Types of muscle contraction and their main characteristics. Topic: types and modes of muscle contractions. Relaxation of the skeletal muscle

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

Skeletal muscles

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

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

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 the stimulus with changes in ion permeability and membrane potential;
• conductivity - the ability to conduct an action potential along the entire fiber;
• contractility- the ability to contract or change voltage when excited;
• elasticity - the ability to develop tension when stretched.

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

Direct irritation of the muscle itself is called direct irritation; irritation of the motor nerve, leading to a contraction of the 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 that of nervous tissue, the application of irritating current electrodes directly to the muscle does not yet provide direct irritation: the current, propagating through the muscle tissue, acts primarily on the endings of the motor nerves located in it and excites them, which leads to contraction. muscles.

Abbreviation types

Isotonic regimen A contraction in which a muscle shortens without tension. Such a contraction is possible when crossing or rupturing the tendon or in an experiment on an isolated (removed from the body) muscle.

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

Auxotonic mode - contraction in which the length of a muscle changes as its tension increases. Such a mode of reductions is observed in the implementation of human labor activity. If the tension of the muscle increases with its shortening, then such a contraction is called concentric and in the case of an increase in muscle tension during its lengthening (for example, when slowly lowering the 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:

• phase of the latent period - starts from the beginning of the action of the stimulus and before the start of shortening;
• contraction phase (shortening phase) - from the beginning of the contraction to the maximum value;
• relaxation phase - from maximum contraction to initial length.

single muscle contraction observed when a short series of nerve impulses of motor neurons enters the muscle. It can be induced by applying a very short (about 1 ms) electrical stimulus to the muscle. Muscle contraction begins after a time interval of up to 10 ms from the onset of exposure to 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 with the development of muscle fatigue. TO 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 action potential generation on the muscle fiber membrane and its single contraction, it can be seen that the action potential always occurs earlier and only then shortening begins to develop, which continues after the end of membrane repolarization. Recall that the duration of the depolarization phase of the action potential of the muscle fiber is 3-5 ms. During this period of time, the fiber membrane is in a state of absolute refractoriness, followed by the restoration of its excitability. Since the duration of shortening is about 50 ms, it is obvious that even during shortening, the muscle fiber membrane must 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 a developing contraction in muscle fibers, new cycles of excitation can be induced on their membrane, followed by summing contractions. This cumulative contraction is called tetanic(tetanus). It can be observed in a single fiber and whole muscle. However, the mechanism of tetanic contraction in natural conditions in the whole muscle has some peculiarities.

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

Tetanus called muscle contraction resulting from the summation of contractions of its motor units caused by the supply of many nerve impulses to them from motor neurons that innervate this muscle. The summation of the efforts developed during the contraction of the fibers of many motor units contributes to an increase in the strength of the tetanic muscle contraction and affects the duration of the contraction.

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

For demonstration various kinds tetanus usually use graphic registration on a kymograph of contractions of an isolated frog gastrocnemius muscle. An example of such a kymogram is shown in Fig. 2.

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

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

With a gradual increase in the frequency of stimulation, the increase in the strength and amplitude of muscle contraction goes only up to a certain limit - the optimum of the response. The frequency of stimulation that causes the greatest response of the muscle is called optimal. A further increase in the frequency of stimulation is accompanied by a decrease in the amplitude and strength of contraction. This phenomenon is called the pessimum of the response, and the frequencies of irritation that exceed 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 provide asynchronous involvement in the process of contraction of a greater or lesser (depending on the number of active motor neurons) number of muscle motor units and the summation of their contractions. The contraction of an integral muscle in the body, but in its nature, is close to smooth-teganic.

To characterize the functional activity of the muscles, the indicators of their tone and contraction are evaluated. Muscle tone is a state of prolonged continuous tension caused by alternating asynchronous contraction of its motor units. At the same time, there may be no visible shortening of the muscle due to the fact that not all are involved in the contraction process, but only those motor units whose properties are best adapted to maintaining muscle tone and the strength of their asynchronous contraction is not enough to shorten the muscle. Reductions 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 a change in the strength and length of the muscle, are called physical.

The mechanism of muscle contraction

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

Rice. 3. The structure of the muscle fiber

The functional unit of the contractile apparatus of a muscle cell is sarcomere The myofibril is made up of sarcomeres. Sarcomeres are separated from each other by Z-plates (Fig. 4). The sarcomeres in the myofibril are arranged in series, therefore contractions of the capcomeres cause contraction of the myofibril and overall shortening of the muscle fiber.

Rice. 4. Scheme of the structure of the sarcomere

The study of the structure of muscle fibers in a light microscope made it possible to reveal their transverse striation, which is due to the special organization of the contractile proteins of protofibrils - actin And myosin. Actin filaments are represented by a double thread twisted into a double helix with a pitch of about 36.5 nm. These filaments, 1 μm long and 6–8 nm in diameter, numbering about 2000, are attached to the Z-plate at one end. 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) 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-disk. A lighter stripe is visible in the center of it. H. At rest, there are no actin filaments. On both sides A- disc visible light isotropic stripes - I-discs formed by actin filaments.

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

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

The use of modern technology has made it possible to establish that the application of electrical stimulation to the area Z-lamina leads to a contraction of the sarcomere, while the size of the disk zone A does not change, and the size of the stripes H 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 region of mutual overlap of actin and myosin filaments changed. These facts allowed X. and A. Huxley to propose a theory of sliding threads 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 - scheme of 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

In 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, the formation of inositol triphosphate, an increase in the intracellular concentration of Ca 2+ ions;

chemomechanical transformation:

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

The transfer of excitation from the motor neuron to the muscle fiber occurs with the help of the mediator acetylcholine (ACh). The interaction of ACh with the cholinergic receptor of the end plate leads to the activation of ACh-sensitive channels and the appearance of an end plate 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 the areas of the cell membrane adjacent to the end plate, AP occurs, which propagates 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 spread of AP inside the muscle fiber along 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 AP signal is converted into a chemical one, i.e., the intracellular concentration of Ca 2+ increases. electrochemical conversion(Fig. 6).

With an increase in the intracellular concentration of Ca 2+ ions, they bind to troponin, which changes the configuration of tropomyosin. The latter will mix into a groove between actin filaments; at the same time, sites are opened on actin filaments 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 Ca 2+ binding. Therefore, 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 conjugation, the head of the myosin transverse bridge is attached to the actin bridge - to the first of several successively 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. The rotation of the head leads to an increase in the elastic elastic traction of the neck of the transverse bridge and an increase in stress. At each specific moment in the process of contraction development, 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 the smoothness of the reduction process. At the fourth and fifth stages, chemomechanical transformation takes place.

Rice. 6. Electromechanical processes in the muscle

The successive reaction of connecting and disconnecting the heads of the cross bridges with the actin filament leads to 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 the sixth stage. The totality of the described processes is the essence of the theory of sliding threads (Fig. 7).

Initially, it was believed that Ca 2+ ions serve as a cofactor for the ATPase activity of myosin. Further research disproved this assumption. In a resting muscle, actin and myosin have practically no ATPase activity. 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 the 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. The reattachment of the myosin head to a new center on the actin filament again leads to the rotation of the head, which is provided by the energy stored in it. In each cycle of connection and disconnection of the myosin head with actin, one ATP molecule is split per bridge. The speed of rotation is determined by the rate of splitting of ATP. Obviously, fast phasic fibers consume significantly more ATP per unit time and store less chemical energy during tonic loading than slow fibers. Thus, in the process of chemomechanical transformation, ATP ensures the separation of the myosin head and the actin filament and provides energy for further interaction of the myosin head with another section of the actin filament. These reactions are possible at calcium concentrations above 10 -6 M.

The described mechanisms of muscle fiber shortening suggest that for relaxation, first of all, it is necessary to lower 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 cisterns. The activation of the calcium pump is carried out by inorganic phosphate, which is formed during the hydrolysis of ATP. and the energy supply of 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 ATP concentration decreases below a critical level and the possibility of separation of the myosin head from the actin filament disappears. There is a phenomenon of rigor mortis with severe rigidity of skeletal muscles.

The functional significance of ATP during skeletal muscle contraction

• ATP hydrolysis under the action of myosin, as a result, 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 active transport of Ca 2+ ions into the lateral cisterns of the sarcoplasmic reticulum, which reduces the level of cytoplasmic calcium to the initial level

Contraction summation and tetanus

If in an experiment an individual muscle fiber or the entire muscle is acted upon by two strong single stimuli rapidly following each other, then the resulting contractions will have a greater amplitude than the maximum contraction during a single stimulus. The contractile effects caused by the first and second stimuli seem to add up. This phenomenon is called the summation of contractions (Fig. 8). It is observed both with direct and indirect stimulation of the muscle.

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

Consider summation in the gastrocnemius muscle of a frog. The duration of the ascending phase of its contraction is approximately 0.05 s. Therefore, to reproduce the first type of summation of contractions on this muscle (incomplete summation), it is necessary that the interval between the first and second stimuli be greater than 0.05 s, and to obtain the second type of summation (the so-called full summation) - less than 0.05 s.

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

Both with full and incomplete summation of contractions, action potentials are not summed up.

Tetanus muscles

If rhythmic stimuli act on a single muscle fiber or on the entire muscle with such frequency that their effects are summed up, a strong and prolonged muscle contraction occurs, called tetanic contraction, or tetanus.

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

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

The faster the muscle fibers contract and relax, the more irritation must occur to cause tetanus.

Muscle fatigue

Fatigue is a temporary decrease in the efficiency of a cell, organ or the whole organism, which occurs as a result of work and disappears after rest.

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

a - dentate tetanus at a stimulation frequency of 18 Hz; 6 - smooth tetanus at an irritation frequency of 35 Hz; M - myogram; R - mark of irritation; B - timestamp 1 s

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

If for a long time an isolated muscle, to which a small load is suspended, is irritated by rhythmic electrical stimuli, then the amplitude of its contractions gradually decreases to zero. The record of contractions recorded at the same time is called the fatigue curve.

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

• during contraction, metabolic products (phosphoric, lactic acids, 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 out of the fibers 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, irritating for a long time, is brought to 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 of ATP and creatine phosphate resynthesis, which is necessary for contraction, is disrupted.

THEM. Sechenov (1903) showed that the restoration of the working capacity of tired muscles of the human hand after a long work of lifting a load is accelerated if work is done with the other hand during the rest period. Temporary restoration of the working capacity of the muscles of a tired hand can also be achieved with other types of motor activity, for example, when muscles work. lower extremities. Unlike simple rest, such rest was named by I.M. Sechenov active. He considered these facts as evidence that fatigue develops primarily in the nerve centers.

rice. 2.4. Electrical stimulation and muscle response. Above shows electrical impulses, below - the response of the muscle

If stimulated with a short electrical impulse, after a short latent period, it occurs. This contraction is called a "single muscle contraction". A single muscle contraction lasts about 10-50 ms, and it reaches its maximum strength after 5-30 ms.

Each individual muscle fiber obeys the all-or-nothing law, i.e., when the strength of stimulation is above the threshold level, a complete contraction occurs with the maximum force for this fiber, and a stepwise increase in the strength of contraction as the strength of irritation increases is impossible. Since a mixed muscle is made up of many fibers with varying levels of sensitivity to excitation, the contraction of the entire muscle can be stepped, depending on the strength of the stimulation, with strong irritations activating deeper muscle fibers.

Superposition and tetanus

A single electrical stimulation (Figure 2.4, top) leads to a single muscle contraction (Figure 2.4, bottom). Two closely following stimulations are superimposed on each other (this is called "superposition", or summation of contractions), which leads to a stronger muscle response, close to the maximum. A series of frequently repeated electrical stimulations causes muscle contractions that increase in strength, as a result of which proper muscle relaxation does not occur. If the frequency of electrical impulses is higher than the fusion frequency, then single stimuli merge into one and cause muscle tetanus (tetanic contraction) - a stable, fairly long-term tension of the contracted muscle.

Abbreviations

Rice. 2.5. Forms of muscle contractions. On the left is a schematic representation of sarcomere shortening, in the middle - changes in strength and length, on the right - an example of contractions

There are various functional forms of muscle contractions (Fig. 2.5).

• At isotonic contraction the muscle is shortened, but its internal tension (tonus!) remains unchanged in all phases of the working cycle. A typical example of isotonic muscle contraction is the dynamic work of flexors and extensors without significant changes in intramuscular tension, such as pulling up.

• At isometric contraction muscle length does not change, and muscle strength is manifested in an increase in its tension. A typical example of isometric contraction is the static muscle activity of lifting weights (holding the barbell).

• Most often, combined variants of muscle contraction are observed. For example, a combination contraction in which the muscles first contract isometrically and then isotonically, as in lifting weights, is called holding contraction.

• Installation (prefabrication) called a contraction in which, on the contrary, after the initial isotonic contraction, an isometric contraction follows. An example is the rotational movement of a hand with a lever - tightening a screw with a wrench or screwdriver.

• Various forms of muscle contractions are distinguished for their description and systematization. In fact, in most dynamic sports movements there is both a shortening of the muscle and an increase in tension (tone) of the muscles - auxotonic contractions.

The terms used here are not typical of the Russian literature on muscle activity. In the domestic literature, it is customary to distinguish the following types of abbreviations.

• concentric contraction- causing shortening of the muscle and moving the place of its attachment to the bone, while the movement of the limb, provided by the contraction of this muscle, is directed against the resistance to be overcome, such as gravity.
• Eccentric contraction- occurs when the muscle lengthens while regulating the speed of movement caused by another force, or in a situation where the maximum force of the muscle is not enough to overcome the opposing force. As a result, the movement occurs in the direction of the external force.
• Isometric contraction- an effort that opposes an external force, at which the length of the muscle does not change and movement in the joint does not occur.
• Isokinetic contraction- muscle contraction at the same speed.
• ballistic movement- fast movement, including: a) concentric movement of the agonist muscles at the beginning of the movement; b) inertial motion during minimal activity; c) eccentric contraction to slow down movement.

Filament sliding mechanism

rice. 2.6 Scheme of cross-linking - the molecular basis of sarcomere contraction

The shortening of the muscle occurs due to the shortening of the sarcomeres that form it, which, in turn, are shortened due to the sliding of actin and myosin filaments relative to each other (and not the shortening of the proteins themselves). The filament slip theory was proposed by Huxley and Hanson (Huxley, 1974; Fig. 2.6). (In 1954, two groups of researchers - X. Huxley with J. Hanson and A. Huxley with R. Niedergerke - formulated a theory explaining muscle contraction by sliding threads. Independently of each other, they found that the length of disk A remained constant in relaxed and shortened sarcomere. This suggested that there are two sets of filaments - actin and myosin, with one entering the gaps between the others, and when the length of the sarcomere changes, these filaments somehow slide over each other. This hypothesis is now accepted by almost everyone.)

Actin and myosin are two contractile proteins that are able to enter into a chemical interaction, leading to a change in their relative position in the muscle cell. In this case, the myosin chain is attached to the actin filament with the help of a number of special "heads", each of which sits on a long springy "neck". When coupling occurs between the myosin head and the actin filament, the conformation of the complex of these two proteins changes, the myosin chains move between the actin filaments, and the muscle as a whole shortens (contracts). However, in order for the chemical bond between the myosin head and the active filament to form, it is necessary to prepare this process, since in a calm (relaxed) state of the muscle, the active zones of the actin protein are occupied by another protein - tropochmyosin, which does not allow actin to interact with myosin. It is precisely in order to remove the tropomyosin “sheath” from the actin filament that calcium ions are rapidly poured out of the cisterns of the sarcoplasmic reticulum, which occurs as a result of the action potential passing through the muscle cell membrane. Calcium changes the conformation of the tropomyosin molecule, as a result of which the active zones of the actin molecule open for the attachment of myosin heads. This attachment itself is carried out with the help of the so-called hydrogen bridges, which very strongly bind two protein molecules - actin and myosin - and are able to stay in such a bound form for a very long time.

To detach the myosin head from actin, it is necessary to expend the energy of adenosine triphosphate (ATP), while myosin acts as ATPase (an enzyme that breaks down ATP). The breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate (P) releases energy, breaks the bond between actin and myosin, and returns the myosin head to initial position. Subsequently, cross-links can form again between actin and myosin.

In the absence of ATP, actin-myosin bonds are not destroyed. This is the cause of rigor mortis (rigor mortis) after death, because the production of ATP in the body stops - ATP prevents muscle rigidity.

Even during muscle contractions without visible shortening (isometric contractions, see above), the cross-linking cycle is activated, the muscle consumes ATP and generates heat. The myosin head repeatedly attaches to the same actin binding site, and the entire myofilament system remains immobile.

Attention: The contractile elements of the muscles actin and myosin themselves are not capable of shortening. Muscle shortening is a consequence of the mutual sliding of myofilaments relative to each other (filament sliding mechanism).

How does the formation of cross-links (hydrogen bridges) translate into motion? A single sarcomere is shortened by approximately 5-10 nm in one cycle, i.e. about 1% of its total length. Due to the rapid repetition of the cross-link cycle, a shortening of 0.4 µm, or 20% of its length, is possible. Since each myofibril consists of many sarcomeres and cross-links are formed in all of them simultaneously (but not synchronously), their total work leads to a visible shortening of the entire muscle. The transmission of the force of this shortening occurs through the Z-lines of myofibrils, as well as the ends of the tendons attached to the bones, as a result of which movement occurs in the joints, through which the muscles realize the movement of parts of the body in space or the promotion of the entire body.

Relationship between sarcomere length and muscle contraction strength

Rice. 2.7. Dependence of the force of contractions on the length of the sarcomere

Muscle fibers develop the greatest force of contraction at a length of 2-2.2 microns. With a strong stretch or shortening of the sarcomeres, the force of contractions decreases (Fig. 2.7). This dependence can be explained by the mechanism of filament sliding: at the specified length of sarcomeres, the overlap of myosin and actin fibers is optimal; with greater shortening, the myofilaments overlap too much, and with stretching, the overlap of the myofilaments is insufficient to develop sufficient contraction strength.

rice. 2.9 Influence of preliminary stretching on the strength of muscle contraction. Pre-stretching increases muscle tension. The resulting curve, which describes the relationship between muscle length and force of contraction under active and passive stretching, demonstrates a higher isometric tension than at rest.

An important factor influencing the strength of contractions is the amount of muscle stretch. Pulling at the end of the muscle and pulling on the muscle fibers is called passive stretching. The muscle has elastic properties, however, unlike a steel spring, the dependence of stress on tension is not linear, but forms an arcuate curve. With an increase in stretching, muscle tension also increases, but up to a certain maximum. The curve describing these relationships is called stretch curve at rest.

This physiological mechanism is explained by the elastic elements of the muscle - the elasticity of the sarcolemma and connective tissue, located parallel to the contractile muscle fibers.

Also, during stretching, the overlap of myofilaments also changes, but this does not affect the stretching curve, since cross-links between actin and myosin are not formed at rest. The pre-stretch (passive stretch) is added to the force of the isometric contractions (active contraction force).

Muscle contraction is vital important function organism 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 contraction of the heart) - peristalsis of the stomach and intestines, changes in the tone of blood vessels, maintaining tone Bladder are caused by smooth muscle contraction. 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 (MU). 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 a skeletal muscle is stimulated by a single impulse of an electric current of superthreshold 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 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 of 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 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- the physiological state of the 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 of the nervous system, due to the fact that some myocytes are able to spontaneously generate rhythmically repeating action potentials.

Excitation and contraction of muscles during natural motor acts is caused by nerve impulses coming from the central nervous system.

Types of muscle contractions

Single muscle tetanic tonic

(myocardium) (skeletal muscle) (smooth muscle)

smooth serrated

optimal

pessimistic

Skeletal muscles under experimental conditions respond to a single stimulus with a single contraction. However, in the whole organism, single contractions are characteristic only of the heart muscle, which contracts in response to single impulses coming to it from the sinus node.

Depending on the frequency of the impulse, the muscle contracts in different ways:

In order to understand the mechanism of tetanus, it is necessary to study a single muscle contraction, which is an indispensable constituent unit of each of them.

The study of a single muscle contraction can be carried out if it is recorded in expanded form using a rapidly rotating kymograph for this (Fig. 2.)

Rice. 2. Periods of single muscle contraction.

I - latent period - 0.01 sec.

II - shortening period - 0.04 sec.

III - relaxation period - 0.05 sec.

muscle contraction period - 0.1 sec.

Under natural conditions, muscles contract under the influence of rhythmic impulses received from the central nervous system. Impulses follow with a frequency greater than the period of a single muscle contraction, i.e. the muscle, not having time to relax, gets the next one. In the muscles, a summation phenomenon occurs, as a result of which they come to a state of prolonged shortening, called tetanus. Experimentally, tetanus can be obtained on the gastrocnemius muscle of a frog when exposed to a rhythmic stimulus.

At a frequency when each subsequent stimulus enters the muscle relaxation phase, it turns out dentate tetanus.

At the frequency of stimulation, when each subsequent impulse enters the phase of muscle shortening, a long continuous contraction occurs, which is called smooth tetanus.

When a muscle is excited and contracted, its excitability changes. As soon as the threshold stimulus acted on the muscles, excitation arose in the muscle, and its excitability disappeared, this will be an absolute refractory phase, i.e. absolute non-excitability, and if at this moment additional muscle irritations are not responded to them, this phase lasts 0.001 - 0.003 seconds. Then the excitability is gradually restored and the muscle responds to new additional, stronger irritations with a weak contraction. This is a relatively refractory phase, it lasts 0.009 - 0.007 seconds. Both of these phases fit into the latent period. After a relatively refractory phase, the excitability in the muscle is not only restored, but also becomes significantly higher than the initial one - the exaltation phase is 0.018 seconds. Then the excitability returns to its original value.

There are the following modes of muscle contractions:

Isotonic- a contraction in which the muscle fibers shorten, but their tension does not change.

Isometric- contraction, in which the length of the fibers does not decrease, but their tension increases.

Auxotonic- contraction, in which both the length and tension of the muscles change. This mode of contraction is typical for working muscles in the whole body. The first two can only be obtained in the experiment.

Purpose of the lesson: To form a clear idea of ​​the basic properties of muscle tissue.

1. Draw synchronous graphs of excitation, excitability, single muscle contraction:

a) striated

b) heart muscle.

2. Explain why the main type of contraction in the striated muscle is tetanic, and in the cardiac muscle it is single.

3. Classify the types and modes of muscle tissue contractions.

Questions for preparation:

1. Contractile function of the muscle cell: biophysical, biochemical basis of contraction and relaxation.

2. Modes of muscle contraction.

3. What is tetanus?

4. What types of tetanus are distinguished? What determines the type of tetanus?

5. Why is the amplitude of a tetanic contraction greater than a single contraction?

6. Features of the structure of striated and smooth muscles.

7. Which muscles have more pronounced elastic and plastic properties?

Work N 1. Recording and analysis of single and tetanic muscle contraction.

Goal of the work:

1. Record single and tetanic muscle contraction.

2. Study the periods of a single muscle contraction.

3. Investigate the influence of the frequency of stimulation on the nature of muscle contraction.

Equipment: dissecting kit, cuvette with gauze, Ringer's solution, electrostimulator, kymograph, tripod with myograph.

Object of study: frog.

Progress.

A muscle preparation is prepared (femur with head and calf muscle with the Achilles tendon) from the frog's hind leg and fix it in a tripod by the myograph. The electrostimulator is connected to the network. The electrodes are directed to the muscle, a threshold stimulus is found. The myograph is brought closer to the kymograph drum, which is turned by hand and the curve of a single contraction is recorded in expanded form.

Gradually accelerating the rhythm of irritation, they record a jagged, and then a smooth tetanus.

Result: Draw or paste the myogram.

LESSON № 5.

TOPIC: Physiological properties of skeletal muscles. Muscle work and strength.

The muscle, contracting and lifting the load, performs external, useful work. The work of the muscles is calculated by the formula W=P∙h, where W is the work of the muscle, P is the weight of the load, h is the height of the load. The work of the muscle with increasing load at the beginning increases, reaches a maximum, and then decreases. When the load is so large that the muscles are not able to lift it with their contraction, the useful work becomes equal to zero. The strength of a muscle does not depend on its length, but is proportional to its cross section. The cross section is understood as the sum of all the cross sections of individual muscle fibers. There are relative (maximum) and absolute muscle strength.

Purpose of the lesson: Get an idea of ​​the work and strength of the muscles.

Homework(in writing):

1. Compare the structure, properties of striated and smooth muscles.

Work number 1. Muscle work under different loads, determination of the absolute and relative strength of the muscle.

The purpose of the work: To show the dependence of the work performed on the magnitude of the load.

Equipment: dissecting kit, cuvette with a napkin, Ringer's solution, electrical stimulator, tripod with myograph, weights, ruler.

Object of study: frog.

Progress.

A muscle preparation is prepared and fixed in a vertical myograph. Assemble a circuit for electric shock stimulation. The irritation of the drug is direct. Select the current strength that causes the maximum contraction of the muscle. Muscle contractions are recorded on a kymograph, the drum of which is rotated by hand. For ease of comparison, muscle contractions should be recorded at a distance of approximately 0.5 -1 cm from one another.

First, irritation is applied to the muscle without a load, the contraction is recorded. Then, a small load is suspended from the lower hook of the myograph and the muscle is irritated with single shocks of electric current and the height of the muscle contraction is recorded on the kymograph drum. Then, gradually increasing the load and irritating the muscle with the same current strength, they record a series of muscle contractions and find the load that the muscle is only able to hold - this wakes up the maximum (relative) muscle strength. To determine the absolute strength of a muscle, you need to find the cross-sectional area of ​​\u200b\u200bthis muscle. To do this, the gastrocnemius muscle is removed from the myograph and cut in half at its widest point. Suppose the muscle is round and the area of ​​the circle is:

Hence, the absolute strength of the muscle is equal to the private division of the maximum strength by the cross-sectional area.

Having determined the strength of the muscle, we proceed to calculate the work. To calculate the work of a muscle under different loads, it is necessary to find the true shortening of the muscle, since the lever on the kymograph records the contraction in an enlarged form. The value of true shortening is as many times less, how many times the length of the entire lever is greater than the length from the axis of rotation to the point of attachment of the load.

Based on the rules of similarity of triangles, the height of the true shortening of the muscles is determined.

Result: Determine the absolute and relative strength of the muscle.

Calculate the work of the muscle. The result is entered in the table.

Conclusion:

LESSON № 6.

TOPIC: Muscle fatigue during work. Theories of muscle fatigue.

Fatigue is a temporary decrease in the efficiency of a muscle, organ or the whole organism as a result of prolonged work and disappearing after a long rest. Fatigue develops at a different rate in different excitable systems. In the “nerve - myoneural synapse - muscle” system, the myoneural synapse gets tired first of all, as the link with the lowest lability. With fatigue, excitability decreases and, finally, there is a complete loss of excitability, leading to the cessation of function.

Purpose of the lesson:

1. Form an idea of ​​what fatigue is and why it occurs.

2. Consider the main theories of the development of fatigue.

Homework (written):

1. List and give a brief description of the theories of muscle fatigue.

2. Explain where and why fatigue initially occurs in the “nerve - synapse - muscle” system.

Questions for preparation:

1. How does the magnitude of muscle contraction change with a gradual increase in load.

2. What is absolute and relative muscle strength. How they are defined.

3. What is fatigue?

4. Mioneural synapse and its characteristics.

5. The concept of optimum and pessimum frequency and strength of the acting stimulus.

WORK #1. Muscle fatigue at work. Localization of fatigue in the neuromuscular preparation.

Goal of the work:

1. Show the dependence of the development of fatigue on the rhythm of stimulation and the magnitude of the load in the whole organism and on an isolated muscle.

2. Establish where fatigue initially occurs in the neuromuscular preparation.

Equipment: dissecting kit, cuvette with a napkin, Ringer's solution, kymograph, tripod with myograph, weights, electrical stimulator, ergograph, dynamometer.

Object of study: frog

Progress:

Influence of frequency of irritation. Two muscle preparations are prepared (one of them is placed in a cuvette with Ringer's solution, and one is fixed in a tripod behind the myograph). Assemble the installation for irritation with electric shock. The electrodes are directed under the sciatic nerve. Find the threshold of muscle excitability. The myograph is brought closer to the kymograph drum.

The muscle is irritated with a frequency of 1 Hz. The fatigue curve is recorded on the kymograph drum.

The muscle preparation is replaced and the experiment is repeated, increasing the frequency of stimulation to 5 Hz. Determine how long this muscle contracted, working in a more frequent rhythm.

When analyzing the obtained kymograms, a gradual increase in the height of contractions is seen, then the height of muscle contractions remains at a constant level for some time. The development of fatigue is characterized by the fact that the range of its contractions gradually decreases, and relaxation remains incomplete, contracture develops.

Influence of the magnitude of the load: Prepare two preparations of the gastrocnemius muscle. One of them is placed in a Petri dish and filled with Ringer's solution, and the other is hung on the hooks of the myograph (experimental conditions are the same as in the first case). A weight of 50 grams is suspended from the lower hook of the myograph. The muscle is irritated with a frequency of 1 Hz. Record the fatigue curve.

The drug is replaced, the load is doubled, and irritation is applied with the same frequency. Record the fatigue curve. When analyzing the kymogram data, it is clear that fatigue develops faster with increasing load.

Localization of fatigue in the neuromuscular preparation. Prepare a neuromuscular preparation. Assemble the installation for irritation with single shocks of electric current. Irritation of the neuromuscular preparation begins with the sciatic nerve - indirect irritation. Continue stimulation until the calf muscle stops contracting. Then the electrodes are transferred to the muscle - direct irritation, electric shocks are applied, while it is noted that the muscle begins to contract again.

Conclusion: Since it is known that the nerve is practically not fatiguable, it can be concluded that the previously observed muscle fatigue during indirect stimulation developed in the myoneural synapse.

1 - When a muscle receives a single stimulation (single electrical stimulus), then there is single and single muscle contraction . This type of contraction is non-physiological for the skeletal muscle, since it always receives nerve fibers a series of impulses. Only the heart muscle contracts according to the principle of single contractions. Experimental recording of a single skeletal muscle contraction consists of three phases: 1. Latent (hidden) period. This is the time from the onset of irritation to the appearance of a contractile effect. Equal to 0.002 s.2. shortening phase. This is the time during which the muscle contracts. It continues for 0.05 s.3. Relaxation phase. Lasts 0.15 s.

2 - The second type of muscle contraction is a prolonged shortening of the muscle or its tension - tetanic and tonic, which can be isometric and isotonic. There are two types of tetanic contractions or tetanus: jagged and smooth (solid). Serrated tetanus observed when the subsequent impulse comes into the phase of muscle relaxation (the state of the muscle is purely laboratory). smooth tetanus takes place when the next pulse hits at the end of the shortening phase.

Modes of muscle contractions:

1) isotonic- a contraction in which the muscle fibers shorten, but the same tension remains (for example, when lifting a load);

2) isometric- a contraction in which the length of the muscle fibers does not change, but the tension in it increases (for example, with pressure resistance);

3) auxotonic- a contraction in which both the tension and the length of the muscle change.

Fatigue is a temporary decrease in muscle performance as a result of work. Fatigue of an isolated muscle can be caused by its rhythmic stimulation. As a result, the force of contractions progressively decreases. The higher the frequency, the strength of irritation, the magnitude of the load, the faster the fatigue develops. The stronger the fatigue of the muscle, the longer the duration of these periods. In some cases, complete relaxation does not occur. In the last century, based on experiments with isolated muscles, 3 theories of muscle fatigue were proposed.

1. Schiff's theory: fatigue is a consequence of the depletion of energy reserves in the muscle.

2. Pfluger's theory: fatigue is due to the accumulation of metabolic products in the muscle.

3. Verworn's theory: fatigue is due to a lack of oxygen in the muscle.

The onset of muscle fatigue depends on the frequency of their contractions. Too frequent contractions cause rapid fatigue

muscle fatigue is the result of not only changes in the functions of the nervous and muscular systems, but also changes in the regulation of the nervous system of all autonomic functions.
Fatigue during dynamic work occurs as a result of changes in metabolism, the activity of the endocrine glands and other organs, and especially the cardiovascular and respiratory systems. A decrease in the efficiency of the cardiovascular and respiratory systems disrupts the blood supply to working muscles, and, consequently, the delivery of oxygen and nutrients and the removal of residual metabolic products.