Skeletal muscles are types of muscle fibers. Physiology of skeletal muscles

Each muscle fiber is a giant multinucleated cell - a symplast formed in the process embryonic development organism by the fusion of many individual cells - myoblasts.

The structure of the muscle fiber is significantly different from the structure of other cells. The most important distinctive features- these are the dimensions, shape, multinucleation, the presence of a contractile apparatus. The structure of the muscle fiber is shown in Fig. 60.

Rice. 60. The most important structural elements of the muscle fiber

Let us dwell on the most important structural elements of the muscle fiber.

Sarcolemma. Outside, the muscle fiber is surrounded by a sheath - sarcolemma with high strength and elasticity. These properties of the sarcolemma are provided by the presence in it a large number elastic fibers of collagen and elastin proteins, forming a dense network.

The sarcolemma has selective permeability, passing into the cell mainly those substances for the transformation of which there are conditions - enzyme systems. There are special transport systems in the sarcolemma, with the help of which, in particular, the difference in the concentration of Na + , K + , Cl ions is maintained inside and outside the muscle fiber, which ensures the formation of a membrane potential on its surface.

Each muscle fiber has a motor nerve ending. The point of attachment of a nerve ending to a muscle fiber is called neuromuscular synapse..

Inside the muscle fiber there are numerous cellular organelles, the most important of which are nuclei, mitochondria, ribosomes, etc. The functions of these organelles are described in Chapter 2 (2.5.1). The space between the organelles is filled with intracellular fluid - sarcoplasm. Among the structural elements of the muscle fiber, the largest volume is occupied by contractile filaments - myofibrils.

Myofibrils. Myofibrils are long, thin filaments that run along a muscle fiber. The number of myofibrils in muscle fibers can range from several tens to one and a half thousand or more. Under the influence of systematic muscle training, especially speed-strength orientation, the number of myofibrils can increase. On the contrary, the restriction of motor activity is accompanied by a decrease in the number of myofibrils. The structure of muscle myofibrils is shown in Fig. 61
Rice. 61. Structure of myofibrils

When viewed through an optical microscope, it can be seen that the myofibrils have a repeating transverse striation - dark and light stripes (discs). Dark discs (A-discs) in the central part have a lighter stripe (H-zone). Light discs (I-discs) intersect in the center with a narrow dark stripe (Z line). The area of ​​myofibrils between the two Z lines is called the sarcomere. The number of sarcomeres in a myofibril depends on the length of the muscle fiber and can reach several hundred. The length of sarcomeres can vary from person to person.

The study of sections of muscle fibers in an electron microscope showed that each myofibril consists of a large number of parallel thick and thin filaments (filaments), which are characterized by a strict mutual distribution. Thick threads are located in the A-disc zone. They are built from the protein myosin. Myosin is the most important contractile protein, accounting for about 55% of the total number of contractile proteins. The myosin molecule has a long fibrillar (elongated) part and a globular (rounded) head. The fibrillar part has a double-stranded polypeptide configuration. The function of the fibrillar part of the myosin molecule is associated with the formation of the structure of a thick myosin filament.

The globular heads of myosin filaments have two active centers, one of which has ATPase activity (the ability to cleave ATP molecules), the other has the ability to bind to active centers on actin filaments (an actin-binding center). The heads of myosin molecules are located on the surface of myosin filaments, forming protrusions (processes). At the same time, they are strictly oriented in space - they are arranged in six longitudinal rows. A thick myosin filament consists, as it were, of two parts that mirror each other. If it is cut in the middle, then two completely identical fragments are formed.

Myosin molecules have the ability to bind Ca 2+ and Mg 2+ ions. Calcium ions are a cofactor for the enzyme ATPase (in its absence, the enzyme is not active). Magnesium ions provide myosin with the ability to bind ATP and ADP molecules.

In the zone of light discs (I-discs) there are thin filaments built from proteins of actin, tropomyosin and troponin. Actin is the second quantitatively contractile protein that forms the basis of actin filaments. Tropomyosin is a structural protein of actin filaments that has a fibrillar shape. Double tropomyosin molecules wrap around actin filaments. Troponin is the regulatory protein of actin filaments. It exists in three forms, one of which blocks the interaction of actin with myosin. Another form is able to bind calcium ions, due to which the conformation of molecules of the first form of troponin changes and the center of actin interaction with myosin opens. The third form of troponin provides attachment of the first two forms to the actin filament. In addition, the thin actin filaments contain the protein actinin. It is contained in the zone of the Z line, which acts as a kind of partition, and ensures the attachment of the ends of actin filaments to it.

One of the most important structural elements of the muscle fiber is sarcoplasmic reticulum. The sarcoplasmic reticulum is an intracellular system of interconnected vesicles and tubules (cistern) that penetrate the cell and are especially densely concentrated in the zone of contact between actin and myosin filaments.

Sarcoplasmic (in the cells of other organs and tissues - endoplasmic) reticulum is present in every cell of the human body. But in the muscle fiber, it performs somewhat unusual functions compared to other cells. Its main role in muscle fiber is to regulate the content of calcium ions near actin and myosin filaments. In a state of relaxation, the reticulum binds Ca 2+ ions, their concentration in the sarcoplasm is approximately 10 -7 mol·liter -1. Under the influence of a motor impulse, calcium ions are released from the reticulum and their concentration rises to 10 -5 mol·liter -1.

The ability of the sarcoplasmic reticulum to bind and release Ca 2+ ions into the cytoplasm is associated with the localization of specific calcium-binding proteins on its inner surface. Ribosomes are also located on the surface of the reticulum - special intracellular formations in which protein synthesis is carried out.

The muscle fiber also has a system of tubular protrusions of the sarcolemma (T-system), directed inside the muscle fiber and located between the myofibrils and the sarcoplasmic reticulum. The T-system ensures the rapid propagation of the excitation wave from the sarcolemma deep into the fiber.

The muscle fiber also contains other intracellular organelles: mitochondria, lysosomes. The functions of these muscle fiber structures have already been described in the chapter "General patterns of metabolism".

The muscle fiber contains not one, but several nuclei, which are located not in the central part of the fiber, but along the perimeter, directly under the sarcolemma

Types of muscle fibers

In skeletal muscles, several types of muscle fibers are distinguished, differing in their motor characteristics, the ratio of various chemical and structural components, features of the structural organization. The main types of muscle fibers are slow-twitch(MS) and rapidly dwindling(BS). Slow twitch fibers due to their higher content of myoglobin are also called red (or type I). Fast-twitching, which are characterized by a lower content of myoglobin, are called white (or type II). It should immediately be noted that it is almost impossible to distinguish between these two types of fibers by color. Some are red in color.

Fast and slow fibers differ more than twice in maximum contraction speed. So, the time of a single contraction of MS reaches 110 ms, and BS - 50 ms. In addition, BS are more than twice as high as MS in terms of their power characteristics.

Significantly different different types fibers according to the level of development of various energy conversion mechanisms. MS fibers have a well-developed mechanism of aerobic ATP resynthesis, which is ensured by big amount mitochondria and a high content of aerobic biological oxidation enzymes, as well as large reserves of aerobic oxidation substrates: glycogen, fats. MC fibers contain more myoglobin protein, due to which they have a larger supply of oxygen and more favorable conditions for the transfer of oxygen from the blood into the fiber.

There are significantly more myofibrils in BS fibers, higher ATP-ase activity, higher concentration of calcium ions.

Two subtypes are distinguished within BS fibers: BS a and BS b. These two subtypes differ mainly in the different level of development of the most important energy conversion mechanisms. In BS a fibers, anaerobic glycolysis is more well developed and the aerobic pathway of ATP resynthesis is somewhat weaker than in MS fibers. They are leading when performing exercises of the so-called. submaximal power, the duration of which ranges from 30 seconds to 2-3 minutes, provided that the work is performed with the maximum intensity for this duration.

In BS b fibers, along with anaerobic glycolysis, the creatine phosphate mechanism of ATP resynthesis is well developed. They are connected when performing exercises of maximum and near-maximal intensity: 100-meter running, exercises with heavy weights, etc.

This does not mean that the exercises of the indicated intensity are performed exclusively by one type of muscle fiber. We are talking about the degree of their involvement in the work, which, of course, is determined by the central nervous system. Performing exercises, the power of which does not exceed 20-25% of the maximum possible for a given individual, is provided only by "red" muscle fibers. During work, the intensity of which is in the range of 25-40% of the maximum, fibers of the BS a are connected to its implementation. If the intensity of the exercise exceeds 40% of the maximum, the fibers of the BS b are involved in the work.

With an increase in exercise intensity within each power zone, the participation of fibers of all types in its provision increases, but to the greatest extent those that are connected to work in a given power range.

Various types muscle fibers also differ in the conditions of innervation. The motor neurons innervating the BS muscle fibers are thicker, they have a more extensive network of nerve endings (axon branches), due to which they innervate a significantly larger number of muscle fibers (from 300 to 500). BS fibers have a larger zone of attachment of the nerve ending to the muscle fiber, which creates more favorable conditions for innervation and the occurrence of an action potential.

Skeletal muscle fibers are not identical in their mechanical and metabolic features. Fiber types differ based on the following characteristics:

Depending on the maximum speed of shortening - fast fibers and slow fibers;

Depending on the main pathway for the formation of ATP - oxidative fibers and glycolytic fibers.

Fast and slow muscle fibers contain myosin isozymes, which break down ATP at different maximum rates; this corresponds to a different maximum speed duty cycle of the cross bridges and, consequently, the shortening of the fiber. High ATPase activity of myosin is characteristic of fast fibers, lower - slow fibers. Although the duty cycle rate is about four times faster in fast fibers than in slow fibers, both types of cross bridges generate the same force.

Another approach to the classification of skeletal muscle fibers is based on differences in the enzymatic mechanisms of ATP synthesis. Some fibers are rich in mitochondria and therefore provide high level oxidative phosphorylation; they are oxidative fibers. The amount of ATP formed in them depends on the supply of blood to the muscle, with which oxygen molecules and energy-rich compounds come. Fibers of this type are surrounded by numerous capillaries. In addition, they contain an oxygen-binding protein - myoglobin, which increases the rate of oxygen diffusion, and also acts as a short-term oxygen depot in muscle tissue. Due to the significant content of myoglobin, oxidative fibers are colored dark red; they are often referred to as red muscle fibers.

In addition, the three types of muscle fibers considered are characterized by different resistance to fatigue. Fast glycolytic fibers tire after a short time, while slow oxidative fibers are very hardy, which allows them to maintain contractile activity for a long time at a practically constant level of tension. Fast oxidative fibers occupy an intermediate place in their ability to resist the development of fatigue (Fig. 30.29).

The characteristics of the three types of skeletal muscle fibers are summarized in Table 1. 30.3.

There are three types of skeletal muscle fibers, depending on the maximum rate of shortening and the predominant mode of ATP formation: slow oxidative, fast oxidative and fast glycolytic.

The difference in the maximum rate of shortening of fast and slow fibers is due to differences in myosin ATPase: fast and slow fibers correspond to high and low ATPase activity.

Fast glycolytic fibers have, on average, a larger diameter than oxidative ones, and therefore develop more significant tension, but get tired faster.

All muscle fibers of one motor unit belong to the same type; most muscles contain all three types of motor units.

The characteristics of the three types of skeletal muscle fibers are summarized in

Professor Suvorova G.N.

Muscle tissues.

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

1) contractile processes in hollow internal organs and vessels

2) movement of body parts relative to each other

3) posture maintenance

4) movement of the organism in space.

Muscle tissue has the following morphofunctional characteristics:

1) Their structural elements have an elongated shape.

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

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

Contains a large number of mitochondria

There are trophic inclusions

Iron-containing protein myoglobin may be present

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

Muscle tissue is divided into two main groups

1) smooth (non-striated)

2) Cross-striped (striated)

Smooth muscle tissue: is of mesenchymal origin.

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

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

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

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

Skeletal - from somite myotomes

Cardiac - from the visceral leaf of the splanchnotome.

Skeletal muscle tissue

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

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

Myoblasts

muscle tubules

The definitive form of myogenesis is the muscle fiber.

The structure of skeletal muscle tissue.

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

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

1. myosymplast

2. myosatellitocytes.

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

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

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

Contractile

Device for transmitting excitation from the sarcolemma

to the contractile apparatus.

Energy

reference

contractile apparatus muscle fiber is represented by myofibrils.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

There are three main types of fibers:

Type I (red)

Type IIB (white)

Type IIA (intermediate)

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

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

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

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

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

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

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

Physical activity is realized as a result of the coordinated actions of the skeletal muscles. Consider the main characteristics of their structure and function.

Human interaction with external environment cannot be carried out without contractions of its muscles. The movements produced at the same time are necessary both for performing the simplest manipulations and for expressing the most subtle thoughts and feelings - through speech, writing, facial expressions or gestures. The mass of muscles is much larger than other organs; they make up 40-50% of body weight. Muscles are "machines" that convert chemical energy directly into mechanical (work) and heat. Their activities, in particular the mechanism of shortening and generating force, can now be explained in sufficient detail at the molecular level using physical and chemical laws.

Fig 1. The structure of skeletal muscles: organization of cylindrical fibers in skeletal muscle attached to bones by tendons.

concept skeletal, or striated muscle belongs to a group of muscle fibers connective tissue (rice. 1). Muscles are usually attached to bones by bundles of collagen fibers. tendons, located at both ends of the muscle. In some muscles, single fibers have the same length as the entire muscle, but in most cases the fibers are shorter and often angled to the longitudinal axis of the muscle. There are very long tendons, they are attached to the bone, remote from the end of the muscle. For example, some of the muscles that move the fingers are located in the forearm; moving our fingers, we feel how the muscles of the hand move. These muscles are connected to the fingers through long tendons.

What is skeletal muscle?

One gram of skeletal muscle tissue contains approximately 100 mg of the "contractile proteins" actin (molecular weight 42,000) and myosin (molecular weight 500,000).

A skeletal muscle, such as the biceps, appears to be a single entity, but is actually made up of several types of tissue. Each muscle consists of long thin cylindrical muscle fibers (cells), elongated along its entire length; so they can be very long. Each multinuclear muscle cell (fiber) is surrounded by parallel muscle fibers, with which it is connected by a layer of connective tissue called endomysium. These fibers are bundled together and held together by a layer of connective tissue called the perimysium. Such a packed group, or bundle, of fibers is called a muscle bundle. Groups of bundles with adjacent vessels and nerves are connected to each other by another layer of connective tissue called epimysium. Gathered together and surrounded by epimysium, the bundles that stretch along the entire length of the skeletal muscle are topped with a layer of connective tissue called fascia.

What is the function of fascia in skeletal muscle?

Fascia is an elastic, dense and durable connective tissue sheath that covers the entire muscle and, going beyond it, forms a fibrous tendon. The fascia is formed by the fusion of all three inner layers of the connective tissue of the skeletal muscle. The fascia separates the muscles from each other, reduces friction during movement and forms a tendon with which the muscle is attached to the bone skeleton. This component of the muscles is usually not given due attention. Nevertheless, many experts believe that for free unrestricted movement of the muscle, and, consequently, the joint, the free movement of the fascia is absolutely necessary.

Rice. 2. Skeletal muscle structure: structural organization of filaments in a skeletal muscle fiber that creates a pattern of transverse bands.

Why is skeletal muscle called striated?

When studied with a light microscope, the main characteristic of skeletal muscle fibers was the alternation of light and dark stripes transverse to the long axis of the fiber. Therefore, skeletal muscles were named striated.

The transverse striation of skeletal muscle fibers is due to the special distribution in their cytoplasm of numerous thick and thin "threads" (filaments) that combine into cylindrical bundles with a diameter of 1-2 microns - myofibrils(rice. 2). The muscle fiber is almost filled with myofibrils, they stretch along its entire length and are connected to tendons at both ends. Myofibrils are made up of contractile filaments (proteins). There are two main contractile microfilaments - myosin and actin. The structural arrangement of these proteins gives skeletal muscle the appearance of alternating light and dark bands. Each dark band (band or disk, A) corresponds to an area where actin and myosin proteins overlap, while a lighter band corresponds to an area where they do not overlap (band or disk, I). Partitions, called Z-plates, divide them into several compartments-sarcomeres - about 2.5 microns long.

What is the structural unit of skeletal muscle tissue?

Structural unit skeletal muscle muscle tissue are muscle cells that differ significantly from other muscle tissues, primarily smooth muscle

Smooth muscle fiber it is a spindle cell diameter from 2 to 10 microns. Unlike multinucleated skeletal muscle fibers, which can no longer divide after differentiation is completed, smooth muscle fibers have a single nucleus and are capable of dividing throughout the life of the organism. Division begins in response to a variety of paracrine signals, often to tissue damage.

The striated muscles of the skeleton consist of many functional units - muscle fibers, which are located in a common connective tissue case. Each fiber of the skeletal muscle is a thin (0.01-0.1 mm in diameter), elongated by 2-3 cm, multinuclear formation - a symplast result of the fusion of many cells. The nuclei in the fiber are located near its surface. Bundles of muscle fibers are surrounded by collagen fibers and connective tissue; collagen is also found between the fibers. At the end of the muscles, collagen, together with connective tissue, forms tendons, which serve to attach the muscles to different parts skeleton. Each fiber is surrounded by a membrane - the sarcolemma, which is similar in structure to the plasma membrane.

The main feature of the muscle fiber is the presence in its cytoplasm - sarcoplasm of a large number of thin filaments - myofibrils, located along the axis of the fiber. Myofibrils consist of alternating light and dark areas - disks, which gives the muscle fiber a transverse striation (banding).

Figure 3. Organization of myosin and actin filaments in a relaxed and contracted sarcomere.

What is a sarcomere?

It is the smallest contractile unit of skeletal muscle.

Let's consider in more detail sarcomere structure, which is shown schematically in pic 3. With the help of a light microscope, one can see regularly alternating transverse light and dark stripes in them. According to the theory of Huxley and Hanson, such a transverse banding of myofibrils is due to the special mutual arrangement of actin and myosin filaments. The middle of each sarcomere is occupied by several thousand “thick” myosin filaments with a diameter of approximately 10 nm. At both ends of the sarcomere are about 2000 "thin" (5 nm thick) actin filaments attached to Z-lamellae like bristles in a brush.

Thick filaments are concentrated in the middle of each sarcomere where they lie parallel to each other; this region looks like a wide dark (anisotropic) band called A-stripe. Both halves of the sarcomere contain a set of thin filaments. One end of each of them is attached to the so-called Z-plate(or Z-line, or Z-band) - a network of intertwining protein molecules - and the other end overlaps with thick filaments. The sarcomere is limited by two consecutive Z-bands. Thus, the thin filaments of two adjacent sarcomeres are anchored on two sides of each Z-band.

Within the A-band of each sarcomere, two more strips are distinguished. In the center of the A-band, a narrow light strip is visible - H-zone. It corresponds to the gap between the opposing ends of the two sets of thin filaments of each sarcomere, i.e. includes only the central parts of thick filaments. In the middle of the H-zone there is a very thin dark M-line. It is a network of proteins that connect the central parts of thick filaments. In addition, titin protein filaments go from the Z-band to the M-line, associated simultaneously with the M-line proteins and with thick filaments. The M-line and titin filaments maintain an orderly organization of thick filaments in the middle of each sarcomere. Thus, thick and thin filaments are not free, loose intracellular structures.

Fig 4. Function of cross bridges. A. Model of contraction mechanism

Let's discuss the actual mechanism of muscle contraction

How do actin and myosin interact?

The active sites of the actin molecule capable of binding the globular heads of myosin are located on it at some distance from each other. When these active sites are open, the myosin head spontaneously binds to the actin filament and forms a cross bridge. When the myosin head is supplied with sufficient energy, the globular head pulls actin towards the center of the sarcomere, which is often referred to as ratcheting. This movement shortens the sarcomere.

Operation of cross bridges (Fig. 4). During contraction, each myosin head can bind a myosin filament to neighboring actin filaments. The movement of the heads creates a combined force, like a "stroke", which advances the actin filaments to the middle of the sarcomere. The bipolar organization of myosin molecules itself ensures the opposite direction of sliding of actin filaments in the left and right halves of the sarcomere. As a result of a single movement of the transverse bridges along the actin filament, the sarcomere is shortened by only 2 x 10 nm, i.e., by approximately 1% of its length. Through the rhythmic detachment and reattachment of myosin heads, the actin filament can be pulled toward the middle of the sarcomere, much like a group of people pulling a long rope by twisting it with their hands. Therefore, when the principle of "pulling the rope" is implemented in many consecutive sarcomeres, the repetitive molecular movements of the cross-bridges result in macroscopic movement. When the muscle relaxes, the myosin heads separate from the actin filaments. Since actin and myosin filaments can easily slide over each other, the resistance of relaxed muscles to stretch is very low. They can be stretched back to their original length with very little effort. Therefore, muscle lengthening during relaxation is passive.

Fig 5. Function of cross bridges. B. Model of the mechanism for generating force by transverse bridges: on the left before, on the right - after the "stroke"

Generation of muscle strength. Due to the elasticity of the transverse bridges, the sarcomere can develop force even without the threads sliding relative to each other, i.e., under strictly isometric experimental conditions. Fig.5.B illustrates such a process of isometric force generation. First, the head of the myosin molecule attaches to the actin filament at a right angle. It then tilts at an angle of approximately 45°, possibly due to attraction between adjacent attachment points on it and on the actin filament. In this case, the head acts as a miniature lever, bringing the internal elastic structure of the transverse bridge (apparently, the “neck” between the head and the myosin filament) into a stressed state. The resulting elastic stretch only reaches about 10 nm. The elastic tension created by an individual cross bridge is so weak that to develop a muscle force of 1 mN, it is necessary to combine the efforts of at least a billion such bridges connected in parallel. They will pull neighboring actin filaments like a team of players pulling a tightrope. Even during isometric contraction, the transverse bridges are not in a continuously stressed state (this is only observed with rigor mortis). In fact, each myosin head separates from the actin filament after only hundredths or tenths of a second; however, after the same short time, a new attachment to it follows. Despite the rhythmic alternation of attachments and detachments with a frequency of about 5–50 Hz, the force developed by the muscle under physiological conditions remains unchanged (with the exception of the flying muscles of insects), since statistically at each moment of time, one and the same number of bridges.

What is a cross bridge cycle?

The cross bridge cycle is a term describing the interaction of the globular head of myosin with the active site of the actin molecule. The formation of a cross bridge is facilitated by two factors: an increase in the intracellular concentration of calcium ions and the presence of adenosine triphosphate (ATP). One cycle of the cross bridge consists of:

activation of the myosin head;

exposure of the active site of the actin molecule in the presence of calcium;

spontaneous formation of a transverse bridge;

rotation of the globular head, accompanied by the advancement of the actin filament and shortening of the sarcomere;

uncoupling of the cross bridge.

The cycle can be repeated or stopped after completion. The rotation of the myosin head is also called the working stroke.

What prevents the spontaneous interaction of myosin and actin after the uncoupling of the transverse bridge? What is the mechanism of the cyclic formation of a transverse bridge - the repeated interaction of the globular head of myosin with the active site of the actin molecule?

To understand all this, it is necessary to take a closer look at the structure of myosin and, especially, actin.

Rice. 6. The structure of myosin

This is a single name for a large family of proteins that have certain differences in the cells of different tissues. Myosin is present in all eukaryotes. About 60 years ago, two types of myosin were known, which are now called myosin I and myosin II. Myosin II was the first of the myosins discovered, and it is he who takes part in muscle contraction. Later, myosin I and myosin V were discovered ( rice. 6 V). Recently, it has been shown that myosin II is involved in muscle contraction, while myosin I and myosin V are involved in the work of the submembrane (cortical) cytoskeleton. More than 10 classes of myosin have been identified so far. On Figure 6 D shows two variants of the structure of myosin, which consists of a head, neck and tail. The myosin molecule consists of two large polypeptides (heavy chains) and four smaller ones (light chains). These polypeptides constitute a molecule with two globular "heads" that contain both kinds of chains, and a long rod ("tail") of two intertwined heavy chains. The tail of each myosin molecule is located along the axis of the thick filament, and two globular heads protrude on the sides. Each globular head has two binding sites: for actin and for ATP. ATP binding sites also have the properties of the ATPase enzyme, which hydrolyzes the bound ATP molecule.

Fig 7. The structure of actin

actin molecule

It is a globular protein consisting of a single polypeptide that polymerizes with other actin molecules and forms two chains that wrap around each other ( rice. 7 A). Such a double helix is ​​the backbone of a thin filament. Each actin molecule has a myosin binding site. In a resting muscle fiber, the interaction between actin and myosin is prevented by two proteins - troponin And tropomyosin(rice. 7 B).

Troponin is a heterotrimeric protein. It consists of troponin T (responsible for binding to a single molecule of tropomyosin), troponin C (binds the Ca 2+ ion), and troponin I (binds actin and inhibits contraction). Each tropomyosin molecule is associated with one heterotrimeric troponin molecule that regulates access to myosin binding sites on seven actin monomers adjacent to the tropomyosin molecule.

What prevents spontaneous interaction between myosin and actin?

Two additional regulatory proteins are located in the grooves of the actin double helix, which prevent the spontaneous interaction of actin and myosin. These proteins, troponin and tropomyosin, play an important role in the process of skeletal muscle contraction. The function of tropomyosin is that at rest it closes (protects) the active sites of the actin filament. Troponin has three binding sites: one serves to bind calcium ions (troponin C), the other is firmly attached to the tropomyosin molecule (troponin T), and the third is associated with actin (troponin I). At rest, these regulatory proteins close the binding sites on the actin molecule and prevent the formation of cross bridges. All these microstructural components, along with mitochondria and other cell organelles, are surrounded by a cell membrane called the sarcolemma.

Rice. 8. Ca 2+ action during myofibril activation.

A. Actin and myosin filaments in the longitudinal section of the fiber. B. They are on its cross section.

Studies using X-ray diffraction analysis (small-angle X-ray scattering) showed that in the absence of Ca 2+, i.e., in the relaxed state of myofibrils, long tropomyosin molecules are located in such a way that they block the attachment of transverse myosin heads to actin filaments. Conversely, when Ca 2+ binds to troponin, tropomyosin enters the groove between the two actin monomers, exposing attachment sites for cross-bridges ( Rice. 8).

If active sites are closed, how do actin and myosin interact?

When the concentration of calcium ions increases inside the cell, they bind to troponin C. This leads to changes in the conformation of troponin. As a result, the three-dimensional structure of tropomyosin also changes and the active site of the actin molecule is exposed. Immediately after this, the myosin head spontaneously binds to the active site of the actin filament, forming a transverse bridge, which begins to move and contributes to the shortening of the sarcomere. The presence or absence of calcium in the cell is partially regulated by the sarcolemma (a specialized cell membrane of skeletal muscle).

What is the function of calcium in skeletal muscle?

Calcium provides the opening of the sections of the actin filament that bind myosin. Calcium ions inside the cell are stored in the SR (sarcoplasmic reticulum) and released after depolarizing stimulation. After release, calcium diffuses and binds to the protein - troponin C. As a result, the conformation of the protein changes, it pulls the tropomyosin molecule and exposes the active sites of the actin molecule. Active sites remain open as long as calcium binding to troponin C continues.

Rice. 9. Scheme of organization of the sarcoplasmic reticulum, transverse tubules and myofibrils.

Storage and release of calcium ions. Relaxed muscle contains more than 1 μmol of Ca 2+ per 1 g of wet weight. If calcium salts were not isolated in special intracellular stores, muscle fibers enriched with its ions would be in a state of continuous contraction.

The source of Ca 2+ entry into the cytoplasm is sarcoplasmic reticulum muscle fibre.

Sarcoplasmic reticulum muscle is homologous to the endoplasmic reticulum of other cells. It is located around each myofibril like a “torn sleeve”, the segments of which are surrounded by A- and I-bands ( Rice. 9). The end parts of each segment expand in the form of so-called lateral sacs(terminal tanks) connected to each other by a series of thinner tubes. In the lateral sacs, Ca 2+ is deposited, which is released after the excitation of the plasma membrane ( rice. 10).

Rice. 10. Scheme of the anatomical structure of the transverse tubules and sarcoplasmic reticulum in an individual skeletal muscle fiber

What's happened transverse tubules (T-tubules)?

Invaginations on the surface of the sarcolemma, located at some distance from each other. Thanks to T-tubules, extracellular fluid can closely contact the internal microstructures of the cell. T-tubules are extensions of the sarcolemma and are also capable of transmitting an action potential to the inner surface of the cell. The sarcoplasmic reticulum (SR) interacts closely with T-tubules.

What is the sarcoplasmic reticulum?

A specialized endoplasmic reticulum, which consists of vesicles oriented along the contractile fibers of skeletal muscle. These vesicles store, release into the intracellular fluid, and reuptake calcium ions. Specialized extended sections of the SR are called end tanks. The terminal cisterns are located in close proximity to the T-tubule and, together with the SR, form a structure called the triad. Structural features of the sarcolemma and triads play an important role in providing the sarcomere with calcium ions necessary for the cross-bridge cycle.

Rice. 11. The role of the sarcoplasmic reticulum in the mechanism of skeletal muscle contraction

Originating in the plasma membrane ( rice. eleven), the action potential quickly spreads along the surface of the fiber and along the membrane of T-tubules deep into the cell. Upon reaching the region of the T-tubules adjacent to the lateral sacs, the action potential activates voltage-dependent "gate" proteins of the T-tubule membrane, physically or chemically coupled to the calcium channels of the lateral sac membrane. Thus, the depolarization of the T-tubule membrane, caused by the action potential, leads to the opening of calcium channels in the membrane of the lateral sacs containing high concentrations of Ca 2+, and Ca 2+ ions are released into the cytoplasm. An increase in the cytoplasmic level of Ca 2+ is usually sufficient to activate all the cross-bridges of the muscle fiber.

The contraction process continues as long as Ca 2+ ions are bound to troponin, i.e. until their concentration in the cytoplasm returns to a low initial value. The membrane of the sarcoplasmic reticulum contains Ca-ATPase, an integral protein that actively transports Ca 2+ from the cytoplasm back to the cavity of the sarcoplasmic reticulum. As just mentioned, Ca 2+ is released from the reticulum as a result of the propagation of the action potential along the T-tubules; it takes much more time for Ca 2+ to return to the reticulum than for its exit. That is why the increased concentration of Ca 2+ in the cytoplasm persists for some time, and the contraction of the muscle fiber continues after the end of the action potential.

Summarize. The contraction is due to the release of Ca 2+ ions stored in the sarcoplasmic reticulum. When Ca 2+ enters back into the reticulum, contraction ends and relaxation begins.

What are the features of the sarcolemma?

The electric charge on the sarcolemma, as well as on other selectively permeable and excitable membranes, is formed due to the unequal distribution of ions. The permeability of the sarcolemma changes upon stimulation of acetylcholine receptors located at the neuromuscular junction. After sufficient stimulation, the sarcolemma can conduct a depolarizing signal (action potential) along its entire length, as well as into the unique T-tubule conduction system.

Rice. 12. The phenomenon of electromechanical coupling

In order to purposefully develop strength, you need to have an idea of ​​\u200b\u200bthe human muscular system. The muscular system is of great importance in the life of the body.

The human skeletal muscles consist of several types of muscle fibers that differ from each other in structural and functional characteristics. Currently, there are four main types of muscle fibers.

Slow phasic fibers of the oxidative type. Fibers of this type are characterized by a high content of myoglobin protein, which is able to bind O2 (similar in its properties to hemoglobin). Muscles that are predominantly composed of fibers of this type are called red because of their dark red color. They perform a very important function of maintaining a person's posture. Limit fatigue in fibers of this type and, consequently, muscles occurs very slowly, due to the presence of myoglobin and a large number of mitochondria. Recovery of function after fatigue occurs quickly.

Fast phasic fibers of the oxidative type. Muscles, which are predominantly composed of this type of fiber, perform rapid contractions without noticeable fatigue, which is explained by the large number of mitochondria in these fibers and the ability to form ATP through oxidative phosphorylation. As a rule, the number of fibers that make up the neuromotor unit in these muscles is less than in the previous group. The main purpose of this type of muscle fibers is to perform fast, energetic movements.

For muscle fibers of all these groups, the presence of one, in extreme cases, several end plates, formed by one motor axon, is characteristic.

Skeletal muscles are an integral part of the human musculoskeletal system. In this case, the muscles perform the following functions:

- provide a certain posture of the human body;

- move the body in space;

- move separate parts of the body relative to each other;

- are a source of heat, performing a thermoregulatory function.

Major groups of skeletal muscles

Human muscles are of two types - smooth and striated. In figures 1 and 2 McComas A. J. Skeletal muscles. - Kyiv: Olympic Literature, 2001. - 107 p. scheme is presented muscular system person.

Figure 1 Figure 2

The main muscles of a person: 1 - muscles that move the hand and fingers; 2 - biceps of the sword; 3 - triceps muscle of the sword; 4 - deltoid muscle; 5 - pectoralis major muscle; 6 - large round muscle; 7 - the latissimus dorsi muscle; 8 - trapezius muscle; 9 - serratus anterior; 10 - sternocleidomastoid muscle; 11 - scalene muscles; 12 - rectus abdominis; 13 - external oblique muscle; 14 - gluteus maximus; 15 - biceps femoris; 16 - semitendinosus muscle; 17 - muscle tensioner of the wide fascia of the thigh; 18 - tailor muscle; 19 - quadriceps femoris; 20 - adductor muscles of the thigh; 21 - triceps muscle of the lower leg (21A - calf muscle, 21b - soleus muscle); 22 - anterior tibial muscle; 23 - foot muscles.

Smooth muscles cover the walls of blood vessels, as well as internal organs. Their work, as a rule, does not depend on the will of man. They shrink relatively slowly, but are very hardy. Skeletal muscles can contract quickly and fatigue relatively quickly. Skeletal muscle is made up of various numbers of muscle cells. This muscle is attached to the skeleton with a tendon at both ends. Muscle fibers are collected in a bundle and surrounded by connective tissue, which passes into the tendon. Human muscles are abundantly supplied with blood vessels and nerves. Special mention should be made of the heart muscle, which consists of muscle fibers. As well as smooth muscles; the heart muscle works without the relative participation of the human will. The endurance of the heart is very great.

The structure and properties of skeletal muscles

The structure of skeletal muscles. Skeletal muscles are made up of a group of muscle bundles. Each of them includes thousands of muscle fibers with a diameter of 20 to 100 microns and a length of up to 12-16 cm. Each fiber is surrounded (covered) by a true cell membrane - the sarcolemma and contains from 1000 to 2000 or more densely packed myofibrils (0.5- 2 µm). Shuvalova N.V. The structure of man. - M.: Olma-press, 2000. - 99 p.

Under a light microscope, myofibrils are formations consisting of dark and light disks regularly alternating with each other.

Disks A are called anisotropic (have birefringence), disks I are called isotropic (almost do not have birefringence). The length of A - disks is constant, the length of I - disks depends on the stage of contraction of the muscle fiber.

In the middle of each isotropic disk is a Z-plate (membrane). These Z-plates divide each myofibril into 20 thousand sections - sacromeres, the length of which is about 2.5 microns. Due to the alternation of isotropic and anisotropic segments, each myofibril has a transverse striation.

In the middle of each sacromere there are about 2500 thick filaments of myosin protein with a diameter of about 10 nm. At both ends of the sacromere, about 2500 thin, about 5 nm in diameter, actin protein filaments are attached to the Z-membrane. Actin filaments with their ends partially enter between myosin filaments.

In the central part of the anisotropic region, actin and myosin filaments do not overlap.

The structural and functional contractile unit of the myofibril is the sacromere - a repeating section of the fibril bounded by two Z plates.

Striated muscles contain 100 mg of contractile proteins, mainly myosin and actin, which form the actomyosin complex. Other contractile proteins include tropomyosin and the troponin complex found in thin filaments.

Muscles also contain myoglobin, glycolytic enzymes, ATP, and a number of other soluble proteins.

Skeletal muscle fibers differ in color. Red fibers are rich in sarcoplasm and contain few myofibrils, while white fibers contain many myofibrils and relatively little sarcoplasm.

Somatic and autonomic nerves terminate in skeletal muscles. The motor nerve branching, ends at each muscle fiber. Only the end of the axial cylinder enters the fiber, which does not penetrate the sarcolemma, but presses it in, forming a special structure - a motor plaque, a neuromuscular synapse, or a motor end plate. Sensory endings in skeletal muscles are represented by a neuromuscular spindle, which are attached to the bone at one end. This is a receptor device containing muscle receptors. Any change in muscle fibers causes a change in the activity of neuromuscular spindle receptors.

Question 39-42

The spinal cord is part of the central nervous system, which is connected with the periphery of the body - skin, muscles and some other internal organs . These connections are carried out in humans through 31-33 pairs of nerves extending from the spinal cord, which is respectively divided into 31-32 segments (segments). Each of these segments innervates a certain part of the body. There are 8 cervical segments, 12 thoracic, 5 lumbar, 5 sacral and 1-3 coccygeal. Information from the periphery enters the spinal cord, and orders are sent from the spinal cord to the muscles to perform certain movements. The central part of the spinal cord is composed of gray matter, which in cross section resembles a butterfly with outstretched wings. The gray matter of the spinal cord is a concentration of a huge number of nerve cells - neurons. There are tens or hundreds of thousands of neurons in each segment, and in total there are more than thirteen million of them in the human spinal cord. The gray matter of the brain is surrounded by white matter, consisting of nerve fibers - processes of neurons. Despite the fact that neurons are very small and usually do not exceed 0.1 millimeters in diameter, the length of their processes sometimes reaches one and a half meters. The "butterfly" of gray matter consists of various cells. In its anterior sections are large motor cells, long fibers coming out of the spinal cord and going to the muscles. As these fibers exit the spinal cord, they gather into bundles called anterior roots. One pair of anterior roots comes out of each segment: one to the right, the other to the left. Sensory fibers included in each segment form a pair of posterior roots. In the spinal cord, some of the sensory fibers go up to the brain. Another part enters the gray matter; here sensory fibers terminate either on motor cells or on small intermediate or intercalary cells, which play a very important role in the functioning of the spinal cord. Irritation of sensitive nerve endings of the skin, muscles, joints, tendons causes a signal propagating along the nerve fiber - a nerve impulse. Impulses coming to the spinal cord along the sensory fibers of the posterior roots excite the intercalary and motor cells; from here, along the motor fibers of the anterior roots, impulses run to the muscles and cause their contraction. This is how simple reflexes work. Reflexes (from the Latin word reflexio - reflection) physiologists called the body's reactions to stimuli carried out through the central nervous system. Therefore, one of the main functions of the spinal cord is reflex. The path along which nerve impulses go from the periphery to the spinal cord and from it to the muscles is called the reflex arc. There are a number of reflexes in which the arcs are well studied. The obtained data of neuropathology are used in practice. For example, when a doctor strikes a tendon near the patient's patella with a hammer, he, by studying the tendon knee reflex, judges the functional state of the conditioned area of ​​the spinal cord. But the spinal cord is not an autonomous reflex system. His work proceeds under the constant control of the brain. The spinal cord is connected to various parts of the brain through pathways - long bundles of white matter nerve fibers. Along one path, signals from the periphery are transmitted upwards to the brain, along others, commands go from top to bottom, from the brain to the spinal cord. Complex coordinated movements are organized and directed by the entire central nervous system. The finest movements of the pianist's hands, perfected by the ballerinas - all this is the result of the action of the flow of impulses from the brain to the spinal cord, and from it to the muscles. So another essential function spinal cord - conductor. A large role in this belongs to the intermediate, or intercalary, neurons. They not only transmit signals from sensory neurons to motor neurons. Intercalated cells receive and process information from various muscles and skin areas. On them, signals from the periphery are also found with impulses from the brain. Intercalary cells send excitatory signals to certain groups of motor cells and simultaneously inhibit the activity of other groups. Thanks to this, the finest coordination of human movements becomes possible.