Types of muscle contractions. Let's swing correctly. Isometric contraction is tension of a muscle without its movement. Isometric contraction of a muscle is accompanied by a change

Muscle contraction is a vital function of the body associated with defensive, respiratory, nutritional, sexual, excretory and other physiological processes. All types of voluntary movements - walking, facial expressions, movements of the eyeballs, swallowing, breathing, etc. are carried out by skeletal muscles. Involuntary movements (except for heart contraction) - peristalsis of the stomach and intestines, changes in the tone of blood vessels, maintenance of bladder tone - are caused by contraction of smooth muscles. The work of the heart is ensured 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 bones and are located parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from blocks (sarcomeres) repeating in the longitudinal direction. The sarcomere is the functional unit of the contractile apparatus of skeletal muscle. The myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a pattern of cross striations.

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. In the spaces between them there are thicker myosin filaments.

Actin filament externally resembles two strings of beads twisted into a double helix, where each bead is a protein molecule actin. Protein molecules lie in the recesses of actin helices at equal distances from each other. troponin, connected to thread-like protein molecules tropomyosin.

Myosin filaments are formed by repeating protein molecules myosin. Each myosin molecule has a head and tail. The myosin head can bind to an actin molecule, forming a so-called cross bridge.

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

Motor unit. The functional unit of skeletal muscle is motor unit (MU). MU is 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 motor unit occur simultaneously (when the corresponding motor neuron is excited). Individual motor units can be excited and contracted independently of each other.

Molecular mechanisms of contractionskeletal muscle

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

Myosin heads attach to actin filament binding centers (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, the actin and myosin filaments move “one step” relative to each other (Fig. 2, B).

Disconnection of actin and myosin and restoration of the head conformation 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, B).

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, the Z-disks of sarcomeres come closer and the myofibril is shortened (Fig. 2, D).

Pairing of excitation and contractionin skeletal muscle

In the resting state, thread sliding in the myofibril does not occur, since the binding centers on the actin surface are closed by tropomyosin protein molecules (Fig. 3, A, B). Excitation (depolarization) of the myofibril and muscle contraction itself are associated with the process of electromechanical coupling, which includes a series of sequential events.

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

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

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

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

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

Skeletal muscle relaxation

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 binding becomes less and less and eventually the actin and myosin filaments are completely disconnected; muscle relaxation occurs.

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

Phases and modes of skeletal muscle contraction

Phases of muscle contraction

When skeletal muscle is irritated by a single pulse of electric current of suprathreshold strength, 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 electromechanical coupling processes occur; 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 serrated and smooth tetanus. B– phases and periods of muscle contraction, B– modes of muscle contraction that occur at different frequencies of muscle stimulation. Change in muscle length shown in blue, muscle action potential- red, muscle excitability- purple.

Modes of muscle contraction

IN natural conditions In the body, a single muscle contraction is not observed, since a series of action potentials occur along the motor nerves innervating the muscle. Depending on the frequency of nerve impulses coming to the muscle, the muscle can contract in one of three modes (Fig. 4, B).

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

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

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

Optimum and pessimum frequency

The amplitude of tetanic contraction depends on the frequency of impulses irritating the muscle. Optimum frequency they call the 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 falls into the refractory phase (Fig. 4, A), as a result of which the amplitude of the tetanus decreases significantly.

Skeletal muscle work

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

- the number of units involved in the reduction;

frequency of contraction of muscle fibers.

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

Types of skeletal muscle work:

• dynamic overcoming work occurs when a 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 yielding operation occurs when a muscle functions but is stretched because the force it makes is not enough to move or hold parts of the body.

During work, the muscle can contract:

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

• isometrics– muscle tension increases, but its length does not change; the muscle contracts isometrically when performing static work;

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

Rule of average loads– the muscle can perform maximum work under moderate loads.

Fatigue– a physiological state of a muscle that develops after prolonged work and is manifested by a decrease in the amplitude of contractions, an extension of the latent period of contraction and the relaxation phase. The causes of fatigue are: depletion of ATP reserves, 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 the synapses of the central nervous system and neuromuscular synapses.

Structural organization and reductionsmooth muscles

Structural organization. Smooth muscle consists of single spindle-shaped cells ( myocytes), which are located in the muscle more or less chaotically. 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 of 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 coupling of excitation and contraction. When the 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 the 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 skeletal muscle, as a result of which relaxation occurs very slowly. Smooth muscles perform prolonged tonic contractions and slow rhythmic movements. Due to the low intensity of ATP hydrolysis smooth muscle optimally adapted for long-term contraction, which does not lead to fatigue and high energy consumption.

Physiological properties of muscles

The general physiological properties of skeletal and smooth muscles are excitability And contractility. Comparative characteristics of skeletal and smooth muscles are given in table. 6.1. The physiological properties and characteristics of the cardiac muscle are discussed in the section “Physiological mechanisms of homeostasis”.

Table 7.1.Comparative characteristics of skeletal and smooth muscles

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

Conductivity smooth muscle tissue manifests itself 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 participation nervous system, due to the fact that some myocytes are capable of spontaneously generating rhythmically repeating action potentials.

Isometric muscle contraction

contraction of a muscle, expressed in increasing its tension while maintaining a constant length (for example, contraction of a muscle of a limb, both ends of which are fixed motionless). In the body to I. m.s. the tension developed by the muscle when trying to lift an overwhelming load is approaching. Wed. Isotonic muscle contraction.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

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Plan

General information

General information

General information

General information

Types of muscle contraction

Isotonic contraction

Isotonic contraction

Isometric contraction

Isometric contraction

Auxotonic contraction

Auxotonic contraction

Auxotonic contraction

Isokinetic contraction

Isokinetic contraction

Types of muscle contractions

Types of muscle contractions. Basic Concepts

Single cut

Single cut

Single cut

Single cut

Single cut

Tetanic contraction

Tetanic contraction

Tetanic contraction

Tetanic contraction

Serrated tetanus

Smooth tetanus

Tetanic contraction

Tetanic contraction

Tetanic contraction

Which differ in cellular and tissue organization, innervation and, to a certain extent, mechanisms of functioning. At the same time, there are many similarities in the molecular mechanisms of muscle contraction between these types of muscles.

Skeletal muscles

Skeletal muscles are the active part of the musculoskeletal system. As a result of the contractile activity of striated muscles, the following occurs:

• movement of the body in space;
• movement of body parts relative to each other;
• maintaining a pose.

In addition, one of the results of muscle contraction is the production of heat.

In humans, as in all vertebrates, skeletal muscle fibers have four important properties:

• excitability- the ability to respond to a stimulus by changes in ionic permeability and membrane potential;
• conductivity - the ability to conduct an action potential along the entire fiber;
• contractility- the ability to contract or change tension when excited;
• elasticity - the ability to develop tensile tension.

Under natural conditions, muscle excitation and contraction are caused by nerve impulses entering the muscle fibers from the nerve centers. To cause excitation in an experiment, electrical stimulation is used.

Direct stimulation of the muscle itself is called direct stimulation; irritation of a motor nerve leading to contraction of a muscle innervated by this nerve (excitation of neuromotor units) is an indirect irritation. Due to the fact that the excitability of muscle tissue is lower than nervous tissue, the application of irritating current electrodes directly to the muscle does not yet provide direct irritation: the current, spreading through the muscle tissue, acts primarily on the endings of the motor nerves located in it and excites them, which leads to contraction muscles.

Types of abbreviation

Isotonic regime- a contraction in which the muscle shortens without creating tension. Such a reduction is possible when a tendon is cut or ruptured or in an experiment on an isolated (removed from the body) muscle.

Isometric mode- a contraction in which muscle tension increases, but the length practically does not decrease. This reduction is observed when trying to lift an overwhelming load.

Auxotonic mode - a contraction in which the length of a muscle changes as its tension increases. This mode of reductions is observed during a person’s labor activity. If the tension of a muscle increases as it shortens, then this contraction is called concentric, and in the case of an increase in muscle tension when lengthening it (for example, when slowly lowering a load) - eccentric contraction.

Types of muscle contractions

There are two types of muscle contractions: single and tetanic.

When a muscle is irritated by a single stimulus, a single muscle contraction occurs, in which the following three phases are distinguished:

• latent period phase - begins from the beginning of the stimulus until the beginning of shortening;
• contraction phase (shortening phase) - from the beginning of contraction to the maximum value;
• relaxation phase - from maximum contraction to initial length.

Single muscle contraction observed when a short series of nerve impulses from motor neurons arrives at the muscle. It can be induced by applying a very short (about 1 ms) electrical stimulus to the muscle. Muscle contraction begins within a time interval of up to 10 ms from the beginning of the stimulus, which is called the latent period (Fig. 1). Then shortening (duration about 30-50 ms) and relaxation (50-60 ms) develop. The entire cycle of a single muscle contraction takes an average of 0.1 s.

The duration of a single contraction in different muscles can vary greatly and depends on the functional state of the muscle. The rate of contraction and especially relaxation slows down as muscle fatigue develops. Fast muscles that have a short-term single contraction include the external muscles of the eyeball, eyelids, middle ear, etc.

When comparing the dynamics of the generation of an action potential on the muscle fiber membrane and its single contraction, it is clear that the action potential always occurs earlier and only then does shortening begin to develop, which continues after the end of membrane repolarization. Let us remember that the duration of the depolarization phase of the muscle fiber action potential is 3-5 ms. During this period of time, the fiber membrane is in a state of absolute refractoriness, followed by restoration of its excitability. Since the duration of shortening is about 50 ms, it is obvious that even during shortening, the muscle fiber membrane should restore excitability and will be able to respond to a new impact with a contraction against the background of an incomplete one. Consequently, against the background of developing contraction in muscle fibers, new cycles of excitation and subsequent cumulative contractions can be caused on their membrane. This cumulative reduction is called tetanic(tetanus). It can be observed in single fiber and whole muscle. However, the mechanism of tetanic contraction in natural conditions in a whole muscle has its own peculiarities.

Rice. 1. Temporal relationships between single cycles of excitation and contraction of skeletal muscle fibers: a - ratio of action potential, release of Ca 2+ into the sarcoplasm and contraction: 1 - latent period; 2 - shortening; 3 - relaxation; b - ratio of action potential, excitability and contraction

Tetanus called a muscle contraction that occurs as a result of the summation of contractions of its motor units caused by the receipt of many nerve impulses from motor neurons innervating them this muscle. The summation of forces developed during contraction of fibers of multiple motor units helps to increase the force of tetanic muscle contraction and affects the duration of contraction.

Distinguish serrated And smooth tetanus. To observe dentate tetanus in an experiment, the muscle is stimulated with electric current pulses at such a frequency that each subsequent stimulus is applied after the shortening phase, but before the end of relaxation. Smooth tetanic contraction develops with more frequent stimulation when subsequent stimuli are applied during the development of muscle shortening. For example, if the muscle shortening phase is 50 ms, the relaxation phase is 60 ms, then to obtain serrated tetanus it is necessary to irritate this muscle with a frequency of 9-19 Hz, to obtain smooth tetanus - with a frequency of at least 20 Hz.

For demonstration various types Tetanus usually uses graphical registration on a kymograph of contractions of the isolated gastrocnemius muscle of the frog. An example of such a kymogram is shown in Fig. 2.

If we compare the amplitudes and forces developed during different modes of muscle contraction, they are minimal with a single contraction, increase with serrated tetanus and become maximum with a smooth tetanic contraction. One of the reasons for this increase in the amplitude and force of contraction is that the increase in the frequency of AP generation on the muscle fiber membrane is accompanied by an increase in the output and accumulation of Ca 2+ ions in the sarcoplasm of muscle fibers, which contributes to greater efficiency of interaction between contractile proteins.

Rice. 2. Dependence of the contraction amplitude on the frequency of stimulation (the strength and duration of the stimuli are unchanged)

With a gradual increase in the frequency of stimulation, the strength and amplitude of muscle contraction increases only to a certain limit - the optimum response. The frequency of stimulation that causes the greatest muscle response is called optimal. A further increase in the frequency of stimulation is accompanied by a decrease in the amplitude and force of contraction. This phenomenon is called response pessimum, and stimulation frequencies exceeding the optimal value are called pessimal. The phenomena of optimum and pessimum were discovered by N.E. Vvedensky.

Under natural conditions, the frequency and mode of sending nerve impulses by motor neurons to the muscle ensure the asynchronous involvement in the contraction process of a larger or smaller (depending on the number of active motor neurons) number of motor units of the muscle and the summation of their contractions. The contraction of an integral muscle in the body is close to smooth-teganic in nature.

To characterize the functional activity of muscles, their tone and contraction are assessed. Muscle tone is a state of prolonged continuous tension caused by alternating asynchronous contraction of its motor units. In this case, visible shortening of the muscle may be absent due to the fact that not all motor units are involved in the contraction process, but only those motor units whose properties the best way adapted to maintaining muscle tone and the strength of their asynchronous contraction is not enough to shorten the muscle. Contractions of such units during the transition from relaxation to tension or when changing the degree of tension are called tonic. Short-term contractions accompanied by changes in muscle strength and length are called physical.

Mechanism of muscle contraction

A muscle fiber is a multinucleated structure surrounded by a membrane and containing a specialized contractile apparatus -myofibrils(Fig. 3). In addition, the most important components of muscle fiber are mitochondria, systems of longitudinal tubules - the sarcoplasmic reticulum and a system of transverse tubules - T-system.

Rice. 3. The structure of muscle fiber

Functional unit of the contractile apparatus muscle cell is sarcomere, The myofibril consists of sarcomeres. Sarcomeres are separated from each other by Z-plates (Fig. 4). Sarcomeres in the myofibril are arranged sequentially, so contractions of the capcomeres cause contraction of the myofibril and overall shortening of the muscle fiber.

Rice. 4. Scheme of the sarcomere structure

Studying the structure of muscle fibers in a light microscope made it possible to identify their transverse striations, which are due to their special organization contractile proteins protofibrils - actin And myosin. Actin filaments are represented by a double filament twisted into a double helix with a pitch of about 36.5 nm. These filaments are 1 µm long and 6-8 nm in diameter, the number of which reaches about 2000, and are attached at one end to the Z-plate. Filamentous protein molecules are located in the longitudinal grooves of the actin helix tropomyosin. With a step of 40 nm, a molecule of another protein is attached to the tropomyosin molecule - troponin.

Troponin and tropomyosin play (see Fig. 3) an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this area is visible as a strip of dark color (due to birefringence) - anisotropic A-disc. A lighter stripe is visible in its center H. At rest, there are no actin filaments. On both sides A- the disk is visible light isotropic stripes - I-discs formed by actin filaments.

At rest, the actin and myosin filaments overlap each other slightly so that the total length of the sarcomere is about 2.5 μm. With electron microscopy in the center H-stripes detected M-line - structure that holds myosin filaments.

Electron microscopy shows that on the sides of the myosin filament there are protrusions called cross bridges. According to modern concepts, the transverse bridge consists of a head and a neck. The head acquires pronounced ATPase activity upon binding to actin. The neck has elastic properties and is a hinged joint, so the head of the cross bridge can rotate around its axis.

The use of modern technology has made it possible to establish that applying electrical stimulation to an area Z-plate leads to reduction of the sarcomere, while the size of the disc zone A does not change, but the size of the stripes N And I decreases. These observations indicated that the length of myosin filaments does not change. Similar results were obtained when the muscle was stretched—the intrinsic length of actin and myosin filaments did not change. As a result of the experiments, it turned out that the area of ​​mutual overlap of actin and myosin filaments changed. These facts allowed X. and A. Huxley to propose the theory of thread sliding to explain the mechanism of muscle contraction. According to this theory, during contraction, the size of the sarcomere decreases due to the active movement of thin actin filaments relative to thick myosin filaments.

Rice. 5. A - diagram of the organization of the sarcoplasmic reticulum, transverse tubules and myofibrils. B - diagram of the anatomical structure of the transverse tubules and sarcoplasmic reticulum in an individual skeletal muscle fiber. B - the role of the sarcoplasmic reticulum in the mechanism of skeletal muscle contraction

During the process of muscle fiber contraction, the following transformations occur in it:

electrochemical conversion:

• PD generation;
• distribution of PD through the T-system;
• electrical stimulation of the contact zone of the T-system and the sarcoplasmic reticulum, activation of enzymes, formation of inositol triphosphate, increase in the intracellular concentration of Ca 2+ ions;

chemomechanical transformation:

• interaction of Ca 2+ ions with troponin, change in the configuration of tropomyosin, release of active centers on actin filaments;
• interaction of the myosin head with actin, rotation of the head and development of elastic traction;
• sliding of actin and myosin filaments relative to each other, reduction in sarcomere size, development of tension or shortening of the muscle fiber.

The transfer of excitation from the motor neuron to the muscle fiber occurs using the mediator acetylcholine (ACh). The interaction of ACh with the endplate cholinergic receptor leads to activation of ACh-sensitive channels and the appearance of an endplate potential, which can reach 60 mV. In this case, the area of ​​the end plate becomes a source of irritating current for the muscle fiber membrane and in areas of the cell membrane adjacent to the end plate, an PD occurs, which spreads in both directions at a speed of approximately 3-5 m/s at a temperature of 36 °C. Thus, the generation of PD is the first stage muscle contraction.

Second stage is the propagation of PD into the muscle fiber through the transverse system of tubules, which serves as a link between the surface membrane and the contractile apparatus of the muscle fiber. The G-system is in close contact with the terminal cisterns of the sarcoplasmic reticulum of two neighboring sarcomeres. Electrical stimulation of the contact site leads to the activation of enzymes located at the contact site and the formation of inositol triphosphate. Inositol triphosphate activates the calcium channels of the membranes of the terminal cisterns, which leads to the release of Ca 2+ ions from the cisterns and an increase in the intracellular concentration of Ca 2+ "from 10 -7 to 10 -5. The set of processes leading to an increase in the intracellular concentration of Ca 2+ is the essence third stage muscle contraction. Thus, at the first stages, the electrical signal of the AP is converted into a chemical one - an increase in the intracellular concentration of Ca 2+, i.e. electrochemical conversion(Fig. 6).

When the intracellular concentration of Ca 2+ ions increases, they bind to troponin, which changes the configuration of tropomyosin. The latter will mix into the groove between the actin filaments; in this case, areas on the actin filaments open with which myosin cross bridges can interact. This displacement of tropomyosin is due to a change in the formation of the troponin protein molecule upon binding of Ca 2+. Consequently, the participation of Ca 2+ ions in the mechanism of interaction between actin and myosin is mediated through troponin and tropomyosin. Thus, fourth stage electromechanical coupling is the interaction of calcium with troponin and the displacement of tropomyosin.

On fifth stage electromechanical coupling occurs when the head of the myosin cross bridge attaches to the actin bridge—to the first of several sequentially located stable centers. In this case, the myosin head rotates around its axis, since it has several active centers that sequentially interact with the corresponding centers on the actin filament. Rotation of the head leads to an increase in the elastic traction of the neck of the cross bridge and an increase in tension. At each specific moment during the development of contraction, one part of the heads of the cross bridges is in connection with the actin filament, the other is free, i.e. there is a sequence of their interaction with the actin filament. This ensures a smooth reduction process. At the fourth and fifth stages, a chemomechanical transformation occurs.

Rice. 6. Electromechanical processes in muscle

The sequential reaction of connection and separation of the heads of the cross bridges with the actin filament leads to the sliding of thin and thick filaments relative to each other and a decrease in the size of the sarcomere and the total length of the muscle, which is sixth stage. The totality of the described processes constitutes the essence of the theory of thread sliding (Fig. 7).

It was initially believed that Ca 2+ ions served as a cofactor for the ATPase activity of myosin. Further research refuted this assumption. In resting muscle, actin and myosin have virtually no ATPase activity. The attachment of the myosin head to actin causes the head to acquire ATPase activity.

Rice. 7. Illustration of the theory of sliding threads:

A. a - muscle at rest: A. 6 - muscle during contraction: B. a. b - sequential interaction of the active centers of the myosin head with centers on the active filament

Hydrolysis of ATP in the ATPase center of the myosin head is accompanied by a change in the conformation of the latter and its transfer to a new, high-energy state. Reattachment of the myosin head to a new center on the actin filament again leads to rotation of the head, which is provided by the energy stored in it. In each cycle of connection and separation of the myosin head with actin, one ATP molecule is cleaved per bridge. The speed of rotation is determined by the rate of ATP breakdown. It is clear that fast phasic fibers consume significantly more ATP per unit time and retain less chemical energy during tonic exercise than slow fibers. Thus, in the process of chemomechanical transformation, ATP provides the separation of the myosin head and the actin filament and provides energy for further interaction of the myosin head with another part of the actin filament. These reactions are possible at calcium concentrations above 10 -6 M.

The described mechanisms of muscle fiber shortening suggest that relaxation first requires a decrease in the concentration of Ca 2+ ions. It has been experimentally proven that the sarcoplasmic reticulum has a special mechanism - a calcium pump, which actively returns calcium to the tanks. The calcium pump is activated by inorganic phosphate, which is formed during the hydrolysis of ATP. and the energy supply for the calcium pump is also due to the energy generated during the hydrolysis of ATP. Thus, ATP is the second most important factor, absolutely necessary for the relaxation process. For some time after death, the muscles remain soft due to the cessation of the tonic influence of motor neurons. Then the ATP concentration decreases below a critical level and the possibility of separation of the myosin head from the actin filament disappears. The phenomenon of rigor mortis occurs with pronounced rigidity of skeletal muscles.

• Hydrolysis of ATP by myosin, as a result of which the cross bridges receive energy for the development of pulling force
• Binding of ATP to myosin, leading to the detachment of cross bridges attached to actin, which creates the possibility of repeating the cycle of their activity
• Hydrolysis of ATP (under the action of Ca 2+ -ATPase) for the active transport of Ca 2+ ions into the lateral cisterns of the sarcoplasmic reticulum, reducing the level of cytoplasmic calcium to the initial level

Summation of contractions and tetanus

If in an experiment two strong single stimulations act on a single muscle fiber or an entire muscle in rapid succession, the resulting contractions will have a greater amplitude than the maximum contraction during a single stimulation. The contractile effects caused by the first and second irritations seem to add up. This phenomenon is called summation of contractions (Fig. 8). It is observed with both direct and indirect muscle irritation.

For summation to occur, it is necessary that the interval between irritations have a certain duration: it must be longer than the refractory period, otherwise there will be no response to the second irritation, and shorter than the entire duration of the contractile response, so that the second irritation affects the muscle before it has time to relax after first irritation. In this case, two options are possible: if the second stimulation arrives when the muscle has already begun to relax, then on the myographic curve the apex of this contraction will be separated from the apex of the first by retraction (Figure 8, G-D); if the second stimulation acts when the first has not yet reached its peak, then the second contraction completely merges with the first, forming a single summed peak (Figure 8, A-B).

Consider the summation in calf muscle frogs. The duration of the ascending phase of its contraction is approximately 0.05 s. Therefore, to reproduce the first type of summation of contractions (incomplete summation) on this muscle, it is necessary that the interval between the first and second stimulation be more than 0.05 s, and to obtain the second type of summation (the so-called complete summation) - less than 0.05 s.

Rice. 8. Summation of muscle contractions 8 response to two stimuli. Timestamp 20ms

With both complete and incomplete summation of contractions, action potentials are not summed up.

Tetanus muscle

If an individual muscle fiber or the entire muscle is subject to rhythmic stimulation with such a frequency that their effects are summed up, a strong and prolonged contraction of the muscle occurs, called tetanic contraction, or tetanus.

Its amplitude can be several times greater than the maximum single contraction. With a relatively low frequency of irritation, it is observed serrated tetanus, at high frequency - smooth tetanus(Fig. 9). With tetanus, the contractile responses of the muscle are summed up, but its electrical reactions - action potentials - are not summed up (Fig. 10) and their frequency corresponds to the frequency of the rhythmic stimulation that caused tetanus.

After the cessation of tetanic irritation, the fibers completely relax, their original length is restored only after some time. This phenomenon is called post-tetanic, or residual, contracture.

The faster the muscle fibers contract and relax, the more frequent the stimulation must be to cause tetanus.

Muscle fatigue

Fatigue is a temporary decrease in the performance of a cell, organ or entire organism that occurs as a result of work and disappears after rest.

Rice. 9. Tetanus of isolated muscle fiber (according to F.N. Serkov):

a — serrated tetanus at a stimulation frequency of 18 Hz; 6 - smooth tetanus at a stimulation frequency of 35 Hz; M - myogram; P — irritation mark; B - time stamp 1 s

Rice. 10. Simultaneous recording of contraction (a) and electrical activity (6) of cat skeletal muscle during tetanic nerve stimulation

If you irritate an isolated muscle for a long time with rhythmic electrical stimuli, to which a small load is suspended, then the amplitude of its contractions gradually decreases to zero. The contraction record recorded in this case is called the fatigue curve.

A decrease in the performance of an isolated muscle during prolonged irritation is due to two main reasons:

• During contraction, metabolic products (phosphoric, lactic acid, etc.) accumulate in the muscle, which have a depressing effect on the performance of muscle fibers. Some of these products, as well as potassium ions, diffuse from the fibers out into the pericellular space and have a depressing effect on the ability of the excitable membrane to generate action potentials. If an isolated muscle placed in a small volume of Ringer's fluid is irritated for a long time and brought to the point of complete fatigue, then it is enough just to change the solution washing it to restore muscle contractions;
• gradual depletion of energy reserves in the muscle. With prolonged work of an isolated muscle, glycogen reserves sharply decrease, as a result of which the process is disrupted. ATP resynthesis and creatine phosphate, necessary for contraction.

THEM. Sechenov (1903) showed that the restoration of the performance of tired muscles of a person’s arm after prolonged work lifting a load is accelerated if the work is done with the other hand during the rest period. Temporary restoration of the working capacity of the muscles of a tired arm can be achieved with other types of motor activity, for example, when working muscles lower limbs. In contrast to simple rest, such rest was called by I.M. Sechenov active. He considered these facts as proof that fatigue develops primarily in the nerve centers.

The contraction is isometric, in which the length of the muscle fibers remains unchanged, but their tension increases.

Large medical dictionary. 2000 .

See what “isometric contraction” is in other dictionaries:

Contraction of a muscle, expressed in increasing its tension while maintaining a constant length (for example, contraction of a muscle of a limb, both ends of which are fixed motionless). In the body to I. m.s. the tension developed by the muscle when attempting... is approaching.

ISOMETRIC CONTRACTION (or CRAMP)- A contraction of a muscle that causes tension but no movement, as when pushing against a wall - there is no actual contraction of the muscle, and its length does not change... Dictionary in psychology

isometric muscle contraction- izometrinis raumens susitraukimas statusas T sritis Kūno kultūra ir sportas apibrėžtis Raumens susitraukimas, kurio metu raumens ilgis beveik nekinta, tik patrumpėja sutraukiančios raumenį skaidulėlės – miofibrilės, tiek pat i štempdamos… …Sporto terminų žodynas

Isometric contraction- (Greek isos - equal, identical, similar; metron - measure) contraction of a muscle with its tension, which, however, did not entail movement and shortening of the muscle. See: Cramps... Encyclopedic Dictionary of Psychology and Pedagogy

Isometric contraction- muscles (isometricus, from the Greek isos equal + metron, meron size, measure) - muscle contraction, when its length remains constant, does not change ...

Isometric contraction m.- (isos equal + metron measure, size) – muscle contraction, in which the length of the muscle fibers remains unchanged, but tension and tone increase... Glossary of terms on the physiology of farm animals

Contraction of a muscle under constant tension, expressed in a decrease in its length and an increase in cross-section. In the body I. m.s. is not observed in its pure form. To purely I. m.s. movement of the unloaded limb is approaching; at… … Great Soviet Encyclopedia

Shortening or tension of muscles in response to irritation caused by motor discharge. neurons. The M. s model has been adopted, according to which, when the surface of the muscle fiber membrane is excited, the action potential first spreads through the system... ... Biological encyclopedic dictionary

MUSCLE CONTRACTION- the main function of muscle tissue is the shortening or tension of muscles in response to irritation caused by the discharge of motor neurons. M. s. underlies all movements of the human body. There are M. s. isometric, when the muscle develops force... ... Psychomotorics: dictionary-reference book

MUSCLES- MUSCLES. I. Histology. Generally morphologically, the tissue of the contractile substance is characterized by the presence of differentiation of its specific elements in the protoplasm. fibrillar structure; the latter are spatially oriented in the direction of their reduction and... ... Great Medical Encyclopedia