooth myocytes. In poorly differentiated smooth myocytes, a granular endoplasmic reticulum and Golgi complex are developed. Thin filaments are oriented along the long axis of the cell. As development progresses, the size of the cell and the number of filaments in the cytoplasm increase. Gradually, the volume of cytoplasm occupied by contractile filaments increases, and their location in the cytoplasm becomes more and more ordered. The proliferative activity of smooth myocytes gradually decreases during myogenesis. This occurs as a result of an increase in the duration of the cell cycle, the exit of cells from the reproduction cycle and the transition to a differentiated state. Differentiating, they synthesize components of the intercellular matrix, basement membrane collagen, and elastin. In definitive cells (myocytes), the synthetic ability is reduced, but does not disappear completely.
Structure and functioning of cells. A smooth myocyte is a spindle-shaped cell 20-500 µm long and 5-8 µm wide. The nucleus is rod-shaped and is located in its central part. When a myocyte contracts, its nucleus bends and even twists (Fig. 9.12-9.14).
The structure of definitive smooth myocytes (leiomyocytes), which are part of the internal organs and the walls of blood vessels, has much in common, but at the same time is characterized by heteromorphy. Thus, in the walls of veins and arteries, ovoid, spindle-shaped, process myocytes 10-40 µm long, sometimes reaching 140 µm, are found. The greatest length smooth myo-
The cells reach up to 500 microns in the uterine wall. The diameter of myocytes ranges from 2 to 20 µm. Depending on the nature of intracellular biosynthetic processes, contractile and secretory myocytes are distinguished. The former are specialized for contraction functions, but at the same time retain secretory activity.
Secretory myocytes resemble fibroblasts in their ultrastructure, but contain in their cytoplasm bundles of thin myofilaments located at the periphery of the cell. The Golgi complex, granular endoplasmic reticulum, many mitochondria, glycogen granules, free ribosomes and polysomes are well developed in the cytoplasm. According to the degree of maturity, such cells are classified as poorly differentiated. Actin filaments form a three-dimensional network in the cytoplasm, elongated predominantly longitudinally. The ends of the filaments are fastened to each other and to the plasmalemma by special cross-linking proteins. These areas are clearly visible on electron micrographs as dense bodies. Myosin monomers are located next to actin filaments. The plasmalemma forms invaginations - caveolae, in which calcium ions are concentrated. The signal to contract usually comes through nerve fibers. The mediator, which is released from their terminals, changes the permeability of the plasma membrane. Calcium ions are released, which entails both the polymerization of myosin and the interaction of myosin with actin.
There is a retraction of actin myofilaments between the myosi-
Rice. 9.12. Structure of a smooth myocyte (diagram):
A, V- when relaxing; b, d- at the greatest contraction; G- with incomplete contraction; v-d- enlarged images of areas outlined in frames on fragments A and b. 1 - plasmalemma; 2 - dense bodies; 3 - core; 4 - endoplasm; 5 - contractile complexes; 6 - mitochondria; 7 - basement membrane; 8 - actin (thin) myofilaments; 9 - myosin (thick) myofilaments
Rice. 9.13. Ultrastructure of a differentiating smooth myocyte in the bronchial wall:
1 - core; 2 - cytoplasm with myofilaments; 3 - Golgi complex, magnification 35,000 (preparation by A. L. Zashikhin)
new, dense spots come closer together, the force is transferred to the plasmalemma, and the entire cell shortens (see Fig. 9.12). When signals from the nervous system cease, calcium ions move from the cytoplasm into caveolae and into the tubules of the endoplasmic reticulum, myosin depolymerizes and the “myofibrils” disintegrate. The contraction stops. Thus, actinomyosin complexes exist in smooth myocytes only during contraction in the presence of free calcium ions in the cytoplasm.
Myocytes are surrounded by a basement membrane. In certain areas, “windows” are formed in it, so the plasma membranes of neighboring myocytes come closer together. Here nexuses are formed, and not only mechanical, but also metabolic connections arise between cells. Elastic and reticular fibers pass over the “cases” of the basement membrane between the myocytes, uniting the cells into a single tissue complex. Smooth myocytes synthesize proteoglycans, glycoproteins, procollagen, proelastin, from which collagen and elastic fibers and the amorphous component of the intercellular matrix are formed. The interaction of myocytes is carried out with the help of cytoplasmic bridges, mutual invaginations, nexuses, desmosomes, and areas of membrane contacts of the surfaces of myocytes.
Regeneration. Smooth muscle tissue of the visceral and vascular types has significant sensitivity to the effects of extreme factors. In activated myocytes, the level of biosynthetic processes increases, the morphological expression of which is the synthesis of contractile proteins, enlargement and hyperchromatosis of the nucleus, hypertrophy of the nucleolus, an increase in the nuclear-cytoplasmic ratio, an increase in the number of free ribosomes and polysomes, active
Rice. 9.14. The structure of smooth muscle tissue (volume diagram) (according to R.V. Krstic, with modifications):
1 - spindle-shaped smooth myocytes; 2 - myocyte cytoplasm; 3 - myocyte nuclei; 4 - plasmalemma; 5 - basement membrane; 6 - superficial pinocytotic vesicles; 7 - intercellular connections; 8 - nerve ending; 9 - collagen fibrils; 10 - microfilaments
tion of enzymes, aerobic and anaerobic phosphorylation, membrane transport. Cellular regeneration is carried out both due to differentiated cells that have the ability to enter the mitotic cycle, and due to the activation of cambial elements (small volume myocytes). Under the influence of a number of damaging factors, a phenotypic transformation of contractile myocytes into secretory ones is noted. This transformation is often observed with damage to the vascular intima, the formation of intimal hyperplasia during the development of atherosclerosis.
Rice. 9.15. Ultrastructure of myopigmentocyte (preparation by N. N. Sarbaeva): 1 - nucleus; 2 - myofilaments, magnification 6000
9.3.2. Muscle tissue of the mesenchymal type as part of organs
Myocytes are united into bundles, between which there are thin layers of connective tissue. Reticular and elastic fibers surrounding the myocytes are woven into these layers. Blood vessels and nerve fibers pass through the layers. The terminals of the latter end not directly on the myocytes, but between them. Therefore, after the arrival of a nerve impulse, the transmitter spreads diffusely, exciting many cells at once. Smooth muscle tissue of mesenchymal origin is represented mainly in the walls of blood vessels and many hollow internal organs.
Smooth muscle tissue within specific organs has different functional properties. This is due to the fact that on the surface of organs there are different receptors for specific biologically active substances. Therefore, their reaction to many medications is not the same. It is possible that the different functional properties of tissues are also associated with the specific molecular organization of actin filaments.
9.3.3. Muscle tissue of neural origin
The muscle tissue of the iris and ciliary body belongs to the fourth type of contractile tissue. Myocytes of this tissue develop from the cells of the neural primordium as part of the inner wall of the optic cup. In a row
Rice. 9.16. Myoepithelial cells in the terminal section of the salivary gland (scheme according to G. S. Katinas):
A- cross section; b- view from the surface. 1 - nuclei of myoepitheliocytes; 2 - processes of myoepithelial cells; 3 - nuclei of secretory epithelial cells; 4 - basement membrane
In vertebrates, the muscular elements of the iris show a variety of divergent differentiation. Thus, myoneural tissue in reptiles and birds is represented by striated multinuclear fibers that are very similar to skeletal-type muscles. In mammals and humans, the main structural and functional unit of the iris muscles is the smooth mononuclear myocyte, or myopigmentocyte. The latter have a pigmented body containing one nucleus, located outside the fusiform contractile part (Fig. 9.15).
The cytoplasm of cells contains a large number of mitochondria and pigment granules, which are similar in size and shape to the granules of the pigment epithelium. Myofilaments in myopigmentocytes are divided into thin (7 nm) and thick (1.5 nm), in size and location they resemble the myofilaments of smooth myocytes. Each myopigmentocyte is surrounded by a basement membrane. Near the cytoplasmic processes of myocytes, unmyelinated nerve fibers are found. Depending on the direction of the processes (perpendicular or parallel to the edge of the pupil), myocytes form two muscles - the constrictor and the dilator of the pupil.
Regeneration. A few studies have shown low regeneration activity after damage or its absence.
9.3.4. Muscle cells of epidermal origin
Myoepithelial cells develop from the epidermal primordium. They are found in the sweat, mammary, salivary and lacrimal glands and share common precursors with their secretory cells. Myoepithelial
the cells are directly adjacent to the epithelial cells proper and have a common basement membrane with them. During regeneration, both cells are also restored from common poorly differentiated precursors. Most myoepithelial cells are stellate in shape. These cells are often called basket cells: their processes cover the terminal sections and small ducts of the glands (Fig. 9.16). In the cell body there is a nucleus and organelles of general importance, and in the processes there is a contractile apparatus, organized as in cells of mesenchymal muscle tissue.
Control questions
1. Genetic classification of muscle tissue. Structural and functional units of different types of muscle tissue.
2. Striated skeletal muscle tissue: development, structure, morphological basis of muscle contraction. Regeneration.
3. Striated cardiac muscle tissue: development, specific structure of various types of cardiomyocytes, regeneration.
4. Types of smooth myocytes: sources of development, topography in the body, regeneration.
Histology, embryology, cytology: textbook / Yu. I. Afanasyev, N. A. Yurina, E. F. Kotovsky, etc. - 6th ed., revised. and additional - 2012. - 800 p. : ill.
A human muscle is an organ of the body (soft tissue) consisting of muscle fibers capable of contracting under the influence of nerve impulses and providing the basic functions of the human body: movement, breathing, nutrition, resistance to stress, etc.
When a muscle contracts (under the influence of nerve impulses), it is distinguished between an actively contracting part - the abdomen - and a passive part, with the help of which it is attached to the bones - the tendon. Considered in general, skeletal muscle is a complex structure consisting of striated muscle tissue, various types of connective (tendon) and nervous (muscle nerves) tissues, endothelium and smooth muscle fibers (vessels).
The structural unit of skeletal muscle is the muscle fiber. It is an elongated, cylindrical cell with multiple nuclei, having a width of 10-100 microns and a length from several millimeters to 30 cm.
A cross-section of the longitudinal fibrous muscle shows that it consists of primary bundles containing 20 - 60 fibers. Each bundle is separated by a connective tissue membrane - the perimysium, and each fiber - by the endomysium. In different muscles there are from several hundred to several hundred thousand fibers with a diameter from 20 to 100 microns and a length of up to 12 - 16 cm.
An individual fiber is covered with a true cell membrane - the sarcolemma. Immediately below it, approximately every 5 microns along the length, are the nuclei. The fibers have a characteristic transverse striation, which is caused by alternating optically more and less dense areas.
The fiber is formed by many (1000 - 2000 or more) densely packed myofibrils (diameter 0.5 - 2 microns), stretching from end to end. Between the myofibrils, mitochondria are located in rows, where the processes of oxidative phosphorylation necessary to supply the muscle with energy occur.
The structural and functional contractile unit of the myofibril is the sarcomere, a repeating section of the fibril bounded by two stripes.
Sarcomeres in the myofibril are separated from each other by Z-plates, which contain the protein beta-actinin. Thin actin filaments extend from the Z plate in both directions. In the spaces between them are thicker myosin filaments.
Actin filament externally resembles two strings of beads twisted into a double helix, where each bead is an actin protein molecule. In the recesses of actin helices, at equal distances from each other, lie molecules of the protein troponin, connected to filamentous molecules of the protein tropomyosin
Myosin filaments are formed by repeating molecules of the protein myosin. Each myosin molecule has a head and a tail. The myosin head can bind to an actin molecule, forming a so-called cross bridge.
The cell membrane of the muscle fiber forms invaginations (transverse tubules), which perform the function of conducting excitation to the membrane of the sarcoplasmic reticulum. The sarcoplasmic reticulum (longitudinal tubes) is an intracellular network of closed tubes and performs the function of depositing Ca++ ions.
Chemical composition of muscle tissue. Human muscle tissue contains 72–80% water and 20–28% dry matter of the muscle mass. Water is part of most cellular structures and serves as a solvent for many substances. Most of the dry residue is formed by proteins and other organic compounds.
1 g of striated muscle tissue contains about 100 mg of contractile proteins, mainly myosin and actin, which form the actinomyosin complex (filament).
The composition of the dry residue of muscles, along with proteins, also includes other substances, among which are nitrogen-containing, nitrogen-free extractive substances and minerals. Of the lipids in muscle tissue, triglycerides are found in the form of fat droplets, as well as cholesterol.
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Muscle tissue combines the ability to contract.
Structural features: contractile apparatus, which occupies a significant part of the cytoplasm of the structural elements of muscle tissue and consists of actin and myosin filaments, which form organelles for special purposes - myofibrils .
Classification of muscle tissue
1. Morphofunctional classification:
1) Striated or striated muscle tissue: skeletal and cardiac;
2) Unstriated muscle tissue: smooth.
2. Histogenetic classification (depending on the sources of development):
1) Somatic type(from myotomes of somites) – skeletal muscle tissue (striated);
2) Coelomic type(from the myoepicardial plate of the visceral layer of the splanchnotome) – cardiac muscle tissue (striated);
3) Mesenchymal type(develops from mesenchyme) – smooth muscle tissue;
4) From cutaneous ectoderm And prechordal plate– myoepithelial cells of glands (smooth myocytes);
5) Neural origin (from the neural tube) - myoneural cells (smooth muscles that constrict and dilate the pupil).
Functions of muscle tissue: movement of a body or its parts in space.
SKELETAL MUSCLE TISSUE
Striated (cross-striped) muscle tissue makes up up to 40% of the mass of an adult, is part of the skeletal muscles, muscles of the tongue, larynx, etc. They are classified as voluntary muscles, since their contractions are subject to the will of the person. These are the muscles that are used when playing sports.
Histogenesis. Skeletal muscle tissue develops from myotome cells, myoblasts. There are head, cervical, thoracic, lumbar, and sacral myotomes. They grow in the dorsal and ventral directions. The branches of the spinal nerves grow into them early. Some myoblasts differentiate in place (form autochthonous muscles), while others, from the 3rd week of intrauterine development, migrate into the mesenchyme and, merging with each other, form muscular tubes (myotubes)) with large centrally oriented nuclei. In myotubes, differentiation of special organelles of myofibrils occurs. Initially they are located under the plasmalemma, and then fill most of the myotube. The nuclei are shifted to the periphery. Cell centers and microtubules disappear, grEPS is significantly reduced. This multi-core structure is called simplast , and for muscle tissue – myosimplast . Some myoblasts differentiate into myosatellitocytes, which are located on the surface of myosymplasts and subsequently take part in the regeneration of muscle tissue.
The structure of skeletal muscle tissue
Let us consider the structure of muscle tissue at several levels of living organization: at the organ level (muscle as an organ), at the tissue level (muscle tissue itself), at the cellular level (the structure of muscle fiber), at the subcellular level (the structure of myofibril) and at the molecular level (the structure of actin and myosin threads).
On the map:
1 - gastrocnemius muscle (organ level), 2 - cross section of the muscle (tissue level) - muscle fibers, between which the RVST: 3 - endomysium, 4 - nerve fiber, 5 - blood vessel; 6 - cross section of muscle fiber (cellular level): 7 - nuclei of muscle fiber - symplast, 8 - mitochondria between myofibrils, blue - sarcoplasmic reticulum; 9 — cross section of myofibril (subcellular level): 10 — thin actin filaments, 11 — thick myosin filaments, 12 — heads of thick myosin filaments.
1) Organ level: structure muscles as an organ.
Skeletal muscle consists of bundles of muscle fibers linked together by a system of connective tissue components. Endomysium– PBCT layers between muscle fibers where blood vessels and nerve endings pass . Perimysium– surrounds 10-100 bundles of muscle fibers. Epimysium– the outer shell of the muscle, represented by dense fibrous tissue.
2) Tissue level: structure muscle tissue.
The structural and functional unit of skeletal striated (striated) muscle tissue is muscle fiber– a cylindrical formation with a diameter of 50 microns and a length from 1 to 10-20 cm. Muscle fiber consists of 1) myosymplast(see its formation above, structure - below), 2) small cambial cells - myosatellite cells, adjacent to the surface of the myosymplast and located in the recesses of its plasmalemma, 3) the basement membrane, which covers the plasmalemma. The complex of plasmalemma and basement membrane is called sarcolemma. The muscle fiber is characterized by transverse striations, the nuclei are shifted to the periphery. Between the muscle fibers there are layers of PBST (endomysium).
3) Cellular level: structure muscle fiber (myosymplast).
The term “muscle fiber” implies “myosymplast”, since myosymplast provides the contraction function, myosatellite cells are involved only in regeneration.
Myosimplast, like a cell, consists of 3 components: a nucleus (more precisely, many nuclei), cytoplasm (sarcoplasm) and plasmolemma (which is covered with a basement membrane and is called sarcolemma). Almost the entire volume of the cytoplasm is filled with myofibrils - special-purpose organelles; general-purpose organelles: grEPS, aEPS, mitochondria, Golgi complex, lysosomes, and also nuclei are shifted to the periphery of the fiber.
In the muscle fiber (myosymplast), functional devices are distinguished: membrane, fibrillar(contractive) and trophic.
Trophic apparatus includes nuclei, sarcoplasm and cytoplasmic organelles: mitochondria (energy synthesis), grEPS and Golgi complex (synthesis of proteins - structural components of myofibrils), lysosomes (phagocytosis of worn-out structural components of the fiber).
Membrane apparatus: each muscle fiber is covered with a sarcolemma, where an outer basement membrane and a plasmalemma (under the basement membrane) are distinguished, which forms invaginations ( T-tubes). To each T- the tube is adjacent to two tanks triad: two L-tubes (aEPS tanks) and one T-tubule (invagination of the plasmalemma). AEPS are concentrated in tanks Ca 2+ required for reduction. Myosatellite cells are adjacent to the plasmalemma on the outside. When the basement membrane is damaged, the mitotic cycle of myosatellite cells starts.
Fibrillar apparatus.Most of the cytoplasm of the striated fibers is occupied by special-purpose organelles - myofibrils, oriented longitudinally, providing the contractile function of the tissue.
4) Subcellular level: structure myofibrils.
When examining muscle fibers and myofibrils under a light microscope, there is an alternation of dark and light areas in them - discs. Dark disks are birefringent and are called anisotropic disks, or A- disks. Light-colored disks are not birefringent and are called isotropic, or I-disks.
In the middle of the disk A there is a lighter area - N- a zone where only thick filaments of the myosin protein are contained. In the middle N-zones (which means A-disk) the darker one stands out M-line consisting of myomesin (necessary for the assembly of thick filaments and their fixation during contraction). In the middle of the disk I there is a dense line Z, which is built from protein fibrillar molecules. Z-line is connected to neighboring myofibrils using the protein desmin, and therefore all the named lines and disks of neighboring myofibrils coincide and a picture of striated muscle fiber is created.
The structural unit of the myofibril is sarcomere (S) — it is a bundle of myofilaments enclosed between two Z-lines. The myofibril consists of many sarcomeres. Formula describing the structure of the sarcomere:
S = Z 1 + 1/2 I 1 + A + 1/2 I 2 + Z 2
5) Molecular level: structure actin And myosin filaments .
Under an electron microscope, myofibrils appear as aggregates of thick, or myosin, and thin, or actin, filaments. Between the thick filaments there are thin filaments (diameter 7-8 nm).
Thick filaments, or myosin filaments,(diameter 14 nm, length 1500 nm, distance between them 20-30 nm) consist of myosin protein molecules, which is the most important contractile protein of muscle, 300-400 myosin molecules in each strand. The myosin molecule is a hexamer consisting of two heavy and four light chains. Heavy chains are two helically twisted polypeptide strands. They bear spherical heads at their ends. Between the head and the heavy chain there is a hinge section with which the head can change its configuration. In the area of the heads there are light chains (two on each). Myosin molecules are arranged in a thick filament in such a way that their heads face outward, protruding above the surface of the thick filament, and the heavy chains form the core of the thick filament.
Myosin has ATPase activity: the released energy is used for muscle contraction.
Thin filaments, or actin filaments,(diameter 7-8 nm), formed by three proteins: actin, troponin and tropomyosin. The main protein by mass is actin, which forms a helix. Tropomyosin molecules are located in the groove of this helix, troponin molecules are located along the helix.
Thick filaments occupy the central part of the sarcomere - A-disc, thin occupy I- discs and partially insert between thick myofilaments. N-zone consists only of thick threads.
At rest interaction of thin and thick filaments (myofilaments) impossible, because The myosin-binding sites of actin are blocked by troponin and tropomyosin. At a high concentration of calcium ions, conformational changes in tropomyosin lead to the unblocking of the myosin-binding regions of actin molecules.
Motor innervation of muscle fiber. Each muscle fiber has its own innervation apparatus (motor plaque) and is surrounded by a network of hemocapillaries located in the adjacent RVST. This complex is called mion. A group of muscle fibers innervated by a single motor neuron is called neuromuscular unit. In this case, the muscle fibers may not be located nearby (one nerve ending can control from one to dozens of muscle fibers).
When nerve impulses arrive along the axons of motor neurons, muscle fiber contraction.
Muscle contraction
During contraction, the muscle fibers shorten, but the length of the actin and myosin filaments in the myofibrils does not change, but they move relative to each other: myosin filaments move into the spaces between actin filaments, actin filaments - between myosin filaments. As a result, the width is reduced I-disk, H-stripes and the length of the sarcomere decreases; width A-disk does not change.
Sarcomere formula at full contraction: S = Z 1 + A+ Z 2
Molecular mechanism of muscle contraction
1. The passage of a nerve impulse through the neuromuscular synapse and depolarization of the plasmalemma of the muscle fiber;
2. The depolarization wave travels along T-tubules (invaginations of the plasmalemma) to L-tubules (cisterns of the sarcoplasmic reticulum);
3. Opening of calcium channels in the sarcoplasmic reticulum and release of ions Ca 2+ into sarcoplasm;
4. Calcium diffuses to the thin filaments of the sarcomere, binds to troponin C, leading to conformational changes in tropomyosin and freeing active centers for binding myosin and actin;
5. Interaction of myosin heads with active centers on the actin molecule with the formation of actin-myosin “bridges”;
6. Myosin heads “walk” along actin, forming new connections between actin and myosin during movement, while the actin filaments are pulled into the space between the myosin filaments towards M-lines, bringing two together Z-lines;
7. Relaxation: Ca 2+ -ATPase of the sarcoplasmic reticulum pumps Ca 2+ from sarcoplasm into cisterns. In the sarcoplasm the concentration Ca 2+ becomes low. Troponin bonds are broken WITH with calcium, tropomyosin closes the myosin-binding sites of thin filaments and prevents their interaction with myosin.
Each movement of the myosin head (attachment to actin and detachment) is accompanied by the expenditure of ATP energy.
Sensory innervation(neuromuscular spindles). Intrafusal muscle fibers, together with sensory nerve endings, form neuromuscular spindles, which are receptors for skeletal muscle. A spindle capsule is formed on the outside. When striated (striated) muscle fibers contract, the tension of the connective tissue capsule of the spindle changes and the tone of the intrafusal (located under the capsule) muscle fibers changes accordingly. A nerve impulse is formed. When a muscle is overstretched, a feeling of pain occurs.
Classification and types of muscle fibers
1. By the nature of the contraction: phasic and tonic muscle fibers. Phasic are capable of performing rapid contractions, but cannot maintain the achieved level of shortening for a long time. Tonic muscle fibers (slow) ensure the maintenance of static tension or tone, which plays a role in maintaining a certain position of the body in space.
2. By biochemical characteristics and color allocate red and white muscle fibers. The color of the muscle is determined by the degree of vascularization and myoglobin content. A characteristic feature of red muscle fibers is the presence of numerous mitochondria, the chains of which are located between the myofibrils. In white muscle fibers there are fewer mitochondria and they are located evenly in the sarcoplasm of the muscle fiber.
3. By type of oxidative metabolism : oxidative, glycolytic and intermediate. Identification of muscle fibers is based on the activity of the enzyme succinate dehydrogenase (SDH), which is a marker for mitochondria and the Krebs cycle. The activity of this enzyme indicates the intensity of energy metabolism. Release muscle fibers A-type (glycolytic) with low SDH activity, WITH-type (oxidative) with high SDH activity. Muscle fibers IN-types occupy an intermediate position. Transition of muscle fibers from A-type in WITH-type marks changes from anaerobic glycolysis to oxygen-dependent metabolism.
For sprinters (athletes, when a quick short contraction is needed, bodybuilders), training and nutrition are aimed at the development of glycolytic, fast, white muscle fibers: they have a lot of glycogen reserves and energy is produced primarily through the anaeolbic pathway (white meat in chicken). Stayers (athletes - marathon runners, in those sports where endurance is required) have a predominance of oxidative, slow, red fibers in the muscles - they have a lot of mitochondria for aerobic glycolysis, blood vessels (they need oxygen).
4. In striated muscles, two types of muscle fibers are distinguished: extrafusal, which predominate and determine the actual contractile function of the muscle and intrafusal, which are part of proprioceptors - neuromuscular spindles.
Factors that determine the structure and function of skeletal muscle are the influence of nervous tissue, hormonal influence, location of the muscle, level of vascularization and motor activity.
CARDIAC MUSCLE TISSUE
Cardiac muscle tissue is located in the muscular layer of the heart (myocardium) and in the mouths of the large vessels associated with it. It has a cellular type of structure and the main functional property is the ability to spontaneous rhythmic contractions (involuntary contractions).
It develops from the myoepicardial plate (visceral layer of the splanchnotome of the mesoderm in the cervical region), the cells of which multiply by mitosis and then differentiate. Myofilaments appear in the cells, which further form myofibrils.
Structure. The structural unit of cardiac muscle tissue is a cell cardiomyocyte. Between the cells there are layers of PBCT with blood vessels and nerves.
Types of cardiomyocytes : 1) typical ( workers, contractile), 2) atypical(conductive), 3) secretory.
Typical cardiomyocytes
Typical (working, contractile) cardiomyocytes– cylindrical cells, up to 100-150 microns long and 10-20 microns in diameter. Cardiomyocytes form the main part of the myocardium, connected to each other in chains by the bases of the cylinders. These zones are called insert discs, in which desmosomal contacts and nexuses (slit-like contacts) are distinguished. Desmosomes provide mechanical cohesion that prevents cardiomyocytes from separating. Gap junctions facilitate the transmission of contraction from one cardiomyocyte to another.
Each cardiomyocyte contains one or two nuclei, sarcoplasm and plasmalemma, surrounded by a basement membrane. There are functional apparatuses, the same as in muscle fiber: membrane, fibrillar(contractile), trophic, and energetic.
Trophic apparatus includes the nucleus, sarcoplasm and cytoplasmic organelles: grEPS and Golgi complex (synthesis of proteins - structural components of myofibrils), lysosomes (phagocytosis of structural components of the cell). Cardiomyocytes, like fibers of skeletal muscle tissue, are characterized by the presence in their sarcoplasm of the iron-containing oxygen-binding pigment myoglobin, which gives them a red color and is similar in structure and function to erythrocyte hemoglobin.
Energy apparatus represented by mitochondria and inclusions, the breakdown of which provides energy. Mitochondria are numerous, lying in rows between fibrils, at the poles of the nucleus and under the sarcolemma. The energy required by cardiomyocytes is obtained by splitting: 1) the main energy substrate of these cells - fatty acids, which are deposited in the form of triglycerides in lipid droplets; 2) glycogen, located in granules located between fibrils.
Membrane apparatus : Each cell is covered with a membrane consisting of a plasmalemma complex and a basement membrane. The shell forms invaginations ( T-tubes). To each T-the tubule is adjacent to one tank (unlike the muscle fiber - there are 2 tanks) sarcoplasmic reticulum(modified aEPS), forming dyad: one L-tube (aEPS tank) and one T-tubule (invagination of the plasmalemma). In AEPS tanks ions Ca 2+ do not accumulate as actively as in muscle fibers.
Fibrillar (contractile) apparatus .Most of the cytoplasm of the cardiomyocyte is occupied by special-purpose organelles - myofibrils, oriented longitudinally and located along the periphery of the cell. The contractile apparatus of working cardiomyocytes is similar to skeletal muscle fibers. When relaxed, calcium ions are released into the sarcoplasm at a low rate, which ensures automaticity and frequent contractions of cardiomyocytes. T-tubules are wide and form dyads (one T-tube and one tank network), which converge in the area Z-lines.
Cardiomyocytes, connecting with the help of intercalary discs, form contractile complexes that contribute to the synchronization of contraction; lateral anastomoses are formed between cardiomyocytes of neighboring contractile complexes.
Function of typical cardiomyocytes: providing the force of contraction of the heart muscle.
Conducting (atypical) cardiomyocytes have the ability to generate and quickly conduct electrical impulses. They form nodes and bundles of the conduction system of the heart and are divided into several subtypes: pacemakers (in the sinoatrial node), transitional cells (in the atrioventricular node) and cells of the His bundle and Purkinje fibers. Conducting cardiomyocytes are characterized by weak development of the contractile apparatus, light cytoplasm and large nuclei. The cells do not have T-tubules or cross-striations because the myofibrils are arranged in a disorderly manner.
Function of atypical cardiomyocytes– generation of impulses and transmission to working cardiomyocytes, ensuring automaticity of myocardial contraction.
Secretory cardiomyocytes
Secretory cardiomyocytes are located in the atria, mainly in the right; characterized by a process form and weak development of the contractile apparatus. In the cytoplasm, near the poles of the nucleus, there are secretory granules containing natriuretic factor, or atriopeptin(a hormone that regulates blood pressure). The hormone causes loss of sodium and water in the urine, dilation of blood vessels, decreased blood pressure, and inhibition of the secretion of aldosterone, cortisol, and vasopressin.
Function of secretory cardiomyocytes: endocrine.
Regeneration of cardiomyocytes. Cardiomyocytes are characterized only by intracellular regeneration. Cardiomyocytes are not capable of division; they lack cambial cells.
SMOOTH MUSCLE TISSUE
Smooth muscle tissue forms the walls of internal hollow organs and blood vessels; characterized by a lack of striations and involuntary contractions. Innervation is carried out by the autonomic nervous system.
Structural and functional unit of non-striated smooth muscle tissue - smooth muscle cell (SMC), or smooth myocyte. The cells are spindle-shaped, 20-1000 µm long and 2 to 20 µm thick. In the uterus, the cells have an elongated process shape.
Smooth myocyte
A smooth myocyte consists of a rod-shaped nucleus located in the center, cytoplasm with organelles and sarcolemma (plasmolemma and basement membrane complex). In the cytoplasm at the poles there is a Golgi complex, many mitochondria, ribosomes, and a developed sarcoplasmic reticulum. Myofilaments are located obliquely or along the longitudinal axis. In SMCs, actin and myosin filaments do not form myofibrils. There are more actin filaments and they are attached to dense bodies, which are formed by special cross-linking proteins. Myosin monomers (micromyosin) are located near the actin filaments. Having different lengths, they are much shorter than thin threads.
Contraction of smooth muscle cells occurs through the interaction of actin filaments and myosin. The signal traveling along the nerve fibers causes the release of a mediator, which changes the state of the plasmalemma. It forms flask-shaped invaginations (caveolae), where calcium ions are concentrated. Contraction of SMCs is induced by the influx of calcium ions into the cytoplasm: caveolae are detached and, together with calcium ions, enter the cell. This leads to the polymerization of myosin and its interaction with actin. Actin filaments and dense bodies come closer together, the force is transferred to the sarcolemma and the SMC is shortened. Myosin in smooth myocytes is able to interact with actin only after phosphorylation of its light chains by a special enzyme, light chain kinase. After the signal stops, calcium ions leave the caveolae; myosin depolarizes and loses its affinity for actin. As a result, the myofilament complexes disintegrate; the contraction stops.
Special types of muscle cells
Myoepithelial cells are derivatives of ectoderm and do not have striations. They surround the secretory sections and excretory ducts of the glands (salivary, mammary, lacrimal). They are connected to glandular cells by desmosomes. By contracting, they promote secretion. In the terminal (secretory) sections, the shape of the cells is branched and stellate. The nucleus is in the center, in the cytoplasm, mainly in the processes, myofilaments are localized, which form the contractile apparatus. These cells also contain cytokeratin intermediate filaments, which emphasizes their similarity to epithelial cells.
Myoneural cells develop from the cells of the outer layer of the optic cup and form the muscle that constricts the pupil and the muscle that dilates the pupil. The structure of the first muscle is similar to SMCs of mesenchymal origin. The muscle that dilates the pupil is formed by cell processes located radially, and the nuclear-containing part of the cell is located between the pigment epithelium and the stroma of the iris.
Myofibroblasts belong to loose connective tissue and are modified fibroblasts. They exhibit the properties of fibroblasts (synthesize intercellular substance) and smooth myocytes (have pronounced contractile properties). As a variant of these cells we can consider myoid cells as part of the wall of the convoluted seminiferous tubule of the testicle and the outer layer of the theca of the ovarian follicle. During wound healing, some fibroblasts synthesize smooth muscle actins and myosins. Myofibroblasts provide contraction of the wound edges.
Endocrine smooth myocytes are modified SMCs that represent the main component of the juxtaglomerular apparatus of the kidneys. They are located in the wall of the arterioles of the renal corpuscle, have a well-developed synthetic apparatus and a reduced contractile apparatus. They produce the enzyme renin, which is located in granules and enters the blood through the mechanism of exocytosis.
Regeneration of smooth muscle tissue. Smooth myocytes are characterized by intracellular regeneration. With an increase in functional load, myocyte hypertrophy and hyperplasia (cellular regeneration) occur in some organs. Thus, during pregnancy, the smooth muscle cells of the uterus can increase 300 times.
Muscle tissue(textus musculares) represent a group of animal and human tissues of different origins that have a common property - contractility. This property is achieved by these tissues due to the presence of special contractile structures in them - myofilaments. The following main types of muscle tissue are distinguished:
smooth (non-striated) muscle tissue and striated (striated) muscle tissue. The latter, in turn, are divided into skeletal muscle tissue and cardiac muscle tissue. Some specialized varieties of other tissues also have the property of contractility. These include the so-called epithelial muscle tissue (in the sweat and salivary glands) and neuroglial muscle tissue (in the iris) (Table 9).
Smooth (unstriated) muscle tissue
Smooth muscle tissue(textus muscularis nonstriatus) develops from mesenchyme. It makes up the motor apparatus of internal organs, blood and lymphatic vessels. Its contractions are slow, tonic in nature. The structural unit of smooth muscle tissue is an elongated spindle-shaped cell - smooth myocyte. It is covered with a plasmalemma, to which the basement membrane and connective tissue fibers adjoin the outside. Inside the cell, in its center, in the myoplasm, there is an elongated nucleus, around which mitochondria and other organelles are located.
Contractile protein filaments were discovered in the myoplasm of myocytes under an electron microscope - myofilaments. Distinguish actin, myosin and intermediate myofilaments. Actin and myosin myofilaments ensure the act of contraction itself, and intermediate ones protect smooth myocytes from excessive expansion during shortening. Myofilaments of smooth myocytes do not form discs, therefore these cells do not have transverse striations, and are called smooth, non-striated. Smooth myocytes regenerate well. They divide by mitosis, can develop from poorly differentiated connective tissue cells, and are capable of hypertrophy. Between the cells there is a supporting stroma of smooth muscle tissue - collagen and elastic fibers that form dense networks around each cell. Smooth muscle cells synthesize the fibers of this stroma themselves.
Striated (striated) muscle tissue
As already mentioned, this group of striated muscle tissues includes skeletal and cardiac muscle tissue. These tissues are united primarily on the basis of the cross-striations of their special organelles - myofibrils. However, in terms of their origin, general structural plan and functional features, these two types of striated muscle tissue differ significantly.
Striated skeletal muscle tissue
Skeletal muscle tissue(textus muscularis striatus sceletalis) develops from segmented mesoderm, more precisely from its central sections, called myotomes. The structural and functional unit of this tissue is multinuclear myosymplasts - striated muscle fibers. From the surface they are covered sarcolemma - a complex formation consisting of a three-layer muscle fiber plasmalemma, a basement membrane and an externally adjacent network of connective tissue fibers. Under the basement membrane, adjacent to the plasma membrane of the muscle fiber, there are special muscle cells - satellites. Inside the muscle fiber, in its sarcoplasm, along the periphery, there are numerous nuclei, and in the center, along the fiber, there are special organelles - myofibrils. Mitochondria and other common organelles in muscle fiber are located around the nuclei and along the myofibrils. Under an electron microscope, myofibrils consist of threads - myofilaments - actnioid, thinner (about 5-7 nm in diameter) and thicker - myosin (about 10-20 nm in diameter).
Actin myofilaments containing the protein actin form isotropic disks (I). These are light-colored, non-birefringent discs. In the center of the disks I passes Z-line -telophragm. This line divides the disk I on two half-discs. In the Z-line area there are so-called triads. Triads consist of tubular elements - T-tubules, formed by pressing the plasma membrane into the muscle fiber. Through these tubes the nerve impulse travels to the myofibrils. In each triad, one T-tubule contacts two terminal cisterns of the sarcoplasmic reticulum, which ensures the release of calcium ions necessary for the contractile act. In the area of the Z-lines of the disk I The ends of actin myofilaments converge. Myosin myofilaments, containing the protein myosin, form anisotropic (A) dark disks that are birefringent. In the center of disk A passes M-line - mesophragm. In M-linny the ends of myosin myofibrils converge and a network of tubules of the sarcoplasmic reticulum is discovered. The alternation of dark and light discs in the myofibrils gives the muscle fiber a transverse striation. The structural unit of myofibrils is the myomer (sarcomere) - this is the section of the myofibril between two Z-lines. Its formula is A+2 1/2 I.
According to modern concepts, each muscle fiber is divided into: contractile apparatus, consisting of multifibrils, including actin and myosin myofilaments; trophic apparatus, which includes sarcoplasm with nuclei and organelles; special membrane apparatus of triads; support apparatus, including the sarcolemma with endomysium and membrane structures of lines Z and M; and finally nervous apparatus, represented by motor neuromuscular endings - motor plaques and sensory nerve endings - neuromuscular spindles.
In skeletal muscle tissue there are whiteand red muscle fibers. White muscle fibers contain little sarcoplasm and myoglobin and many multifibrils. On a cross section, densely located myofibrils are clearly visible in white muscle fibers. They provide a strong but short-lasting contraction. Red muscle fibers contain a lot of sarcoplasm and therefore a lot of myoglobin and little myofibrils. On a cross section, in such muscle fibers the myofibrils are arranged loosely in groups, forming polygons called Conheim fields. These fields are separated from each other by layers of sarcoplasm. Red muscle fibers contain many mitochondria and are capable of long-term contraction. Each skeletal muscle, as an organ, contains both white and red muscle fibers. However, their ratio in different muscle groups is not the same.
Each muscle fiber is surrounded on the outside by a layer of loose fibrous connective tissue called endomysium(endomysium). Groups of muscle fibers are surrounded perimysium(perimysium), and the muscle itself is a dense connective tissue membrane - epimysium(epimysium).
Striated skeletal muscle tissue is capable of regeneration. Contraction of muscle tissue is interpreted from the perspective of the sliding theory: actin myofilaments move in and slide between myosin ones.
