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. At 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 propagating 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 contraction force, 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.
Human muscles in relation to his total mass are approximately 40%. Their main function in the body is to provide movement through the ability to contract and relax. For the first time, muscle structure (8th grade) begins to be studied at school. There, knowledge is given at a general level, without much in-depth. The article will be of interest to those who want to go a little beyond this framework.
Muscle structure: general information
Muscle tissue is a group that includes striated, smooth and cardiac varieties. Differing in origin and structure, they are united based on the function they perform, that is, the ability to contract and lengthen. In addition to the listed varieties, which are formed from mesenchyme (mesoderm), the human body also has muscle tissue of ectodermal origin. These are the myocytes of the iris.
The structural, general structure of the muscles is as follows: they consist of an active part, called the abdomen, and tendon ends (tendon). The latter are formed from dense connective tissue and perform the function of attachment. They have a characteristic whitish-yellow color and shine. In addition, they have significant strength. Usually, with their tendons, muscles are attached to the links of the skeleton, the connection with which is movable. However, some can also attach to the fascia, to various organs (eyeball, laryngeal cartilage, etc.), to the skin (on the face). The blood supply to muscles varies and depends on the loads they experience.
Regulating muscle function
Their work is controlled, like other organs, by the nervous system. Its fibers in the muscles end as receptors or effectors. The former are also located in the tendons and have the form of terminal branches of the sensory nerve or neuromuscular spindle, which has a complex structure. They react to the degree of contraction and stretching, as a result of which a person develops a certain feeling, which, in particular, helps to determine the position of the body in space. Effector nerve endings (also known as motor plaques) belong to the motor nerve.
The structure of the muscles is also characterized by the presence in them of the endings of fibers of the sympathetic nervous system (autonomic).
The structure of striated muscle tissue
It is often called skeletal or striated. The structure of skeletal muscle is quite complex. It is formed by fibers that have a cylindrical shape, a length from 1 mm to 4 cm or more, and a thickness of 0.1 mm. Moreover, each is a special complex consisting of myosatellitocytes and myosymplast, covered with a plasma membrane called sarcolemma. Adjacent to it outside is a basement membrane (plate), formed from the finest collagen and reticular fibers. Myosymplast consists of a large number of ellipsoidal nuclei, myofibrils and cytoplasm.
The structure of this type of muscle is distinguished by a well-developed sarcotubular network, formed from two components: ER tubules and T-tubules. The latter play an important role in accelerating the conduction of action potentials to microfibrils. Myosatellite cells are located directly above the sarcolemma. The cells have a flattened shape and a large nucleus, rich in chromatin, as well as a centrosome and a small number of organelles; there are no myofibrils.
The sarcoplasm of skeletal muscle is rich in a special protein - myoglobin, which, like hemoglobin, has the ability to bind with oxygen. Depending on its content, the presence/absence of myofibrils and the thickness of the fibers, two types of striated muscles are distinguished. The specific structure of the skeleton, muscles - all these are elements of a person’s adaptation to upright walking, their main functions are support and movement.
Red muscle fibers
They are dark in color and rich in myoglobin, sarcoplasm and mitochondria. However, they contain few myofibrils. These fibers contract quite slowly and can remain in this state for a long time (in other words, in working condition). The structure of skeletal muscle and the functions it performs should be considered as parts of a single whole, mutually determining each other.
White muscle fibers
They are light in color, contain a much smaller amount of sarcoplasm, mitochondria and myoglobin, but are characterized by a high content of myofibrils. This means that they contract much more intensely than red ones, but they also “get tired” quickly.
The structure of human muscles differs in that the body contains both types. This combination of fibers determines the speed of muscle reaction (contraction) and their long-term performance.
Smooth muscle tissue (unstriated): structure
It is built from myocytes located in the walls of lymphatic and blood vessels and forming the contractile apparatus in the internal hollow organs. These are elongated cells, spindle-shaped, without transverse striations. Their arrangement is group. Each myocyte is surrounded by a basement membrane, collagen and reticular fibers, among which are elastic. Cells are connected by numerous nexuses. The structural features of the muscles of this group are that one nerve fiber (for example, the pupillary sphincter) approaches each myocyte, surrounded by connective tissue, and the impulse is transported from one cell to another using nexuses. The speed of its movement is 8-10 cm/s.
Smooth myocytes have a much slower contraction rate than myocytes of striated muscle tissue. But energy is also used sparingly. This structure allows them to make long-term contractions of a tonic nature (for example, sphincters of blood vessels, hollow, tubular organs) and fairly slow movements, which are often rhythmic.
Cardiac muscle tissue: features
According to the classification, it belongs to the striated muscle, but the structure and functions of the heart muscles are noticeably different from skeletal muscles. Cardiac muscle tissue consists of cardiomyocytes, which form complexes by connecting with each other. The contraction of the heart muscle is not subject to the control of human consciousness. Cardiomyocytes are cells that have an irregular cylindrical shape, with 1-2 nuclei and a large number of large mitochondria. They are connected to each other by insertion disks. This is a special zone that includes the cytolemma, areas of attachment of myofibrils to it, desmos, nexuses (through them the transmission of nervous excitation and ion exchange between cells occurs).
Classification of muscles depending on shape and size
1. Long and short. The first ones are found where the greatest range of motion occurs. For example, upper and lower limbs. And the short muscles, in particular, are located between individual vertebrae.
2. Broad muscles (stomach in the photo). They are mainly located on the body, in the cavity walls of the body. For example, superficial muscles of the back, chest, abdomen. With a multilayer arrangement, their fibers, as a rule, go in different directions. Therefore, they provide not only a wide variety of movements, but also strengthen the walls of body cavities. In the broad muscles, the tendons are flat and occupy a large surface area; they are called sprains or aponeuroses.
3. Circular muscles. They are located around the openings of the body and, through their contractions, narrow them, as a result of which they are called “sphincters”. For example, the orbicularis oris muscle.
Complex muscles: structural features
Their names correspond to their structure: two-, three- (pictured) and four-headed. The structure of muscles of this type is different in that their beginning is not single, but divided into 2, 3 or 4 parts (heads), respectively. Starting from different points of the bone, they then move and unite into a common abdomen. It can also be divided transversely by the intermediate tendon. This muscle is called digastric. The direction of the fibers can be parallel to the axis or at an acute angle to it. In the first case, the most common, the muscle shortens quite strongly during contraction, thereby providing a large range of movements. And in the second, the fibers are short, located at an angle, but they are much larger in number. Therefore, the muscle shortens slightly during contraction. Its main advantage is that it develops great strength. If the fibers approach the tendon only on one side, the muscle is called unipennate, if on both sides it is called bipennate.
Auxiliary apparatus of muscles
The structure of human muscles is unique and has its own characteristics. For example, under the influence of their work, auxiliary devices are formed from the surrounding connective tissue. There are four of them in total.
1. Fascia, which is nothing more than a shell of dense, fibrous fibrous tissue (connective). They cover both single muscles and entire groups, as well as some other organs. For example, kidneys, neurovascular bundles, etc. They influence the direction of traction during contraction and prevent the muscles from moving to the sides. The density and strength of fascia depends on its location (they differ in different parts of the body).
2. Synovial bursae (pictured). Many people probably remember their role and structure from school lessons (Biology, 8th grade: “Muscle structure”). They are peculiar bags, the walls of which are formed by connective tissue and are quite thin. Inside they are filled with fluid such as synovium. As a rule, they are formed where the tendons come into contact with each other or experience great friction against the bone during muscle contraction, as well as in places where the skin rubs against it (for example, the elbows). Thanks to the synovial fluid, gliding improves and becomes easier. They develop mainly after birth, and over the years the cavity increases.
3. Synovial vagina. Their development occurs within the osteofibrous or fibrous canals that surround the long muscle tendons where they slide along the bone. In the structure of the synovial vagina, two petals are distinguished: the inner one, covering the tendon on all sides, and the outer one, lining the walls of the fibrous canal. They prevent the tendons from rubbing against the bone.
4. Sesamoid bones. Typically, they ossify within the ligaments or tendons, strengthening them. This facilitates the work of the muscle by increasing the shoulder of force application.
Muscles form the active part of the musculoskeletal system. They are attached to the bones of the skeleton, act on bone levers, and set them in motion. Therefore they are also called skeletal muscles.
Skeletal muscles built from striated muscle tissue. They perform the following functions: 1) maintain the position of the body and its parts in space; 2) provide movement of the body (running, walking and other types of movements);
3) move body parts relative to each other; 4) carry out breathing and swallowing movements; 5) participate in the articulation of speech and the formation of facial expressions; 6) generate heat; 7) convert chemical energy into mechanical energy.
There are about 600 muscles in the human body. The total mass of skeletal muscles in newborn children averages 22% of body weight; at 17–18 years old it reaches 35–40%. In older and older people, the relative mass of skeletal muscles decreases to 25–30%. In trained athletes, muscles can account for up to 50% of the total body weight.
The main functional properties of muscles: 1) excitability - the ability to quickly respond to the action of a stimulus with excitation, as a result of which the muscle is able to contract; 2) conductivity - the ability to conduct excitation from nerve endings to the contractile structures of muscle fibers;
3) contractility - the ability to contract, shorten or change tension.
Excitation and contraction of muscles occur under the influence of nerve impulses coming along the nerves from the central nervous system, from the brain and spinal cord. In order for a muscle to be excited and respond by contracting, the strength of the nerve impulse must be of sufficient magnitude. The force of stimulation that can cause muscle contraction is called threshold irritation.
The wave of excitation that arises in the muscle quickly spreads throughout the muscle, as a result the muscle contracts and acts on the bone levers, causing them to move.
In the muscle there are abdomen, consisting of striated muscle tissue, and tendon ends (tendons), formed by dense fibrous connective tissue. With the help of tendons, muscles are attached to the bones of the skeleton (Fig. 28).
Rice. 28. Scheme of origin and attachment of muscles:
1 – muscle, 2 – tendon, 3 – bone
However, some muscles can also attach to other organs (skin, eyeball).
The end of the muscle located closer to the midplane of the body. usually called the beginning of the muscle the other end, spaced from the median plane, is called muscle attachment. The origin of the muscle usually remains stationary as the length of the muscle changes. This place on the bone is called a fixed point. The attachment point of the muscle located on the bone that is set in motion is called the moving point.
The main working tissue of skeletal muscle is striated muscle tissue. Its main structural and functional element is the complex muscle fiber. Muscle fibers - these are multinucleate formations. One fiber can have more than 100 rice cores. 29). The length of the muscle fibers reaches several centimeters.
On the outside, the muscle fiber is undermined by the sheath - sarcolemma. In the cytoplasm of the muscle fiber - sarcoplasm, along with cellular organelles of a general nature, there are also specialized organelles - myofibrils. These are the main structures of muscle fiber, consisting of the contractile proteins actin and myosin. Each myofibril consists of contractile sections - sarcomeres. At the boundaries of sarcomeres, protein molecules are located across the muscle fiber. These areas attached to the sarcolemma are called telophragm. In the middle of the sarcomeres are mesophragm, also representing a transverse protein network. Actin filaments are attached to the telophragm, and myosin filaments are attached to the mesophragm.
Due to the different structure of protein molecules and the refraction of light rays, light and dark areas are visible in the sarcomeres and at their boundaries in the muscle fibers, creating the impression of striations.
Muscle contraction is based on the sliding of actin and myosin filaments relative to each other. Actin filaments, moving towards each other when excited, reduce the length of sarcomeres.
Muscle contractility manifests itself either in its shortening, or in tension, at which the length of the muscle fibers does not change. In the body, muscle contraction occurs under the influence of nerve impulses that the muscle receives from the central nervous system along the nerves that connect to it.
Motor nerve fibers, approaching muscle fibers, form endings on them - motor plates. Nerve impulses arriving at the area of neuromuscular endings stimulate the release of a biologically active substance - acetylcholine, which causes an action potential. The action potential spreads across the muscle fiber membrane, the membranes of the sarcoplasmic reticulum, causing the release of calcium ions into the sarcoplasm, the formation of actomiazin, and the breakdown of ATP molecules. The energy released in this process is used to slide protein filaments and contract the muscle.
Receptors in skeletal muscles are represented by neuromuscular spindles. Each neuromuscular spindle is surrounded by a connective tissue capsule and contains specialized muscle fibers on which sensory nerve endings - receptors - are located. They sense muscle stretches and transmit nerve impulses to the central nervous system.
Each muscle consists of a large number of muscle fibers interconnected by thin layers of loose fibrous connective tissue in bundles. Groups of bundles are covered with a thicker and denser connective tissue membrane and form a muscle. The connective tissue fibers surrounding the muscle fibers and their bundles, extending beyond the muscle, form the tendon. The tendons of different muscles are not the same. In muscles located on the limbs, the tendons are usually narrow and long. The tendons of the muscles involved in the formation of the walls of the cavities are wide, they are called aponeuroses.
Muscles are rich in blood vessels, through which the blood brings nutrients and oxygen to them, and carries out metabolic products. The source of energy for muscle contraction is glycogen. In the process of its breakdown, adenosine triphosphate acid (ATP) is produced, which is the source of energy for muscle contraction.
1. What percentage of the total body weight is muscle in a newborn child, in adolescence, in old people?
2. What functions do skeletal muscles perform?
Related information.
Lecture 4 . Physiology of muscle tissue
Muscle tissue performs the following functions:
Ensuring motor activity - goal-directed behavior is the most effective form of adaptation.
Providing special functions inherent only to humans is, first of all, communicative function, expressed in the form of oral and written speech.
Performing the respiratory function - excursion of the chest and diaphragm.
Participation in heat generation processes - thermoregulatory tone, muscle tremors.
A specific property of all types of muscles is contractility – the ability to contract, that is, to shorten or develop tension. To realize this ability, the muscle uses two additional properties - excitability And conductivity .
Skeletal muscles are also called arbitrary , since their reduction can be controlled at will. They are completely devoid of automaticity and do not able to work without control impulses from the central nervous system. Smooth muscles do not contract of their own accord, which is why they are also called involuntary .
Morphofunctional characteristics of skeletal muscle . Skeletal muscle consists of multinucleated muscle fibers. The fiber thickness ranges from 10 to 100 microns. The length of the fibers ranges from a few mm to several centimeters.
The number of muscle fibers becomes constant at 4-5 months of postnatal development. Subsequently, only the diameter and length of the fibers increase (for example, under the influence of training - functional hypertrophy).
The muscle fiber is covered with sarcolemma. The sarcoplasm of muscle fiber contains the following intracellular elements: nuclei, mitochondria, proteins, fat droplets, glycogen granules, phosphate-containing substances, various small molecules and electrolytes. T-tubules extend from the surface of the sarcolemma into the fiber, which ensure its interaction with the sarcoplasmic reticulum. The sarcoplasmic reticulum is a system of interconnected cisterns and extending from them into longitudinal direction of the tubules located between myofibrils. The extreme cisterns of the reticulum are connected to the T-tubules. The tanks contain calcium ions necessary for the contraction process. Inside the muscle fiber stretches a mass of threads - myofibrils, which are part of the mechanism of the contraction process. Each myofibril consists of protofibrils, which are located parallel to each other and are of a protein nature.
There are two types of intramuscular threads: thin actin and fat myosin . Actin filaments consist of two subunits - fibers twisted in the form of a spiral, each of which is formed by connected molecules of the globular protein actin. In addition to actin, thin filaments include regulatory proteins tropomyosin And troponin . These proteins in unexcited muscle interfere with the connection between actin and myosin, so the muscle rest is in a relaxed state condition.
Fig.1. Scheme of the spatial organization of contractile and regulatory proteins in striated muscle.
Each myosin filament is surrounded by six actin filaments. These filaments form a kind of cylinder, inside of which the myosin filament is located. The cross bridges of the myosin filament are directed in different directions, so they interact with all actin protofibrils. In turn, each actin filament contacts three myosin filaments.
