home → Treadmill Biochemical mechanisms of muscle contraction and relaxation. The mechanism of muscle contraction. Regulation of muscle contraction and relaxation Mechanism of muscle contraction biochemistry

Biochemical mechanisms of muscle contraction and relaxation. The mechanism of muscle contraction. Regulation of muscle contraction and relaxation Mechanism of muscle contraction biochemistry

BIOCHEMISTRY OF MUSCLES AND MUSCLE CONTRACTION. The mechanism of muscle contraction and relaxation. The most important feature of muscle functioning is that during the process of muscle contraction there is a direct conversion of the chemical energy of ATP into the mechanical energy of muscle contraction. Biochemically, they differ in the mechanisms of energy supply for muscle contraction.

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Lecture 7. Topic: BIOCHEMISTRY OF MUSCLES AND MUSCLE CONTRACTION

Questions:

2. The structure of myofibrils.

1. General characteristics of muscles. The structure of muscle cells.

The study of muscles is the most important section of biochemistry, which is of exceptional importance for sports biochemistry.

The most important feature of muscle functioning is that during the process of muscle contraction, the chemical energy of ATP is directly converted into the mechanical energy of muscle contraction. This phenomenon has no analogues in technology and is inherent only in living organisms.

When studying skeletal muscles using a light microscope, transverse striations were discovered in them; hence their name striated.

Skeletal muscle consists of a tendon head, with which the muscle begins on a bone, a muscle belly, consisting of fibers, and a tendon tail, with which the muscle ends on another bone (Fig.).

Muscle fiber structural unit of muscle. There are three types of muscle fibers: white fast twitch ( VT ), intermediate ( FR ) and slow twitch ( ST ). Biochemically, they differ in the mechanisms of energy supply for muscle contraction. They are innervated by different motor neurons, which determines the non-simultaneous activation of the work and the different speed of contraction of the fibers. Different muscles have different combinations of fiber types.

Muscle fibers

Tendon

Drawing. Muscle

Each muscle consists of several thousand muscle fibers, united by connective layers and the same membrane. The muscle is a multicomponent complex. To understand the structure of a muscle, you should study all levels of its organization and the structures that make up its composition.

Animals and humans have two main types of muscles:striated and smooth, and striated muscles are divided into two typesskeletal and cardiac. Smooth muscles are characteristic of internal organs and blood vessels.

Striated muscles are made up of thousands of muscle cells and fibers. The fibers are united by connective tissue layers and the same shell fascia . Muscle fibers myocytes - are highly elongated multinucleated cells of giant sizes from 0.1 to 10 cm long and about 0.1 × 0.2 mm thick.

A myocyte consists of all the essential components of a cell. A feature of muscle fiber is that inside this cell contains a large number of contractile elements myofibrils Like other cells of the body, myocytes contain a nucleus, and striated muscle cells have several nuclei, ribosomes, mitochondria, lysosomes, and a cytoplasmic reticulum.

Cytoplasmic reticulumcalled in these cellssarcoplasmic reticulum.It is connected through special tubes called T-tubules to the cell membrane sarcolemma. Of particular note in the sarcoplasmic reticulum are vesicles called cisternae. They contain a large amount of calcium ions. Using a special enzyme, calcium is pumped into the tanks. This mechanism is called the calcium pump and is necessary for muscle contraction.

Cytoplasm or the sarcoplasm of myocytes contains a large number of proteins. There are many active enzymes here, among which the most important areglycolytic enzymes, creatine kinase. Protein is important myoglobin, retains oxygen in the muscles.

In addition to proteins, the cytoplasm of muscle cells contains phosphogenes ATP, ADP, AMP, and also creatine phosphate, necessary for normalsupplying the muscle with energy.

The main carbohydrate in muscle tissue is glycogen. Its concentration reaches 3%. Free glucose in the sarcoplasm occurs in low concentrations. Accumulates in muscles trained for endurance reserve fat.

On the outside, the sarcolemma is surrounded by threads of collagen protein. The muscle fiber stretches and returns to its original state due to elastic forces arising in the collagen sheath.

2. The structure of myofibrils.

Contractile elements myofibrils occupy most of the volume of myocytes. In untrained muscles, myofibrils are scattered, while in trained muscles they are grouped into bundles called fields of Conheim.

Microscopic examination of the structure of myofibrils showed that they have a diameter of about 1 μm and consist of alternating light and dark areas or disks. In muscle cells, myofibrils are arranged in such a way that the light and dark areas of adjacent myofibrils coincide, which creates a transverse striation of the entire muscle fiber visible under a microscope.

The use of an electron microscope with very high magnification made it possible to decipher the structure of myofibrils and establish the reasons for the presence of light and dark areas in them. It was discovered that myofibrils are complex structures, built in turn from a large number of muscle filaments of spirit typesthick and thin.Thick ones are twice as thick as thin ones, 15 and 7 nm, respectively.

Myofibrils consist of alternating bundles of parallel thick and thin filaments, whose ends overlap each other.

The section of the myofibril, consisting of thick filaments and the ends of thin filaments located between them, is birefringent. Under a microscope, these areas appear dark and are calledanisotropic or dark disks (A-disks).

Thin sections consist of thin threads and look light because they are not birefringent and easily transmit light. Such areas are calledisotropic or light disks ( I-discs).

Z Z Z

I-disc A-disc

Drawing. Scheme of the structure of myofibril

In the middle of a bundle of thin threads (disc I ) a thin plate of protein is located transversely, which fixes the position of muscle filaments in space and at the same time ordering the location of A- and I -discs of many myofibrils. This plate is clearly visible under a microscope and is called Z-plate or Z-line.

Disks A have a lighter stripe in the middle, the H zone, intersected by a darker M zone.

The area between neighboring Z - called lines sarcomere Each myofibril consists of several hundred sarcomeres (up to 1000-1200).

sarcomere

I-disc A-disc I-disc

Drawing. Muscle structure at different levels of organization: a muscle fiber; b location of the myofibril in the resting muscle

Each sarcomere includes: 1) a network of transverse tubes, oriented at an angle of 90° to the longitudinal axis of the fiber and connecting to the outer surface of the cell; 2) sarcoplasmic reticulum, constituting 8×10% of the cell volume; 3) several mitochondria.

Discs I consist only of thin filaments, and disks A consist of two types of filaments. Zone H contains only thick filaments, line Z holds thin filaments together. Between the thick and thin filaments there are cross bridges (adhesions) about 3 nm thick; the distance between these bridges is 40 nm.

A study of the chemical composition of myofibrils showed that thin and thick filaments are formed by proteins. The rod-shaped myosin molecule consists of two identical main chains (200 kDa each) and four light chains (20 kDa each), the total mass of myosin is about 500 kDa.

Thick filaments are made of protein myosin. These proteins form a double helix with a globular head at the end attached to a very long rod.The rod is a double-stranded α-helical superhelix.

Myosin heads have ATPase activity, that is, the ability to break down ATP. The second section of myosin provides connection between thick filaments and thin filaments. The general structure of myosin is shown in the figure.

tail

Drawing. Schematic representation of a myosin molecule

Thin filaments are made of proteinsactin, troponin and tropomyosin.

The main protein in this case actin . It has two important properties:

forms fibrillar actin capable of rapid polymerization;
actin is capable of connecting to myosin heads via cross bridges.

Actin water-soluble globular protein with a molecular weight of 42 kDa; this form of actin is designated as G -actin. In muscle fiber, actin is in a polymerized form, which is designated as F -actin. Thin muscle filaments are formed by double-stranded actin structures interconnected by non-covalent bonds.

Other thin filament proteins help actin carry out its functions.

Troponin (Tn), whose molecular weight is about 76 kDa. It is a spherical molecule consisting of three different subunits, named according to their functions: tropomyosin-binding (Tn-T), inhibitory (Tn-1) and calcium-binding (Tn-C). Each thin filament component is connected to two other non-covalent bonds:

F -actin tropomyosin
Tn-1 Tn-T

In muscle, where all the components considered are assembled together in a thin filament (Fig.), tropomyosin blocks the attachment of the myosin head to nearby molecules of globular actin of thin filaments ( F-actin).

Myosin molecules combine to form filaments consisting of approximately 400 rod-shaped molecules linked to each other in such a way that pairs of myosin molecule heads lie 14.3 nm apart; they are arranged in a spiral (Fig.). Myosin filaments are joined tail to tail.

Drawing. Packing of myosin molecules during thick filament formation

Myosin performs three biologically important functions:

At physiological values of ionic strength and pH, myosin molecules spontaneously form a fiber.

Myosin has catalytic activity, i.e. it is an enzyme. In 1939, V.A. Engelhardt and M.N. Lyubimov discovered that myosin is capable of catalyzing the hydrolysis of ATP. This reaction is a direct source of free energy necessary for muscle contraction.

Myosin binds the polymerized form of actin, the main protein component of thin myofibrils. It is this interaction, as will be shown below, that plays a key role in muscle contraction.

The structure and mechanism of contraction of skeletal muscles.

3. The mechanism of muscle contraction and relaxation.

Mobility is a characteristic property of all life forms. Directed movement occurs during the divergence of chromosomes during cell division, active transport of molecules, movement of ribosomes during protein synthesis, contraction and relaxation of muscles. Muscle contraction is the most advanced form of biological mobility. Any movement, including muscle movement, is based on general molecular mechanisms.

In humans, there are several types of muscle tissue. Striated muscle tissue makes up the skeletal muscles (skeletal muscles that we can contract voluntarily). Smooth muscle tissue is part of the muscles of internal organs: the gastrointestinal tract, bronchi, urinary tract, blood vessels. These muscles contract involuntarily, regardless of our consciousness.

In this chapter we will look at the structure and processes of contraction and relaxation of skeletal muscles, since they are of greatest interest for the biochemistry of sports.

Mechanism muscle contractionhas not yet been fully disclosed.

The following is known for certain.

1. The source of energy for muscle contraction is ATP molecules.

2. ATP hydrolysis is catalyzed during muscle contraction by myosin, which has enzymatic activity.

3. The trigger mechanism for muscle contraction is an increase in the concentration of calcium ions in the sarcoplasm of myocytes, caused by a nerve motor impulse.

4. During muscle contraction, cross bridges or adhesions appear between thin and thick strands of myofibrils.

5. During muscle contraction, thin filaments slide along thick filaments, which leads to shortening of myofibrils and the entire muscle fiber as a whole.

There are many hypotheses explaining the mechanism of muscle contraction, but the most substantiated is the so-calledhypothesis (theory) of “sliding threads” or “rowing hypothesis”.

In a resting muscle, thin and thick filaments are in a separated state.

Under the influence of a nerve impulse, calcium ions leave the cisterns of the sarcoplasmic reticulum and attach to the thin filament protein troponin. This protein changes its configuration and changes the configuration of actin. As a result, a cross bridge is formed between the actin of the thin filaments and the myosin of the thick filaments. This increases the ATPase activity of myosin. Myosin breaks down ATP and, due to the energy released, the myosin head rotates like a hinge or oar of a boat, which leads to the sliding of muscle filaments towards each other.

Having made a turn, the bridges between the threads are broken. The ATPase activity of myosin decreases sharply, and ATP hydrolysis stops. However, with the further arrival of the nerve impulse, the cross bridges are formed again, since the process described above is repeated again.

Each contraction cycle uses up 1 molecule of ATP.

Muscle contraction is based on two processes:

helical coiling of contractile proteins;

cyclically repeating formation and dissociation of a complex between the myosin chain and actin.

Muscle contraction is initiated by the arrival of an action potential at the end plate of the motor nerve, where the neurohormone acetylcholine is released, the function of which is to transmit impulses. First, acetylcholine interacts with acetylcholine receptors, resulting in propagation of an action potential along the sarcolemma. All this causes an increase in the permeability of the sarcolemma for cations Na+ , which rush into the muscle fiber, neutralizing the negative charge on the inner surface of the sarcolemma. Connected to the sarcolemma are the transverse tubes of the sarcoplasmic reticulum, through which the excitation wave propagates. From the tubes, the excitation wave is transmitted to the membranes of vesicles and cisterns, which entwine myofibrils in areas where actin and myosin filaments interact. When a signal is transmitted to the cisterns of the sarcoplasmic reticulum, the latter begin to release the Ca contained in them 2+ . Released Ca 2+ binds to Tn-C, which causes conformational shifts that are transmitted to tropomyosin and then to actin. Actin seems to be released from the complex with the components of thin filaments in which it was located. Next, actin interacts with myosin, and the result of this interaction is the formation of adhesions, which makes it possible for the thin filaments to move along the thick ones.

The generation of force (shortening) is determined by the nature of the interaction between myosin and actin. The myosin rod has a movable hinge, in the area of which rotation occurs when the globular head of myosin binds to a certain area of actin. It is these turns, occurring simultaneously in numerous areas of interaction between myosin and actin, that cause the retraction of actin filaments (thin filaments) into the H-zone. Here they contact (at maximum shortening) or even overlap each other, as shown in the figure.

b
V

Drawing. Reduction mechanism: A state of rest; b moderate reduction; V maximum reduction

The energy for this process is supplied by the hydrolysis of ATP. When ATP attaches to the head of the myosin molecule, where the active center of myosin ATPase is localized, no connection is formed between the thin and thick filaments. The resulting calcium cation neutralizes the negative charge of ATP, promoting proximity to the active center of myosin ATPase. As a result, myosin phosphorylation occurs, i.e., myosin is charged with energy, which is used to form adhesions with actin and to advance the thin filament. After the thin filament advances one “step,” ADP and phosphoric acid are split off from the actomyosin complex. A new ATP molecule then attaches to the myosin head, and the whole process is repeated with the next head of the myosin molecule.

ATP consumption is also necessary for muscle relaxation. After the termination of the motor impulse Ca 2+ passes into the cisterns of the sarcoplasmic reticulum. Tn-C loses the calcium associated with it, which results in conformational shifts in the troponin-tropomyosin complex, and Tn- I closes actin active sites again, making them unable to interact with myosin. Ca concentration 2+ in the area of contractile proteins becomes below the threshold, and muscle fibers lose the ability to form actomyosin.

Under these conditions, the elastic forces of the stroma, deformed at the time of contraction, take over, and the muscle relaxes. In this case, thin threads are extracted from the space between the thick threads of disk A, zone H and disk I acquire their original length, lines Z move away from each other to the same distance. The muscle becomes thinner and longer.

Hydrolysis rate ATP during muscular work it is huge: up to 10 micromol per 1 g of muscle in 1 minute. General reserves ATP small, therefore, to ensure normal muscle function ATP must be restored at the same rate at which it is consumed.

Muscle relaxationoccurs after the cessation of a long-term nerve impulse. At the same time, the permeability of the wall of the sarcoplasmic reticulum tanks decreases, and calcium ions, under the action of the calcium pump, using the energy of ATP, go into the tanks. The concentration of calcium ions in the sarcoplasm quickly decreases to the initial level. The proteins again acquire the conformation characteristic of the resting state.

Thus, both the process of muscle contraction and the process of muscle relaxation are active processes that consume energy in the form of ATP molecules,

There are no myofibrils in smooth muscles. Thin filaments are attached to the sarcolemma, thick filaments are located inside the fibers. Calcium ions also play a role in contraction, but enter the muscle not from the cisterns, but from the extracellular substance, since smooth muscles do not have cisterns with calcium ions. This process is slow and therefore smooth muscles work slowly.

Drawing. Diagram of the location of thick and thin fibers in smooth muscle fibers.

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Category: "Biochemistry". Morphological organization of skeletal muscle. The role of intracellular structures in the life of a muscle cell. Structural organization and molecular structure of myofibrils. Chemical composition of muscle. The role of ATP in the contraction and relaxation of muscle fibers. The mechanism of muscle contraction. The sequence of chemical reactions in a muscle during its contraction. Muscle relaxation.

The specific function of muscles is to provide motor function - contraction and relaxation. In connection with the performance of this important function, the structure of the muscle cell and its chemical composition has a number of specific features.
70-80% of muscle mass is water, 20-26% dry residue.
Characteristic for muscles is a high protein content of 16.5-20.9%. This is due to the fact that in addition to proteins inherent in other cells, muscles have specific contractile proteins, which make up 45% of all proteins in a muscle cell. The remaining mass of proteins consists of sarcoplasmic proteins (about 30%) and stromal proteins (15% of the total).
Skeletal muscle consists of bundles of fibers enclosed in a common connective sheath, the sarcolemma. Within each fiber are about a hundred or more myofibrils, long specialized organelles of the muscle cell that perform contraction functions. Each myofibril consists of several parallel threads, the so-called filaments of two types - thick and thin, which are located hexagonally in it; each thick filament is surrounded by six thin ones. The structural connection between the filaments is carried out only by regularly spaced “cross bridges”. When contracting and relaxing, the thin filaments slide along the thick ones and do not change their length. In this case, the bonds between the two types of filaments are destroyed and arise again. Thick filaments are mainly composed of the protein myosin, and thin filaments are made of actin. The contractile protein myosin is characterized by a high molecular weight (more than 440,000).
A feature of myosin is that it has areas with enzymatic activity (ATPase activity), which manifests itself in the presence of Ca2+. Under the influence of myosin, ATP is broken down into ADP and inorganic phosphate (H3PO4). The released energy is used for muscle contraction.
Actin– contractile protein, with a lower molecular weight (about 420,000). It can exist in two forms: globular (G-actin) and fibrillar (F-actin). F-actin is a polymer of G-actin. F - actin - activates ATP - myosin ace, which creates a driving force that causes thin and thick filaments to slide relative to each other. In addition to these two main proteins, the contractile system contains regulatory proteins localized in thin (actin filaments) - tropomyosin B and troponin, consisting of three subunits: J, C and T.
Tropomyosin B has a filamentous helical structure and is located in the groove of the helical chain of F-actin. Troponin is associated with tropomyosin B and can form complexes with actin and myosin.
The tropomyosin B-troponin complex is called relaxation protein, as it is associated with the process of relaxation of the contracted fibril. Two more proteins have been isolated from thin filaments: and – actin, which are apparently proteins that strengthen the complex structure of thin filaments. Approximately, the myofibril contains myosin, actin, tropomyosin and troponin in relation to the total protein of 55, 25, 15 and 5%, respectively. There are two other muscle proteins worth noting: myostromin And myoglobin. Myostromins form the basis of the muscle stroma; they are sparingly soluble proteins that cannot be extracted from the muscle by saline solutions. The muscle stroma has elasticity, which is essential for muscle relaxation after contraction. Myoglobin- a protein containing iron and similar in structure and function to the protein of erythrocytes - hemoglobin. It has a significantly greater affinity for oxygen than hemoglobin and, accumulating oxygen brought by the blood, serves as a reserve reservoir of oxygen in the muscle.
Of the non-protein substances, it should be noted, in addition to ATP, first of all creatine phosphate(KF) and glycogen. CP is the first powerful reserve of resynthesis (recovery) of ATP, spent on muscle contractions. Glycogen– the main reserve carbohydrate source of muscle energy. Muscle contains a number of intermediate products of carbohydrate metabolism: (pyruvic acid, lactic acid, etc.) and a large number of mineral ions. The highest content in the muscle is K+ and PO4--, slightly less Na +, Mg ++, Ca ++, Cl -, Fe3+, SO4--_.
Inside the muscle fiber, under the sarcolemma, there is sarcoplasm - a liquid protein solution surrounding the contractile elements of the muscle fiber - myofibrils, as well as other structural components - organelles that perform a specific function. This is first of all - sarcoplasmic reticulum And T-system directly related to muscle contraction. Sarcoplasmic reticulum is directly related to muscle contraction and relaxation, regulating the release of its elements and the reverse transport of Ca2+ in the muscle fiber. The T-system transmits a change in the electrical potential of the surface membrane to the elements of the reticulum, which leads to the release of Ca ions that enter the fibrils and trigger the process of muscle contraction. Mitochondria - contain enzymes of oxidative processes that produce the main source of energy for muscle contraction - ATP.
Muscle contraction is based on the longitudinal movement of myosin and actin filaments relative to each other without changing the length of the filaments themselves. The connection between the filaments is carried out using “cross bridges” - myosin heads protruding from the surface of the myosin filament and capable of interacting with actin. The stimulus for turning on the complex mechanism of muscle contraction is a nerve impulse transmitted to the muscle cell by the motor nerve, quickly spreading through the sarcolemma and causing the release of acetylcholine at the end of the motor nerve (synapse), a chemical intermediary (mediator) in the transmission of nervous excitation. The release of acetylcholine onto the surface of the cell membrane creates a potential difference between its outer and inner surfaces, associated with a change in its permeability to Na+ and K+ ions. At the moment of depolarization of the sarcolemma, the T-system of the muscle cell is also depolarized. Since the T-system is in contact with all fibrils of the fiber, the electrical impulse propagates simultaneously to all its sarcomeres. Changes in the T-system are immediately transmitted to the reticulum membranes closely adjacent to it, causing an increase in their permeability, resulting in the release of calcium into the sarcoplasm and myofibrils. Contraction occurs when the Ca2+ concentration in the space between the actin and myosin filaments increases to 10-5 M.
Ca2+ ions join troponin C (calmodulin), which entails a change in the conformation of the entire complex; tropomyosin deviates from the myosin head by about 20°, opening the active centers of actin that can connect with myosin (charged with ATP energy and located in a complex with ADP and Fn in the presence of Mg++), forming the actomyosin complex.
The conformation of the globular part of the myosin molecule (head) changes, which deviates at a certain angle, approximately 45° from the direction of the axis of the myosin filament and moves the thin actin filament behind it: contraction occurs. A conformational change in myosin leads to the hydrolysis of ATP under the action of its ATPase. ADP and phosphate group are released into the medium. Another ATP molecule takes their place. As a result, the original state is restored and the operating cycle can be repeated. The frequency of the working cycle and its duration are determined by the concentration of Ca2+ and the presence of ATP.
After the cessation of the motor impulse, reverse transport of Ca2+ ions occurs into the sarcoplasmic reticulum, its concentration between actin and myosin filaments drops below 10-7 M, and muscle fibers lose the ability to form actomyosin, shorten and develop pulling tension in the presence of ATP.
The muscle relaxes. Reverse transport of Ca2+ is carried out using the energy obtained from the breakdown of ATP by the enzyme Ca2+ - ATPase. The transfer of each Ca2+ ion requires 2 ATP molecules. Thus, the energy for contraction and relaxation is provided by the supply of ATP. Consequently, ATP reserves must be constantly renewed between contractions. Muscles have very powerful and sophisticated mechanisms for replenishing (resynthesising) spent ATP and maintaining its concentration at the required, optimal level to ensure work of varying duration and power.
This goal, along with the high initial ATP, is served by the high activity of respiratory enzymes and the ability of the muscle to increase the level of the oxidative process many times in a relatively short time (1-3 minutes). Increased blood supply to the muscles during work increases the flow of oxygen and nutrients.
In the initial period, oxygen bound to myoglobin can be used. The possibility of ATP resynthesis is also ensured by the internal mechanisms of the cell - a high level of creatine phosphate, as well as a high concentration of glycogen and the activity of glycolytic enzymes.

Animals and humans have two main types of muscles:

striated (attached to the bones, i.e., to the skeleton, and therefore also called skeletal; they also secrete the heart muscle, which has its own characteristics);
smooth (muscles of the walls of hollow organs and skin).

The structure of muscle cells

Striated muscle consists of numerous elongated muscle cells. Motor nerves enter the muscle fiber at various points and transmit an electrical impulse to it, causing contraction. Muscle fiber is usually considered as a multinucleated cell of giant size, covered with an elastic membrane - the sarcolemma. The diameter of a functionally mature striated muscle fiber is usually between 10 and 100 µm, and the length of the fiber often corresponds to the length of the muscle.

A number of structures are found in the sarcoplasm of muscle fibers: mitochondria, microsomes, ribosomes, tubules and cisterns of the sarcoplasmic reticulum, various vacuoles, lumps of glycogen and lipid inclusions that play the role of reserve energy materials, etc.

In each muscle fiber in the semi-liquid sarcoplasm along the length of the fiber there are located, often in the form of bundles, many thread-like formations - myofibrils (their thickness is usually less than 1 micron), which, like the entire fiber as a whole, have transverse striations. The transverse striation of the fiber, depending on the optical heterogeneity of protein substances localized in all myofibrils at the same level, is easily detected when examining skeletal muscle fibers in a polarizing or phase-contrast microscope (Fig. 2).

The repeating element of the striated myofibril is the sarcomere - a section of the myofibril, the boundaries of which are narrow 2-lines. Each myofibril consists of several hundred sarcomeres. The average sarcomere length is 2.5-3.0 µm. In the middle of the sarcomere there is a zone 1.5-1.6 µm long, dark in a phase-contrast microscope. In polarized light it exhibits strong birefringence. This zone is usually called disk A (anisotropic disk). In the center of disk A there is a line M, which can only be observed in an electron microscope. The middle part of disk A is occupied by zone H of weaker birefringence. Finally, there are isotropic disks, or I disks, with very weak birefringence. In a phase contrast microscope, they appear lighter than disks A. The length of disks I is about 1 µm. Each of them is divided into two equal halves by a Z-membrane, or Z-line. According to modern concepts, A disks contain thick filaments, consisting mainly of the myosin protein, and thin filaments, usually consisting of the second component of the actomyosin system, the actin protein. Thin (actin) filaments begin within each sarcomere at the Z-line, extend through disk I, penetrate into disk A and are interrupted in the region of zone H.

Rice. 2. Photograph of a microslide of striated muscle tissue

Rice. 3. Scheme of the structure of the sarcomere

When examining thin sections of muscle under an electron microscope, it was discovered that the protein threads were strictly ordered. Thick filaments with a diameter of 12-16 nm and a length of approximately 1.5 µm are arranged in the shape of a hexagon with a diameter of 40-50 nm and extend across the entire disk A. Between these thick filaments are thin filaments with a diameter of 8 nm, extending from the 2-line to a distance of about 1 µm (Fig. 3). A study of the muscle in a state of contraction showed that disks I almost disappear in it, and the area of overlap of thick and thin filaments increases (in skeletal muscle in a state of contraction, the sarcomere is shortened to 1.7-1.8 µm).

According to the model proposed by E. Huxley and R. Niedergerke, as well as H. Huxley and J. Henson, when myofibrils contract, one system of filaments penetrates into another, i.e., the filaments begin to slide over each other, which is the cause of muscle abbreviations.

WITHmuscle fiber structure and contraction.

Muscle contraction in a living system is a mechanochemical process. Modern science considers it the most perfect form of biological mobility. Biological objects “developed” the contraction of muscle fiber as a way to move in space (which significantly expanded their life capabilities).

Muscle contraction is preceded by a tension phase, which is the result of work carried out by converting chemical energy into mechanical energy directly and with good efficiency (30-50%). The accumulation of potential energy in the tension phase brings the muscle into a state of possible, but not yet realized, contraction.

Animals and humans have (and humans believe that they have already been well studied) two main types of muscles: striated and smooth. Striated muscles or skeletal are attached to the bones (except for striated fibers of the cardiac muscle, which differ from skeletal muscles in composition). Smooth muscles support the tissues of internal organs and skin and form the muscles of the walls of blood vessels, as well as the intestines.

In the biochemistry of sports they study skeletal muscles, “specifically responsible” for sports results.

A muscle (as a macro formation belonging to a macro object) consists of individual muscle fibers(micro formations). There are thousands of them in a muscle; accordingly, muscle effort is an integral value that sums up the contractions of many individual fibers. There are three types of muscle fibers: white fast-twitch , intermediate And red slow-twitch. Types of fibers differ in the mechanism of their energy supply and are controlled by different motor neurons. Muscle types differ in the ratio of fiber types.

A separate muscle fiber - a thread-like acellular formation - simplast. The symplast “does not look like a cell”: it has a highly elongated shape with a length of 0.1 to 2-3 cm, in the sartorius muscle up to 12 cm, and a thickness of 0.01 to 0.2 mm. The symplast is surrounded by a shell - sarcolemma, to the surface of which the endings of several motor nerves approach. Sarcolemma is a two-layer lipoprotein membrane (10 nm thick) reinforced by a network of collagen fibers. When they relax after contraction, they return the symplast to its original shape (Fig. 4).

Rice. 4. Individual muscle fiber.

On the outer surface of the sarcolemma-membrane, an electrical membrane potential is always maintained, even at rest it is equal to 90-100 mV. The presence of potential is a necessary condition for controlling muscle fiber (like a car battery). The potential is created due to the active (meaning with the expenditure of energy - ATP) transfer of substances through the membrane and its selective permeability (according to the principle - “whoever I want, I’ll let him in or let him out”). Therefore, inside the simplast, some ions and molecules accumulate in higher concentrations than outside.

The sarcolemma is well permeable to K + ions - they accumulate inside, and Na + ions are removed outside. Accordingly, the concentration of Na + ions in the intercellular fluid is greater than the concentration of K + ions inside the symplast. A pH shift to the acidic side (during the formation of lactic acid, for example) increases the permeability of the sarcolemma for high-molecular substances (fatty acids, proteins, polysaccharides), which normally do not pass through it. Low molecular weight substances (glucose, lactic and pyruvic acids, ketone bodies, amino acids, short peptides) easily pass (diffuse) through the membrane.

Internal contents of simplast – sarcoplasm– This is a colloidal protein structure (the consistency resembles jelly). In a suspended state, it contains glycogen inclusions, fat droplets, and various subcellular particles are “built in”: nuclei, mitochondria, myofibrils, ribosomes and others.

Contractile “mechanism” inside the symplast – myofibrils. These are thin (Ø 1 - 2 microns) muscle filaments, long - almost equal to the length of the muscle fiber. It has been established that in the symplasts of untrained muscles, the myofibrils are not located in an orderly manner, along the symplast, but with scatter and deviations, and in trained ones, the myofibrils are oriented along the longitudinal axis and are also grouped into bundles, like in ropes. (When spinning artificial and synthetic fibers, the macromolecules of the polymer are not initially located strictly along the fiber and, like athletes, they are “persistently trained” - oriented correctly - along the axis of the fibers, by repeated rewinding: see the long workshops at ZIV and Khimvolokno).

Under a light microscope, it can be observed that the myofibrils are indeed “striated.” They alternate light and dark areas - disks. Dark rims A (anisotropic) proteins contain more than light discs I (isotropic). Light discs crossed by membranes Z (telophragms) and a section of myofibril between two Z - called membranes sarcomere. The myofibril consists of 1000 – 1200 sarcomeres (Fig. 5).

The contraction of a muscle fiber as a whole consists of individual contractions sarcomeres. Contracting each separately, the sarcomeres together create an integral force and perform mechanical work to contract the muscle.

The length of the sarcomere varies from 1.8 µm at rest to 1.5 µm during moderate and up to 1 µm during full contraction. The disks of sarcomeres, dark and light, contain protofibrils (myofilaments) - protein thread-like structures. They are found in two types: thick (Ø – 11 – 14 nm, length – 1500 nm) and thin (Ø – 4 – 6 nm, length – 1000 nm).

Rice. 5. Myofibril area.

Light wheels ( I ) consist only of thin protofibrils, and dark disks ( A ) – from protofibrils of two types: thin, fastened together by a membrane, and thick, concentrated in a separate zone ( H ).

When the sarcomere contracts, the length of the dark disk ( A ) does not change, and the length of the light disk ( I ) decreases as thin protofibrils (light disks) move into the spaces between thick ones (dark disks). On the surface of protofibrils there are special outgrowths - adhesions (about 3 nm thick). In the “working position” they form an engagement (cross bridges) between thick and thin threads of protofibrils (Fig. 6). When contracting Z -membranes rest against the ends of thick protofibrils, and thin protofibrils can even wrap around thick ones. During supercontraction, the ends of the thin filaments in the center of the sarcomere are curled, and the ends of the thick protofibrils are crushed.

Rice. 6. Formation of adhesions between actin and myosin.

Energy supply to muscle fibers is carried out using sarcoplasmic reticulum(aka - sarcoplasmic reticulum) – systems of longitudinal and transverse tubes, membranes, bubbles, compartments.

In the sarcoplasmic reticulum, various biochemical processes occur in an organized and controlled manner; the network covers everything together and each myofibril separately. The reticulum includes ribosomes, they carry out the synthesis of proteins, and mitochondria - “cellular energy stations” (as defined in the school textbook). Actually mitochondria embedded between myofibrils, which creates optimal conditions for energy supply to the process of muscle contraction. It has been established that in trained muscles the number of mitochondria is greater than in the same untrained muscles.

Chemical composition of muscles.

Water with leaves 70 - 80% of the muscle weight.

Squirrels. Proteins account for from 17 to 21% of muscle weight: approximately 40% of all muscle proteins are concentrated in myofibrils, 30% in sarcoplasm, 14% in mitochondria, 15% in sarcolemma, the rest in nuclei and other cellular organelles.

Muscle tissue contains enzymatic myogenic proteins groups, myoalbumin– reserve protein (its content gradually decreases with age), red protein myoglobin– chromoprotein (it is called muscle hemoglobin, it binds more oxygen than blood hemoglobin), and also globulins, myofibrillar proteins. More than half of the myofibrillar proteins are myosin, about a quarter - actin, the rest is tropomyosin, troponin, α- and β-actinins, enzymes creatine phosphokinase, deaminase and others. Muscle tissue contains nuclearsquirrels– nucleoproteins, mitochondrial proteins. In proteins stroma, entwining muscle tissue - the main part - collagen And elastin sarcolemmas, as well as myostromins (associated with Z -membranes).

Inpre-soluble nitrogen compounds. Human skeletal muscles contain various water-soluble nitrogen compounds: ATP, from 0.25 to 0.4%, creatine phosphate (CrP)– from 0.4 to 1% (with training, its amount increases), their breakdown products are ADP, AMP, creatine. In addition, muscles contain a dipeptide carnosine, about 0.1 - 0.3%, involved in restoring muscle performance during fatigue; carnitine, responsible for the transport of fatty acids across cell membranes; amino acids, and among them glutamine predominates (does this explain the use of monosodium glutamate, read the composition of seasonings, to give food the taste of meat); purine bases, urea and ammonia. Skeletal muscle also contains about 1.5% phosphatides, which participate in tissue respiration.

Nitrogen-free connections. Muscles contain carbohydrates, glycogen and its metabolic products, as well as fats, cholesterol, ketone bodies, and mineral salts. Depending on the diet and the degree of training, the amount of glycogen varies from 0.2 to 3%, while training increases the mass of free glycogen. Storage fats accumulate in muscles during endurance training. Protein-bound fat makes up approximately 1%, and muscle fiber membranes can contain up to 0.2% cholesterol.

Minerals. Minerals in muscle tissue make up approximately 1 - 1.5% of muscle weight; these are mainly potassium, sodium, calcium, and magnesium salts. Mineral ions such as K + , Na + , Mg 2+ , Ca 2+ , Cl - , HP0 4 ~ play a vital role in the biochemical processes during muscle contraction (they are included in “sports” supplements and mineral water).

Biochemistry of muscle proteins.

The main contractile protein of muscles is myosin refers to fibrillar proteins (Molecular weight about 470,000). An important feature of myosin is the ability to form complexes with ATP and ADP molecules (which allows you to “take” energy from ATP), and with the protein actin (which makes it possible to maintain contraction).

The myosin molecule has a negative charge and specifically interacts with Ca ++ and Mg ++ ions. Myosin, in the presence of Ca++ ions, accelerates the hydrolysis of ATP, and thus exhibits enzymatic adenosine triphosphate activity:

myosin-ATP+H2O → myosin + ADP + H3PO4 + work(energy 40 kJ/mol)

The myosin protein is formed by two identical, long polypeptide α-chains, twisted like a double helix, Fig. 7. Under the action of proteolytic enzymes, the myosin molecule breaks into two parts. One of its parts is capable of binding to actin through adhesions, forming actomyosin. This part is responsible for adenosine triphosphatase activity, which depends on the pH of the environment, the optimum is pH 6.0 - 9.5, as well as the concentration of KCl. The actomyosin complex disintegrates in the presence of ATP, but in the absence of free ATP it is stable. The second part of the myosin molecule also consists of two twisted helices; due to an electrostatic charge, they bind the myosin molecules into protofibrils.

Rice. 7. Structure of actomyosin.

The second most important contractile protein is actin(Fig. 7). It can exist in three forms: monomeric (globular), dimeric (globular) and polymeric (fibrillar). Monomeric globular actin, when its polypeptide chains are tightly packed into a compact spherical structure, is associated with ATP. By splitting ATP, actin monomers - A, form dimers, including ADP: A - ADP - A. Polymeric fibrillar actin is a double helix consisting of dimers, Fig. 7.

Globular actin transforms into fibrillar actin in the presence of K + and Mg ++ ions, and fibrillar actin predominates in living muscles.

Myofibrils contain a significant amount of protein tropomyosin, which consists of two α-helical polypeptide chains. In resting muscles, it forms a complex with actin and blocks its active centers, since actin is able to bind to Ca ++ ions, which remove this blockade.

At the molecular level, thick and thin protofibrils of the sarcomere interact electrostatically, since they have special areas - outgrowths and protrusions - where a charge is formed. In the A-disk region, thick protofibrils are built from a bundle of longitudinally oriented myosin molecules, thin protofibrils are arranged radially around thick ones, forming a structure similar to a multi-strand cable. In the central M-band of thick protofibrils, myosin molecules are connected by their “tails”, and their protruding “heads” - outgrowths are directed in different directions and are located along regular spiral lines. In fact, opposite them in the fibrillar actin spirals at a certain distance from each other, monomeric actin globules are also protruding. Each protrusion has active center, due to which the formation of adhesions with myosin is possible. Z-membranes of sarcomeres (like alternating pedestals) hold thin protofibrils together.

Biochemistry of contraction and relaxation.

The cyclic biochemical reactions that occur in the muscle during contraction ensure the repeated formation and destruction of adhesions between the “heads” - the outgrowths of the myosin molecules of thick protofibrils and the protrusions - the active centers of thin protofibrils. The work of forming adhesions and moving the actin filament along the myosin filament requires both precise control and significant energy expenditure. In reality, at the moment of fiber contraction, about 300 adhesions are formed per minute in each active center - protrusion.

As we noted earlier, only ATP energy can be directly converted into mechanical work of muscle contraction. ATP hydrolyzed by the enzymatic center of myosin forms a complex with the entire myosin protein. In the ATP-myosin complex, myosin, saturated with energy, changes its structure, and with it the external “dimensions” and, in this way, performs mechanical work to shorten the growth of the myosin filament.

In resting muscle, myosin is still bound to ATP, but through Mg++ ions without hydrolytic cleavage of ATP. The formation of adhesions between myosin and actin at rest is prevented by the complex of tropomyosin with troponin, which blocks the active centers of actin. The blockade is maintained and ATP is not broken down while Ca++ ions are bound. When a nerve impulse arrives at a muscle fiber, it is released pulse transmitter– neurohormone acetylcholine. Na+ ions neutralize the negative charge on the inner surface of the sarcolemma and depolarize it. In this case, Ca++ ions are released and bind to troponin. In turn, troponin loses its charge, causing the active centers - the protrusions of actin filaments - to be unblocked and adhesions arise between actin and myosin (since the electrostatic repulsion of thin and thick protofibrils has already been removed). Now, in the presence of Ca ++, ATP interacts with the center of enzymatic activity of myosin and is cleaved, and the energy of the transforming complex is used to reduce the adhesion. The chain of molecular events described above is similar to an electric current recharging a microcapacitor; its electrical energy is immediately converted into mechanical work on the spot and needs to be recharged again (if you want to move on).

After the rupture of the adhesive, ATP is not cleaved, but again forms an enzyme-substrate complex with myosin:

M–A + ATP -----> M – ATP + A or

M–ADP–A + ATP ----> M–ATP + A + ADP

If at this moment a new nerve impulse arrives, then the “recharging” reactions are repeated; if the next impulse does not arrive, the muscle relaxes. The return of a contracted muscle upon relaxation to its original state is ensured by the elastic forces of proteins in the muscle stroma. Putting forward modern hypotheses of muscle contraction, scientists suggest that at the moment of contraction, actin filaments slide along myosin filaments, and their shortening is also possible due to changes in the spatial structure of contractile proteins (changes in the shape of the helix).

At rest, ATP has a plasticizing effect: by combining with myosin, it prevents the formation of its adhesions with actin. By breaking down during muscle contraction, ATP provides energy for the process of shortening the adhesions, as well as the work of the “calcium pump” - the supply of Ca ++ ions. The breakdown of ATP in muscle occurs at a very high rate: up to 10 micromoles per 1 g of muscle per minute. Since the total reserves of ATP in the muscle are small (they may only be enough for 0.5-1 sec of work at maximum power), to ensure normal muscle activity, ATP must be restored at the same rate at which it is broken down.

In this lecture we will look at the structure and processes of contraction and relaxation of skeletal muscles, since they are of greatest interest for the biochemistry of sports.

Mechanism muscle contraction has not yet been fully disclosed.

The following is known for certain.

1. The source of energy for muscle contraction is ATP molecules.

2. ATP hydrolysis is catalyzed during muscle contraction by myosin, which has enzymatic activity.

3. The trigger mechanism for muscle contraction is an increase in the concentration of calcium ions in the sarcoplasm of myocytes, caused by a nerve motor impulse.

4. During muscle contraction, cross bridges or adhesions appear between thin and thick strands of myofibrils.

5. During muscle contraction, thin filaments slide along thick filaments, which leads to shortening of myofibrils and the entire muscle fiber as a whole.

There are many hypotheses explaining the mechanism of muscle contraction, but the most substantiated is the so-called hypothesis (theory) of “sliding threads” or “rowing hypothesis”.

In a resting muscle, thin and thick filaments are in a separated state.

Under the influence of a nerve impulse, calcium ions leave the cisterns of the sarcoplasmic reticulum and attach to the thin filament protein, troponin. This protein changes its configuration and changes the configuration of actin. As a result, a cross bridge is formed between the actin of the thin filaments and the myosin of the thick filaments. This increases the ATPase activity of myosin. Myosin breaks down ATP and, due to the energy released, the myosin head rotates like a hinge or oar of a boat, which leads to the sliding of muscle filaments towards each other.

Each contraction cycle uses up 1 molecule of ATP.

Muscle contraction is based on two processes:

Helical coiling of contractile proteins;

Cyclically repeating formation and dissociation of a complex between the myosin chain and actin.

Muscle contraction is initiated by the arrival of an action potential at the end plate of the motor nerve, where the neurohormone acetylcholine is released, the function of which is to transmit impulses. First, acetylcholine interacts with acetylcholine receptors, resulting in propagation of an action potential along the sarcolemma. All this causes an increase in the permeability of the sarcolemma for Na + cations, which rush into the muscle fiber, neutralizing the negative charge on the inner surface of the sarcolemma. Connected to the sarcolemma are the transverse tubes of the sarcoplasmic reticulum, through which the excitation wave propagates. From the tubes, the excitation wave is transmitted to the membranes of vesicles and cisterns, which entwine myofibrils in areas where actin and myosin filaments interact. When a signal is transmitted to the cisterns of the sarcoplasmic reticulum, the latter begin to release the Ca 2+ contained in them. The released Ca 2+ binds to Tn-C, which causes conformational shifts that are transmitted to tropomyosin and then to actin. Actin seems to be released from the complex with the components of thin filaments in which it was located. Next, actin interacts with myosin, and the result of this interaction is the formation of adhesions, which makes it possible for the thin filaments to move along the thick ones.

Drawing. Reduction mechanism: A– state of rest; b– moderate reduction; V– maximum reduction

ATP consumption is also necessary for muscle relaxation. After the cessation of the motor impulse, Ca 2+ passes into the cisterns of the sarcoplasmic reticulum. Tn-C loses the calcium bound to it, resulting in conformational shifts in the troponin-tropomyosin complex, and Tn-I again closes the active centers of actin, making them unable to interact with myosin. The Ca 2+ concentration in the region of contractile proteins becomes below the threshold, and muscle fibers lose their ability to form actomyosin.

Under these conditions, the elastic forces of the stroma, deformed at the time of contraction, take over, and the muscle relaxes. In this case, thin threads are removed from the space between the thick threads of disk A, zone H and disk I acquire their original length, lines Z move away from each other to the same distance. The muscle becomes thinner and longer.

Hydrolysis rate ATP during muscular work it is huge: up to 10 micromol per 1 g of muscle in 1 minute. General reserves ATP small, therefore, to ensure normal muscle function ATP must be restored at the same rate at which it is consumed.

Muscle relaxation occurs after the cessation of a long-term nerve impulse. At the same time, the permeability of the wall of the sarcoplasmic reticulum tanks decreases, and calcium ions, under the action of the calcium pump, using the energy of ATP, go into the tanks. The removal of calcium ions into the reticulum tanks after the cessation of the motor impulse requires significant energy expenditure. Since the removal of calcium ions occurs towards a higher concentration, i.e. against the osmotic gradient, then two molecules of ATP are spent on removing each calcium ion. The concentration of calcium ions in the sarcoplasm quickly decreases to the initial level. The proteins again acquire the conformation characteristic of the resting state.

Thus, both the process of muscle contraction and the process of muscle relaxation are active processes that consume energy in the form of ATP molecules,

Smooth muscles do not have myofibrils, which consist of several hundred sarcomeres. Thin filaments are attached to the sarcolemma, thick filaments are located inside the fibers. Calcium ions also play a role in contraction, but enter the muscle not from the cisterns, but from the extracellular substance, since smooth muscles do not have cisterns with calcium ions. This process is slow and therefore smooth muscles work slowly.

This is interesting: