What ions are needed for muscle contraction. Skeletal muscles

27.08.202127.05.2017

We have repeatedly had the opportunity to notice that the same metal performs several biochemical duties: iron transports oxygen and electrons, copper participates in similar processes, zinc promotes the hydrolysis of polypeptides and the decomposition of bicarbonates, etc.

But calcium breaks all records in this regard. Calcium ions form protective shells in corals, the accumulations of which reach enormous sizes; calcium is necessary for the functioning of enzymes that provide muscle activity; calcium regulates the blood coagulation system, activates some enzymes; it is also part of the bones and teeth of vertebrates, etc.

The calcium cycle is facilitated by the different solubility of its carbonate salts: CaCO 3 carbonate is slightly soluble in water, and Ca(HCO 3) 2 bicarbonate is quite soluble, and its concentration in solution depends on the concentration of carbon dioxide and, therefore, on the partial pressure of this gas over the solution ; therefore, when the carbonic waters of mountain springs flow to the surface of the earth and lose carbon dioxide (carbon dioxide), calcium carbonate precipitates, forming crystalline aggregates (stalactites and stalagmites in caves). Microorganisms carry out a similar process, extracting bicarbonate from sea water and using carbonate to build protective shells.

In organisms of higher animals, calcium also performs functions associated with the creation of mechanically strong structures. In the bones, calcium is contained in the form of salts, similar in composition to the mineral apatite 3Ca 3 (PO 4) 2 * CaF 2 (Cl). The chlorine symbol in brackets indicates the partial substitution of chlorine for fluorine in this mineral.

The formation of bone tissue occurs under the influence of vitamins of group D; these vitamins, in turn, are synthesized in organisms under the influence of ultraviolet radiation from the sun. A significant amount of vitamin D is found in fish oil, therefore, with a deficiency of vitamin B baby food calcium is not absorbed in the intestines and symptoms of rickets develop; doctors prescribe as medicine fish fat or pure preparations of vitamin D. An excess of this vitamin is very dangerous: it can cause the reverse process - the dissolution of bone tissue!

From food products, calcium is found in milk, dairy products (especially a lot of it in cottage cheese, since milk protein casein is associated with calcium ions), as well as in plants.

Proteins having a small molecular weight (about 11,000) and contained in the muscles of fish show the ability to actively capture calcium ions. Some of them (for example, carp albumin) have been studied extensively; their composition turned out to be unusual: they contain a lot of the amino acids alanine and phenylalanine and do not contain histidine, cysteine and arginine at all - almost unchanged components of other proteins.

For complex compounds of the calcium ion, the formation of bridges is characteristic - the ion binds mainly carboxyl and carbonyl groups in the resulting complex.

The coordination number of the calcium ion is large and reaches eight. This feature of it, apparently, underlies the action of the enzyme ribonuclease, which catalyzes the process of hydrolysis of nucleic acids (RNA), which is important for the body, accompanied by the release of energy. It is assumed that the calcium ion forms a rigid complex, bringing together a water molecule and a phosphate group; arginine residues surrounded by a calcium ion contribute to the fixation of the phosphate group. It is polarized by calcium and is more easily attacked by the water molecule. As a result, the phosphate group is cleaved from the nucleotide. It was also proved that the calcium ion in this enzymatic reaction cannot be replaced by other ions with the same oxidation state.

Calcium ions also activate other enzymes, in particular α-amylase (catalyses the hydrolysis of starch), but in this case, calcium can still be replaced under artificial conditions with a three-charged neodymium metal ion.

Calcium is also the most important component of that amazing biological system, which is most like a machine - the muscle system. This machine produces mechanical work from the chemical energy contained in food substances; its coefficient useful action high; it can almost instantly be transferred from a state of rest to a state of motion (moreover, no energy is consumed at rest); its specific power is about 1 kW per 1 kg of mass, the speed of movements is well regulated; the machine is quite suitable for long-term work requiring repetitive movements, the service life is about 2.6 * 10 6 operations. Approximately so described the muscle prof. Wilkie in a popular lecture, adding that a machine ("linear motor") can serve as food.

It was very difficult for scientists to figure out what happens inside this "linear motor", how a chemical reaction generates purposeful movement, and what role calcium ions play in all this. It is currently established that muscle consists of fibers (elongated cells) surrounded by a membrane (sarcolemma). In muscle cells there are myofibrils - the contractile elements of the muscle, which are immersed in a liquid - sarcoplasm. Myofibrils are made up of segments called sarcomeres. Sarcomeres contain a system of two types of filaments - thick and thin.

The thick filaments are made up of the protein myosin. Myosin molecules are elongated particles with a thickening at one end - a head. The heads protrude above the surface of the filamentous molecule and can be located at different angles to the axis of the molecule. The molecular weight of myosin is 470,000.

Thin filaments are formed by actin protein molecules that have a spherical shape. The molecular weight of actin is 46,000. Actin particles are arranged in such a way that a long double helix is obtained. Every seven actin molecules are connected by a filamentous molecule of the tropomyosin protein, which carries (closer to one of the ends) a spherical molecule of another protein, troponin (Fig. 19). A thin filament of skeletal muscle contains up to 400 actin molecules and up to 60 tropomyosin molecules. Thus, the work of the muscle is based on the interaction of parts built from four proteins.

Perpendicular to the axes of the threads are protein formations - z-plates, to which thin threads are attached at one end. Thick threads are placed between thin ones. In a relaxed muscle, the distance between the z-plates is approximately 2.2 microns. Muscle contraction begins with the fact that, under the influence of a nerve impulse, the protrusions (heads) of myosin molecules are attached to thin filaments and so-called cross-links, or bridges, appear. The heads of thick filaments on both sides of the plate are inclined in opposite directions, therefore, turning, they draw in a thin thread between the thick ones, which leads to a contraction of the entire muscle fiber.

The source of energy for muscle work is the reaction of hydrolysis of adenosine triphosphate (ATP); the presence of this substance is necessary for the functioning of the muscular system.

In 1939, V. A. Engelgardt and M. N. Lyubimova proved that myosin and its complex with actin - actomyosin are catalysts that accelerate the hydrolysis of ATP in the presence of calcium and potassium ions, as well as magnesium, which in general often facilitates hydrolytic reactions. The special role of calcium is that it regulates the formation of crosslinks (bridges) between actin and myosin. The ATP molecule attaches to the head of the myosin molecule in thick filaments. Then some kind of chemical change occurs, bringing this complex into an active, but unstable state. If such a complex comes into contact with an actin molecule (on a thin thread), then energy will be released due to the ATP hydrolysis reaction. This energy causes the bridge to deviate and pull the thick thread closer to the protein plate, i.e., cause the contraction of the muscle fiber. Next, a new ATP molecule joins the actin-myosin complex, and the complex immediately disintegrates: actin is separated from myosin, the bridge no longer connects the thick thread with the thin one - the muscle relaxes, and myosin and ATP remain bound into a complex that is in an inactive state.

Calcium ions are contained in the tubules and vesicles surrounding a single muscle fiber. This system of tubes and vesicles, formed by thin membranes, is called the sarcoplasmic reticulum; it is immersed in a liquid medium in which the threads are located. Under the influence of a nerve impulse, the permeability of membranes changes, and calcium ions, leaving the sarcoplasmic reticulum, enter the surrounding fluid. It is assumed that calcium ions, when combined with troponin, affect the position of the filamentous tropomyosin molecule and transfer it to a position in which the active ATP-myosin complex can attach to actin. Apparently, the regulatory influence of calcium ions extends via tropomyosin filaments to seven actin molecules at once.

After muscle contraction, calcium is very quickly (fractions of a second) removed from the fluid, again leaving for the vesicles of the sarcoplasmic reticulum, and the muscle fibers relax. Consequently, the mechanism of operation of the "linear motor" consists in alternately pushing a system of thick myosin filaments into the space between thin actin filaments attached to protein plates, and this process is regulated by calcium ions periodically emerging from the sarcoplasmic reticulum and again leaving it.

Potassium ions, the content of which in the muscle is much greater than the content of calcium, contribute to the transformation of the globular form of actin into a filamentous - fibrillar form: in this state, actin interacts more easily with myosin.

From this point of view, it becomes clear why potassium ions increase the contraction of the heart muscle, why they are necessary in general for the development of the muscular system of the body.

Calcium ions are active participants in the process of blood coagulation. There is no need to say how important this process is for the preservation of the life of the organism. If blood were to lack the ability to clot, a minor scratch would pose a serious threat to life. But in a normal body, bleeding from small wounds stops after 3-4 minutes. A dense clot of fibrin protein forms on damaged tissues, clogging the wound. A study of the formation of a blood clot has shown that complex systems are involved in its creation, including several proteins and special enzymes. At least 13 factors must act in concert to the right move the whole process.

When a vessel is damaged circulatory system the protein thromboplastin enters the blood. Calcium ions take part in the action of this protein on a substance called prothrombin (i.e., "source of thrombin"). Another protein (from the class of globulins) accelerates the conversion of prothrombin to thrombin. Thrombin acts on fibrinogen, a high molecular weight protein (its molecular weight is about 400,000), whose molecules have a filamentous structure. Fibrinogen is produced in the liver and is a soluble protein. However, under the influence of thrombin, it first turns into a monomeric form, and then polymerizes, and an insoluble form of fibrin is obtained - the same clot that stops bleeding. In the process of formation of insoluble fibrin, calcium ions again participate.

Minerals are part of all living tissues. However, the normal functioning of tissues is ensured not only by the presence of certain mineral salts in them, but also by their strictly defined ratio. Minerals maintain the necessary osmotic pressure in biological fluids and ensure the constancy of the acid-base balance in the body. Consider the main minerals.

Potassium found mainly in cells sodium- in the interstitial fluid. For the normal functioning of the body, a strictly defined ratio of sodium and potassium particles is required. The proper ratio of these ions ensures the normal excitability of the nervous and muscle tissues. Sodium plays an important role in maintaining a constant osmotic pressure. With a low content of potassium in the myocardium (muscular tissue of the heart), the contractile function of the heart is disturbed. But with an excess of potassium, the activity of the heart is also disturbed. daily requirement adult: sodium - 4-6 g, potassium - 2-3 g.

Calcium is part of the bones in the form of phosphorus salts. Its ions ensure normal brain activity and skeletal muscle. The presence of calcium is necessary for blood clotting. Excess calcium increases the frequency and strength of heart contractions, and at super-high concentrations in the body, it can cause cardiac arrest. The daily requirement of an adult for calcium is 0.7-0.8 g.

Phosphorus is part of all cells and interstitial fluids. It plays an important role in the metabolism of proteins, fats, carbohydrates and vitamins. This substance is an indispensable component of energy-rich substances. Salts of phosphoric acids maintain the constancy of the acid-base balance of blood and other tissues. The daily requirement of an adult for phosphorus is 1.5-2 g.

Chlorine found in the body mainly in combination with sodium and is part of the hydrochloric acid of gastric juice. Chlorine is essential for the normal functioning of cells. The daily requirement of an adult for chlorine is 2-4 g.

Iron is integral part hemoglobin and some enzymes. Providing oxygen transport, it takes part in oxidative processes. The daily requirement for iron for men is 10 mg, for women - 18 mg.

Bromine found in small amounts in the blood and other tissues. By enhancing inhibition in the cerebral cortex, it contributes to the normal relationship between the processes of excitation and inhibition.

Iodine- an essential component of thyroid hormone. The lack of this substance in the body causes a violation of many functions. The daily requirement for iodine for healthy adults is 0.15 mg (150 mcg).

Sulfur included in many proteins. It is found in some enzymes, hormones, vitamins and other compounds that play important role in metabolism. In addition, sulfuric acid is used by the liver to neutralize certain substances.

For the normal functioning of the body, in addition to the listed substances, magnesium, zinc, etc. are important. Some of them (aluminum, cobalt, manganese, etc.) are part of the body in such small quantities that they are called microelements. A varied diet usually fully provides the body with all the minerals.

Muscle contraction is a complex process consisting of a number of stages. The main constituents here are myosin, actin, troponin, tropomyosin and actomyosin, as well as calcium ions and compounds that provide energy to the muscles. Consider the types and mechanisms muscle contraction. We will study what stages they consist of and what is necessary for a cyclic process.

muscles

Muscles are combined into groups that have the same mechanism of muscle contraction. On the same basis, they are divided into 3 types:

striated muscles of the body;
striated muscles of the atria and cardiac ventricles;
smooth muscles of organs, vessels and skin.

The striated muscles are part of the musculoskeletal system, being part of it, since in addition to them, it includes tendons, ligaments, and bones. When the mechanism of muscle contractions is implemented, the following tasks and functions are performed:

the body is moving;
body parts move relative to each other;
the body is supported in space;
heat is generated;
the cortex is activated by afferentation from receptive muscle fields.

From smooth muscles consists of:

locomotor apparatus internal organs, which includes the lungs and digestive tube;
lymphatic and circulatory systems;
urinary system.

Physiological properties

As with all vertebrates, there are three most important properties of skeletal muscle fibers in the human body:

contractility - contraction and change in voltage during excitation;
conductivity - the movement of potential throughout the fiber;
excitability - response to an irritant by changing the membrane potential and ion permeability.

Muscles are excited and begin to contract from those coming from the centers. But under artificial conditions, it can then be irritated directly (direct irritation) or through the nerve innervating the muscle (indirect irritation).

Types of abbreviations

The mechanism of muscle contraction involves the conversion of chemical energy into mechanical work. This process can be measured in an experiment with a frog: it calf muscle loaded with a small weight, and then irritated with light electrical impulses. A contraction in which the muscle becomes shorter is called isotonic. At isometric contraction no shortening occurs. Tendons do not allow shortening during development. Another auxotonic mechanism of muscle contractions involves conditions of intense loads, when the muscle is shortened in a minimal way, and the strength is developed to the maximum.

Structure and innervation of skeletal muscles

The striated skeletal muscles include many fibers located in the connective tissue and attached to the tendons. In some muscles, the fibers are located parallel to the long axis, while in others they have an oblique appearance, attaching to the central tendon cord and to the pinnate type.

The main feature of the fiber is the sarcoplasm of a mass of thin filaments - myofibrils. They include light and dark areas, alternating with each other, and in neighboring striated fibers are at the same level - on cross section. This results in transverse striping throughout the muscle fiber.

The sarcomere is a complex of dark and two light discs and is delimited by Z-shaped lines. Sarcomeres are the contractile apparatus of the muscle. It turns out that the contractile muscle fiber consists of:

contractile apparatus (system of myofibrils);
trophic apparatus with mitochondria, Golgi complex and weak;
membrane apparatus;
support apparatus;
nervous apparatus.

Muscle fiber is divided into 5 parts with its structures and functions and is an integral part of muscle tissue.

innervation

This process in striated muscle fibers is realized through nerve fibers, namely the axons of the motor neurons of the spinal cord and brain stem. One motor neuron innervates several muscle fibers. The complex with a motor neuron and innervated muscle fibers is called neuromotor (NME), or (DE). The average number of fibers innervated by one motor neuron characterizes the value of the MU of the muscle, and the reciprocal value is called the density of innervation. The latter is large in those muscles where the movements are small and "thin" (eyes, fingers, tongue). On the contrary, its small value will be in muscles with “rough” movements (for example, the trunk).

Innervation can be single and multiple. In the first case, it is realized by compact motor endings. This is usually characteristic of large motor neurons. (called in this case physical, or fast) generate AP (action potentials) that apply to them.

Multiple innervation occurs, for example, in external eye muscles. No action potential is generated here, since there are no electrically excitable sodium channels in the membrane. In them, depolarization spreads throughout the fiber from synaptic endings. This is necessary in order to activate the mechanism of muscle contraction. The process here is not as fast as in the first case. That is why it is called slow.

Structure of myofibrils

Muscle fiber research today is carried out on the basis of X-ray diffraction analysis, electron microscopy, as well as histochemical methods.

It is calculated that each myofibril, whose diameter is 1 μm, includes approximately 2500 protofibrils, that is, elongated polymerized protein molecules (actin and myosin). Actin protofibrils are twice thinner than myosin ones. At rest, these muscles are located in such a way that actin filaments penetrate with their tips into the gaps between myosin protofibrils.

A narrow light band in disc A is free of actin filaments. And the Z membrane holds them together.

Myosin filaments have transverse protrusions up to 20 nm long, in the heads of which there are about 150 myosin molecules. They depart bipolar, and each head connects the myosin to the actin filament. When there is a force of actin centers on myosin filaments, the actin filament approaches the center of the sarcomere. At the end, myosin filaments reach the Z line. Then they occupy the entire sarcomere, and actin filaments are located between them. In this case, the length of the I disk is reduced, and at the end it disappears completely, along with which the Z line becomes thicker.

So, according to the theory of sliding threads, the reduction in the length of the muscle fiber is explained. The "cog wheel" theory was developed by Huxley and Hanson in the mid-twentieth century.

Mechanism of muscle fiber contraction

The main thing in the theory is that it is not the filaments (myosin and actin) that shorten. Their length remains unchanged even when the muscles are stretched. But bundles of thin threads, slipping, come out between thick threads, the degree of their overlap decreases, thus reducing.

The molecular mechanism of muscle contraction through the sliding of actin filaments is as follows. Myosin heads connect the protofibril to the actin fibril. When they tilt, sliding occurs, moving the actin filament to the center of the sarcomere. Due to the bipolar organization of myosin molecules on both sides of the filaments, conditions are created for the sliding of actin filaments in different sides.

When the muscles relax, the myosin head moves away from the actin filaments. Thanks to easy sliding, relaxed muscles resist stretching much less. Therefore, they are passively elongated.

Stages of reduction

The mechanism of muscle contraction can be briefly divided into the following stages:

A muscle fiber is stimulated when an action potential arrives from motor neurons at the synapses.
An action potential is generated at the muscle fiber membrane and then propagated to the myofibrils.
An electromechanical pairing is performed, which is a transformation of the electrical PD into mechanical sliding. This necessarily involves calcium ions.

Calcium ions

For a better understanding of the process of fiber activation by calcium ions, it is convenient to consider the structure of an actin filament. Its length is about 1 μm, thickness - from 5 to 7 nm. It is a pair of twisted filaments that resemble an actin monomer. Approximately every 40 nm there are spherical troponin molecules, and between the chains - tropomyosin.

When calcium ions are absent, that is, myofibrils relax, long tropomyosin molecules block the attachment of actin chains and myosin bridges. But when calcium ions are activated, tropomyosin molecules sink deeper, and the areas open up.

Then myosin bridges attach to actin filaments, and ATP is split, and muscle strength develops. This is made possible by the action of calcium on troponin. In this case, the molecule of the latter is deformed, thereby pushing through the tropomyosin.

When the muscle is relaxed, it contains more than 1 µmol of calcium per 1 gram of fresh weight. Calcium salts are isolated and kept in special storages. Otherwise, the muscles would contract all the time.

The storage of calcium occurs as follows. On different parts of the muscle cell membrane inside the fiber there are tubes through which the connection with the environment outside the cells takes place. This is a system of transverse tubes. And perpendicular to it is a system of longitudinal ones, at the ends of which there are bubbles (terminal tanks) located in close proximity to the membranes of the transverse system. Together they form a triad. It is in the vesicles that calcium is stored.

Thus, AP propagates inside the cell, and electromechanical coupling occurs. Excitation penetrates the fiber, passes into the longitudinal system, releases calcium. Thus, the mechanism of contraction of the muscle fiber is carried out.

3 processes with ATP

In the interaction of both threads in the presence of calcium ions, ATP plays a significant role. When the mechanism of muscle contraction of the skeletal muscle is realized, the energy of ATP is used to:

operation of the sodium and potassium pump, which maintains a constant concentration of ions;
these substances on opposite sides of the membrane;
sliding threads that shorten myofibrils;
work of the calcium pump, acting for relaxation.

ATP is found in the cell membrane, myosin filaments, and the membranes of the sarcoplasmic reticulum. The enzyme is cleaved and utilized by myosin.

ATP consumption

It is known that myosin heads interact with actin and contain elements for splitting ATP. The latter is activated by actin and myosin in the presence of magnesium ions. Therefore, the cleavage of the enzyme occurs when the myosin head attaches to actin. In this case, the more cross-bridges, the higher the rate of splitting will be.

ATP mechanism

After the movement is completed, the AFT molecule provides energy for the separation of myosin and actin involved in the reaction. Myosin heads separate, ATP is broken down to phosphate and ADP. At the end, a new ATP molecule is attached, and the cycle resumes. This is the mechanism of muscle contraction and relaxation at the molecular level.

Cross-bridge activity will continue only as long as ATP hydrolysis occurs. If the enzyme is blocked, the bridges will not reattach.

With the onset of the death of the organism, the level of ATP in the cells falls, and the bridges remain stably attached to the actin filament. This is the stage of rigor mortis.

ATP resynthesis

Resynthesis can be implemented in two ways.

Through enzymatic transfer from the phosphate group of creatine phosphate to ADP. Since the reserves in the cell of creatine phosphate are much larger than ATP, resynthesis is realized very quickly. At the same time, through the oxidation of pyruvic and lactic acids, resynthesis will be carried out slowly.

ATP and CF can disappear completely if resynthesis is disturbed by poisons. Then the calcium pump will stop working, as a result of which the muscle will irreversibly contract (that is, contracture will occur). Thus, the mechanism of muscle contraction will be disrupted.

Physiology of the process

Summarizing the above, we note that the contraction of the muscle fiber consists in the shortening of the myofibrils in each of the sarcomeres. The filaments of myosin (thick) and actin (thin) are connected at their ends in a relaxed state. But they begin sliding movements towards each other when the mechanism of muscle contraction is realized. Physiology (briefly) explains the process when, under the influence of myosin, the necessary energy is released to convert ATP to ADP. In this case, the activity of myosin will be realized only with a sufficient content of calcium ions accumulating in the sarcoplasmic reticulum.

Muscle contraction is a vital function of the body associated with defensive, respiratory, nutritional, sexual, excretory and other physiological processes. All kinds of voluntary movements - walking, facial expressions, movements of the eyeballs, swallowing, breathing, etc. are carried out by skeletal muscles. Involuntary movements (except for the contraction of the heart) - peristalsis of the stomach and intestines, changes in the tone of blood vessels, maintaining the tone of the bladder - are caused by contraction of smooth muscles. The work of the heart is provided by the contraction of the cardiac muscles.

Structural organization of skeletal muscle

Muscle fiber and myofibril (Fig. 1). Skeletal muscle consists of many muscle fibers that have points of attachment to the bones and are parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from longitudinally repeating blocks (sarcomeres). The sarcomere is the functional unit of the contractile apparatus of the skeletal muscle. Myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a pattern of transverse striation.

Sarcomere and filaments. Sarcomeres in the myofibril are separated from each other by Z-plates, which contain the protein beta-actinin. In both directions, thin actin filaments. Between them are thicker myosin filaments.

The actin filament looks like two strands of beads twisted into a double helix, where each bead is a protein molecule. actin. In the recesses of actin helices, protein molecules lie at equal distances from each other. troponin attached to filamentous protein molecules tropomyosin.

Myosin filaments are made up of repeating protein molecules. myosin. Each myosin molecule has a head and tail. The myosin head can bind to the actin molecule, forming the so-called cross bridge.

The cell membrane of the muscle fiber forms invaginations ( transverse tubules), which perform the function of conducting excitation to the membrane of the sarcoplasmic reticulum. Sarcoplasmic reticulum (longitudinal tubules) is an intracellular network of closed tubules and performs the function of depositing Ca ++ ions.

motor unit. The functional unit of skeletal muscle is motor unit(DE). DE - a set of muscle fibers that are innervated by the processes of one motor neuron. Excitation and contraction of the fibers that make up one MU occur simultaneously (when the corresponding motor neuron is excited). Individual MUs can fire and contract independently of each other.

Molecular mechanisms of contractionskeletal muscle

According to thread slip theory, muscle contraction occurs due to the sliding movement of actin and myosin filaments relative to each other. The thread sliding mechanism includes several successive events.

Myosin heads attach to actin filament binding sites (Fig. 2, A).

The interaction of myosin with actin leads to conformational rearrangements of the myosin molecule. The heads acquire ATPase activity and rotate 120°. Due to the rotation of the heads, actin and myosin filaments move "one step" relative to each other (Fig. 2b).

The dissociation of actin and myosin and the restoration of the conformation of the head occurs as a result of the attachment of an ATP molecule to the myosin head and its hydrolysis in the presence of Ca++ (Fig. 2, C).

The cycle "binding - change in conformation - disconnection - restoration of conformation" occurs many times, as a result of which actin and myosin filaments are displaced relative to each other, Z-discs of sarcomeres approach each other and the myofibril shortens (Fig. 2, D).

Conjugation of excitation and contractionin skeletal muscle

At rest, filament sliding does not occur in the myofibril, since the binding centers on the actin surface are closed by tropomyosin protein molecules (Fig. 3, A, B). Excitation (depolarization) of myofibrils and proper muscle contraction are associated with the process of electromechanical coupling, which includes a number of successive events.

As a result of neuromuscular synapse firing on the postsynaptic membrane, an EPSP occurs, which generates the development of an action potential in the area surrounding the postsynaptic membrane.

Excitation (action potential) spreads along the myofibril membrane and reaches the sarcoplasmic reticulum due to the system of transverse tubules. Depolarization of the sarcoplasmic reticulum membrane leads to the opening of Ca++ channels in it, through which Ca++ ions enter the sarcoplasm (Fig. 3, C).

Ca++ ions bind to the troponin protein. Troponin changes its conformation and displaces tropomyosin protein molecules that closed the actin binding centers (Fig. 3d).

Myosin heads join the opened binding centers, and the process of contraction begins (Fig. 3, E).

For the development of these processes, a certain period of time (10–20 ms) is required. The time from the moment of excitation of the muscle fiber (muscle) to the beginning of its contraction is called latent period of contraction.

Relaxation of the skeletal muscle

Muscle relaxation is caused by the reverse transfer of Ca++ ions through the calcium pump into the channels of the sarcoplasmic reticulum. As Ca++ is removed from the cytoplasm open centers there is less and less binding, and eventually the actin and myosin filaments are completely disconnected; muscle relaxation occurs.

Contracture called persistent prolonged contraction of the muscle, which persists after the cessation of the stimulus. Short-term contracture may develop after a tetanic contraction as a result of the accumulation of a large amount of Ca ++ in the sarcoplasm; long-term (sometimes irreversible) contracture can occur as a result of poisoning, metabolic disorders.

Phases and modes of skeletal muscle contraction

Phases of muscle contraction

When stimulating the skeletal muscle with a single impulse electric current above threshold force, a single muscle contraction occurs, in which 3 phases are distinguished (Fig. 4, A):

latent (hidden) period of contraction (about 10 ms), during which the action potential develops and the processes of electromechanical coupling take place; muscle excitability during a single contraction changes in accordance with the phases of the action potential;

shortening phase (about 50 ms);

relaxation phase (about 50 ms).

Rice. 4. Characteristics of a single muscle contraction. Origin of dentate and smooth tetanus.

B- phases and periods of muscular contraction,
B- modes of muscle contraction that occur at different frequencies of muscle stimulation.

Change in muscle length shown in blue action potential in muscle- red, muscle excitability- purple.

Modes of muscle contraction

Under natural conditions, a single muscle contraction is not observed in the body, since a series of action potentials go along the motor nerves that innervate the muscle. Depending on the frequency of nerve impulses coming to the muscle, the muscle can contract in one of three modes (Fig. 4b).

Single muscle contractions occur at a low frequency electrical impulses. If the next impulse comes to the muscle after the completion of the relaxation phase, a series of successive single contractions occurs.

At a higher frequency of impulses, the next impulse may coincide with the relaxation phase of the previous contraction cycle. The amplitude of contractions will be summed up, there will be dentate tetanus- prolonged contraction, interrupted by periods of incomplete relaxation of the muscle.

With a further increase in the frequency of impulses, each subsequent impulse will act on the muscle during the shortening phase, resulting in smooth tetanus- prolonged contraction, not interrupted by periods of relaxation.

Frequency Optimum and Pessimum

The amplitude of tetanic contraction depends on the frequency of impulses irritating the muscle. Optimum frequency they call such a frequency of irritating impulses at which each subsequent impulse coincides with the phase of increased excitability (Fig. 4, A) and, accordingly, causes tetanus of the greatest amplitude. Pessimum frequency called a higher frequency of stimulation, at which each subsequent current pulse enters the refractoriness phase (Fig. 4, A), as a result of which the tetanus amplitude decreases significantly.

Skeletal muscle work

The strength of skeletal muscle contraction is determined by 2 factors:

the number of MUs participating in the reduction;

the frequency of contraction of muscle fibers.

The work of the skeletal muscle is accomplished by a coordinated change in tone (tension) and length of the muscle during contraction.

Types of work of the skeletal muscle:

dynamic overcoming work occurs when the muscle, contracting, moves the body or its parts in space;

static (holding) work performed if, due to muscle contraction, parts of the body are maintained in a certain position;

dynamic inferior work occurs when the muscle is functioning but is being stretched because the effort it makes is not enough to move or hold the body parts.

During the performance of work, the muscle can contract:

isotonic- the muscle shortens under constant tension (external load); isotonic contraction is reproduced only in the experiment;

isometric- muscle tension increases, but its length does not change; the muscle contracts isometrically when performing static work;

auxotonically- muscle tension changes as it shortens; auxotonic contraction is performed during dynamic overcoming work.

Average load rule- the muscle can perform maximum work with moderate loads.

Fatigue – physiological state muscle, which develops after a long work and is manifested by a decrease in the amplitude of contractions, lengthening of the latent period of contraction and relaxation phase. The causes of fatigue are: depletion of ATP, accumulation of metabolic products in the muscle. Muscle fatigue during rhythmic work is less than synapse fatigue. Therefore, when the body performs muscular work, fatigue initially develops at the level of CNS synapses and neuromuscular synapses.

Structural organization and reductionsmooth muscles

Structural organization. Smooth muscle is composed of single spindle-shaped cells ( myocytes), which are located in the muscle more or less randomly. The contractile filaments are arranged irregularly, as a result of which there is no transverse striation of the muscle.

The mechanism of contraction is similar to that in skeletal muscle, but the rate of filament sliding and the rate of ATP hydrolysis are 100–1000 times lower than in skeletal muscle.

The mechanism of conjugation of excitation and contraction. When a cell is excited, Ca++ enters the cytoplasm of the myocyte not only from the sarcoplasmic reticulum, but also from the intercellular space. Ca++ ions, with the participation of the calmodulin protein, activate an enzyme (myosin kinase), which transfers the phosphate group from ATP to myosin. Phosphorylated myosin heads acquire the ability to attach to actin filaments.

Contraction and relaxation of smooth muscles. The rate of removal of Ca ++ ions from the sarcoplasm is much less than in the skeletal muscle, as a result of which relaxation occurs very slowly. Smooth muscles make long tonic contractions and slow rhythmic movements. Due to the low intensity of ATP hydrolysis, smooth muscles are optimally adapted for long-term contraction, which does not lead to fatigue and high energy consumption.

Physiological properties of muscles

The common physiological properties of skeletal and smooth muscles are excitability And contractility. Comparative characteristics of skeletal and smooth muscles are given in Table. 6.1. Physiological properties and features of the cardiac muscles are discussed in the section "Physiological mechanisms of homeostasis".

Table 7.1.Comparative characteristics of skeletal and smooth muscles

Property	Skeletal muscles	Smooth muscles
Depolarization rate		slow
Refractory period	short	long
The nature of the reduction	fast phasic	slow tonic
Energy costs
Plastic
Automation
Conductivity
innervation	motoneurons of the somatic NS	postganglionic neurons of the autonomic NS
Movements carried out	arbitrary	involuntary
Sensitivity to chemicals
Ability to divide and differentiate

Plastic smooth muscles is manifested in the fact that they can maintain a constant tone both in a shortened and in a stretched state.

Conductivity smooth muscle tissue is manifested in the fact that excitation spreads from one myocyte to another through specialized electrically conductive contacts (nexuses).

Property automation smooth muscle is manifested in the fact that it can contract without the participation nervous system, due to the fact that some myocytes are able to spontaneously generate rhythmically repeating action potentials.

Mobility is a characteristic property of all life forms. Directed movement occurs when chromosomes separate 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 common molecular mechanisms.

There are several types of muscle tissue in humans. 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 lecture, we will consider the structure and processes of contraction and relaxation of skeletal muscles, since they are of the greatest interest for the biochemistry of sports.

Mechanism muscle contraction has not been fully disclosed to date.

The following is well known.

1. ATP molecules are the source of energy for muscle contraction.

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 the thin and thick filaments of myofibrils.

5. During muscle contraction, thin threads slide along thick ones, 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 reasonable is the so-called hypothesis (theory) of "sliding threads" or "rowing hypothesis".

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

Under the influence of a nerve impulse, calcium ions leave the cisterns of the sarcoplasmic reticulum and attach to the protein of thin filaments - troponin. This protein changes its configuration and changes the configuration of actin. As a result, a transverse bridge is formed between the actin of thin filaments and myosin of thick filaments. This increases the ATPase activity of myosin. Myosin breaks down ATP and, due to the energy released in this case, the myosin head rotates like a hinge or a boat oar, 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 sharply decreases, and ATP hydrolysis stops. However, with further arrival of the nerve impulse, the transverse bridges are again formed, since the process described above is repeated again.

In each contraction cycle, 1 molecule of ATP is consumed.

Muscle contraction is based on two processes:

helical twisting of contractile proteins;

cyclically repeating formation and dissociation of the 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, which leads to the 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. The transverse tubules of the sarcoplasmic reticulum are connected to the sarcolemma, along which the excitation wave propagates. From the tubules, the excitation wave is transmitted to the membranes of the vesicles and cisterns, which braid the myofibrils in the areas where the interaction of actin and myosin filaments occurs. When a signal is transmitted to the cisterns of the sarcoplasmic reticulum, the latter begin to release the Ca 2+ located in them. The released Ca 2+ binds to Tn-C, which causes conformational shifts that are transmitted to tropomyosin and then to actin. Actin, as it were, is 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 thin filaments to move along thick ones.

Force generation (shortening) is due to the nature of the interaction between myosin and actin. The myosin rod has a movable hinge, in the region of which the rotation occurs when the globular head of myosin is bound to a certain area of actin. It is these rotations, occurring simultaneously in numerous sites of interaction between myosin and actin, that are the reason for the retraction of actin filaments (thin filaments) into the H-zone. Here they contact (at maximum shortening) or even overlap with each other, as shown in the figure.

Drawing. Reduction mechanism: A- a state of rest; b– moderate contraction; V- maximum contraction

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 located, no connection is formed between the thin and thick filaments. The calcium cation that appears neutralizes the negative charge of ATP, promoting convergence with the active center of myosin ATPase. As a result, phosphorylation of myosin occurs, i.e., myosin is charged with energy, which is used to form adhesions with actin and to move a thin filament. After the thin thread advances one "step", ADP and phosphoric acid are cleaved from the actomyosin complex. Then a new ATP molecule is attached to the myosin head, and the whole process is repeated with the next head of the myosin molecule.

The consumption of ATP is also necessary for muscle relaxation. After the termination of the action of the motor impulse, Ca 2+ passes into the cisterns of the sarcoplasmic reticulum. Th-C loses its associated calcium, resulting in conformational shifts in the troponin-tropomyosin complex, and Th-I again closes actin active sites, making them unable to interact with myosin. The concentration of Ca 2+ 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, the Z lines move away from each other by the same distance. The muscle becomes thinner and longer.

Hydrolysis rate ATP during muscular work is huge: up to 10 micromoles per 1 g of muscle in 1 min. General stocks ATP are small, therefore, to ensure the normal functioning of the muscles ATP should be restored at the same rate as it is consumed.

Muscle relaxation occurs after the cessation of the receipt of a long nerve impulse. At the same time, the permeability of the wall of the cisterns of the sarcoplasmic reticulum decreases, and calcium ions, under the action of the calcium pump, using the energy of ATP, go into the cisterns. The removal of calcium ions into the reticulum cisterns after the cessation of the motor impulse requires significant energy expenditure. Since the removal of calcium ions occurs in the direction of a higher concentration, i.e. against the osmotic gradient, then two ATP molecules are spent to remove each calcium ion. The concentration of calcium ions in the sarcoplasm rapidly decreases to the initial level. Proteins reacquire the conformation characteristic of the resting state.