Selected chapters from the book "Modern Strength Training. Theory and Practice." Skeletal muscles

CLASSIFICATION OF MUSCLE FIBERS.

Morphological classification

Cross-striped (cross-striated)

Smooth (non-striated)

Classification by type of control of muscle activity

Cross-striped muscle tissue of skeletal type.

Smooth muscle tissue internal organs.

Cardiac-type striated muscle tissue

CLASSIFICATION OF SKELETAL MUSCLE FIBERS

STRIPED MUSCLES represent the most specialized apparatus for carrying out rapid contractions. There are two types of striated muscles - skeletal and cardiac. SKELETAL muscles are composed of muscle fibers, each of which is a multinucleated cell resulting from the fusion of a large number of cells. Depending on contractile properties, color and fatigue, muscle fibers are divided into two groups - RED and WHITE. The functional unit of muscle fiber is the myofibril. Myofibrils occupy almost the entire cytoplasm of the muscle fiber, pushing the nuclei to the periphery.

RED MUSCLE fibers (type 1 fibers) contain a large number of mitochondria with high activity of oxidative enzymes. The strength of their contractions is relatively small, and the rate of energy consumption is such that they have enough aerobic metabolism (they use oxygen). They are involved in movements that do not require significant effort, such as maintaining a posture.

WHITE MUSCLE FIBERS (type 2 fibers) are characterized by high activity of glycolytic enzymes, significant contractile force and such a high rate of energy consumption for which aerobic metabolism is no longer sufficient. Therefore, motor units consisting of white fibers provide fast but short-term movements that require jerking efforts.

CLASSIFICATION OF SMOOTH MUSCLES

Smooth muscles are divided into VISCERAL(UNITARY) AND MULTI-UNITARY. VISCERAL SMOOTH muscles are found in all internal organs, ducts of the digestive glands, blood and lymphatic vessels, and skin. TO MULIPIUNITARY include the ciliary muscle and the iris muscle. The division of smooth muscles into visceral and multiunitary is based on the different densities of their motor innervation. IN VISCERAL SMOOTH MUSCLES, motor nerve endings are present on a small number of smooth muscle cells.

FUNCTIONS OF SKELETAL AND SMOOTH MUSCLES.

FUNCTIONS AND PROPERTIES OF SMOOTH MUSCLES

1. ELECTRICAL ACTIVITY. Smooth muscles are characterized by unstable membrane potential. Fluctuations in membrane potential, regardless of neural influences, cause irregular contractions that maintain the muscle in a state of constant partial contraction - tone. The membrane potential of smooth muscle cells does not reflect the true value of the resting potential. When the membrane potential decreases, the muscle contracts; when it increases, it relaxes.

2. AUTOMATION. Action potentials of smooth muscle cells are autorhythmic in nature, similar to the potentials of the conduction system of the heart. This indicates that any smooth muscle cells are capable of spontaneous automatic activity. Automaticity of smooth muscles, i.e. the ability for automatic (spontaneous) activity is inherent in many internal organs and vessels.

3. RESPONSE TO TENSION. In response to stretch, smooth muscle contracts. This is because stretching reduces the cell membrane potential, increases AP frequency and, ultimately, smooth muscle tone. In the human body, this property of smooth muscles serves as one of the ways to regulate the motor activity of internal organs. For example, when the stomach is filled, its wall stretches. An increase in the tone of the stomach wall in response to its stretching helps maintain the volume of the organ and better contact of its walls with incoming food. In blood vessels, stretching caused by fluctuations in blood pressure.

4. PLASTICITY b. Voltage variability without a natural connection with its length. Thus, if a smooth muscle is stretched, its tension will increase, but if the muscle is held in the state of elongation caused by stretching, then the tension will gradually decrease, sometimes not only to the level that existed before the stretch, but also below this level.

5. CHEMICAL SENSITIVITY. Smooth muscles are highly sensitive to various physiologically active substances: adrenaline, norepinephrine. This is due to the presence of specific receptors on the smooth muscle cell membrane. If you add adrenaline or norepinephrine to a preparation of intestinal smooth muscle, the membrane potential increases, the frequency of AP decreases and the muscle relaxes, i.e., the same effect is observed as when the sympathetic nerves are excited.

FUNCTIONS AND PROPERTIES OF SKELETAL MUSCLES

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

1) provide a certain posture of the human body;

2) move the body in space;

3) move individual parts of the body relative to each other;

4) are a source of heat, performing a thermoregulatory function.

Skeletal muscle has the following essential PROPERTIES:

1)EXCITABILITY- the ability to respond to a stimulus by changing ionic conductivity and membrane potential.

2) CONDUCTIVITY- the ability to conduct the action potential along and deep into the muscle fiber along the T-system;

3) CONTRACTABILITY- the ability to shorten or develop tension when excited;

4) ELASTICITY- the ability to develop tension when stretching.

The membrane potential of striated muscle fibers is (-80) - (-90) mV, and the threshold level of depolarization is about -50 mV AP, arising on the postsynaptic membrane of the muscle fiber, spreads through the sarcolemma (the membrane that surrounds the muscle fiber) in both directions from the place of formation (synapse). It is transmitted by sarcolemma electrogenically (similar to the transmission of PD without "Yakushev's nerve fiber). The duration of AP in most skeletal muscles is 2-3 ms. In connection with this, as well as with the need for greater polarization of the membrane for the occurrence of a spike (MP KR = 40 mV), the speed of propagation of AP by the muscle fiber membrane is about 3-5 m1s. A short time after the arrival of PD, the muscle fiber begins to contract. To understand the mechanism of muscle contraction, it is necessary to become familiar with its microstructure.

Muscle fiber structure

The muscle fiber does not exceed 0.1 mm in diameter, and its length can range from a few millimeters to 12 cm (Fig. 20).

Under a light microscope, an alternation of dark and light stripes (transverse darkening) is visible. Dark disks (anisotropic disks - A) have double interlayering, while light disks (isotropic disks - I) do not have this property. The part of the muscle fiber from the middle of one isotropic disc to the middle of the other is called sarcomere. The length of a sarcomere in a muscle at rest is about 2 µm, and when contracted with maximum force it is slightly more than 1 µm. (Fig. 20 shows a sarcomere bounded on both sides by 2-lines; I - isotropic disk; A - anisotropic disk; H - area with reduced anisotropy. Transverse section of the myofibril (d) gives an idea of ​​the hexagonal distribution of thick and thin myofilaments).

Sarcolem. The muscle fiber membrane - sarcolemma - is formed by a typical plasma membrane, reinforced by connective tissue fibers. The latter, combining at the ends of the muscle fibers, form tendons, with the help of which the muscle is attached to the bones.

Sarcoplasm. The sarcoplasm of the muscle fiber contains a typical set of organelles. But one of them deserves special attention - sarcoplasmic

Rice. 20. into the muscle (A) includes muscle fibers (b), each of which contains myofibrils (c). Myofibril (g) is formed from thick and thin myofilaments (g, d)

niureticulum (NR). This is a widely branched network consisting of cisterns and tubes bounded by two-layer protein-lipid membranes (Fig. 21). The sarcoplasmic reticulum performs an important function in initiating muscle contraction as a Ca2+ depot.

Rice. 21.(according to B.I. Khodorov): A- distribution of tubes (T-system) and SR inside the sarcomere; b- triad: during the propagation of AP by the T-tube, Ca2 is released from the CP tank, which, by binding to troponin in the troponin-tropomyosin complex, eliminates the inhibitory effect on actin myofilament. Cross bridges of myosin filaments can now interact with actin filaments. The relaxation process is associated with the active return of Ca2+ to the tanks

It is also necessary to mention the presence of protein in sarcoplasm myoglobin, which serves as an oxygen depot inside the fiber.

Contractile protofibrils. Contractile protofibrils are arranged in an orderly manner inside the muscle fiber in the sarcoplasm. There are two types of protofibrils: thick (15-17 nm thick) and thin (about 6 nm thick). Thin protofibrils are located in the I-zone and with protein actin filaments. Thick threads that are located in zone A are called myosin(see Fig. 20).

More than two hundred myosin molecules are involved in the formation of myosin filaments (twisted in pairs, with a protruding head). The heads are directed at an angle from the center towards the thin threads (reminiscent of a “brush” for washing dishes). The myosin head contains the ATPase enzyme, and the ATP molecule is located on the head itself.

Lactin filaments composed of two actin filaments of globular actin molecules, they look like beads. Thin threads have active centers, located at a distance of 40 nm from each other, to which myosin heads can attach. In addition to actin, thin filaments also contain other proteins - the troponin complex (calmodulin), which is located above the active centers, covering them, which prevents the connection of actin with myosin.

Thin filaments pass through the middle of the I-zone into two nearby sarcomeres. In the middle of this zone is located X-membrane, what separates sarcomeries from each other. Thus, the contents of each sarcomere are isolated by the sarcolemma and Z-membranes.

Mechanism of muscle contraction

Initiation of muscle contraction. Spreading along the outer membrane, the PD enters the muscle fiber (see Fig. 21), here it is transmitted to the membrane of the sarcoplasmic reticulum, where it opens electrically excitable calcium channels. Because the calcium concentration in the sarcoplasm is less than 10~7 mol1L, and in the sarcoplasmic reticulum - more than 10 4 mol1L, an intensive release of its ions in the sarcoplasm begins.

The released calcium initiates muscle contraction. A calcium level sufficient to initiate muscle contraction is achieved 12-15 ms after the arrival of the nerve impulse. This is the hidden, latent time of muscle contraction. Due to the fact that the speed of propagation of AP by the sarcolemma is greater than the time required for the release of Ca2" from the sarcoplasmic reticulum, all fibrils of the muscle region innervated by one nerve contract simultaneously.

Ca2+ plays a certain role in the initiation of muscle contraction after entry into the sarcoplasm. calmodulin. By attaching Ca2+, calmodulin promotes the activation of Atphase and the use of ATP energy to connect the active center of the actin filament with the myosin head, as well as muscle shortening (Fig. 22). When calmodulin (troponin C) combines with calcium, the active center of actin is released, as a result of which the myosin head attaches to it. These processes occur if the concentration of free calcium in the sarcoplasm increases 100 times or more: from 10"7 to 10~5 mol1L.

"Hinge mechanism." Due to the combination of these processes, the following occurs:

a) pulling the myosin filaments towards the atin filaments;

b) charging myosin with energy, which is used to rotate the myosin head.

Rice. 22. A- cross bridges in a state of muscle fiber relaxation; 6 - during contraction (arrows indicate the direction of movement of actin protofibrils (and) two halves of the sarcomere); V- model of stress development in cross bridges

the time of their contraction (on the left - in a state of relaxation, on the right - during contraction of the muscle fiber). 4 - neck of the transverse bridge; 5 - head of the cross bridge

After this, the phosphorus and adenosine diphosphoric acid (ADP) formed leave, and a new ATP molecule joins in their place, which leads to the rupture of the connection between myosin and the active center of actin.

When a muscle contracts:

a) actin and myosin filaments are practically not shortened;

b) the interaction of actin with myosin leads to the mutual entry of threads into the spaces between them;

c) two adjacent 7-membranes come closer to each other, and with the strongest possible contraction, the distance between them can be reduced by almost half;

d) when the length of the muscle decreases, the sarcomere expands, since the sarcoplasm contained inside the sarcomere does not shrink;

d) similar processes simultaneously occur in all sarcomeres of the muscle fiber, so both ends of the muscle are pulled towards the center.

At present, the mechanism that ensures the entry of actomyosin filaments into each other is still completely unknown. The generally accepted hypothesis of a “hinge mechanism” (see Fig. 22). After the myosin head connects to the active center of actin, it rotates by 45°. Due to the rupture of the bridge, the neck of the myosin head straightens, acquiring its original position. For such movements this system received the name hinge mechanism. During rotation, myosin is advanced by actin one “step”, or “stroke”, equal to 20 nm. The arrival of a new portion of Ca2+ leads to a repetition of the “step,” but now with a different head, which turns out to be opposite the new active center of actin, since they are located at a distance of about 40 nm from each other. Due to the fact that myosin filaments have a bipolar organization of heads, their parallel “rows” ensure the sliding of actin filaments along the sarcomere (from the membrane to its middle).

Muscle relaxation.

These processes ("steps") will be repeated as long as the sarcoplasm contains free Ca2" (in a concentration of more than 10-5 mol1L) and ATP. If there is no new wave of depolarization, calcium quickly returns back to the cisterns of the sarcoplasmic reticulum. It is pumped out of the sarcoplasm against the concentration gradient using the Ca2+ pump located on the membrane of the sarcoplasmic reticulum. The operation of this pump, which requires a large amount of ATP (2 ATP molecules are used to remove each Ca2+), is activated by calcium itself, or more precisely, by an increase in its concentration in the sarcoplasm. A consequence of calcium pumping. from sarcoplasm - breaking all actin and myosin bonds and muscle relaxation.

Energy of muscle contraction

ATP in muscle is necessary for:

1) reduction (formation of bridges);

2) relaxation (breaking bridges);

3) operation of the Ca2+ pump;

4) operation of the K* - pump (to eliminate disturbed ion gradients due to the arrival of excitation).

However, there is relatively little ATP in muscle sarcoplasm. it is only enough for a few muscle contractions (about eight single contractions). At the same time, under natural conditions, muscles can contract for a long time, which becomes possible only due to the activation of the mechanisms of ATP re-synthesis - creatine phosphokinase, glycolytic, and aerobic oxidation.

The sequence of “switching on” of the indicated ATP resynthesis pathways is as follows. First, immediately after ATP hydrolysis, its restoration begins due to creatine phosphate (CP):

ADF + CF<=>ATP + CP.

The creatine phosphokinase pathway is inertialess (it is triggered immediately by ADP that is formed) and can provide muscle contraction within a few seconds. At the same time, the glycolytic pathway is activated. The formation of ATP during the glycolysis of carbohydrates occurs with the participation of enzymes, the activity of which increases gradually from the beginning of muscle contractions. But after 15-20 s they become active enough to pick up the baton of ATP resynthesis when CP is depleted. The disadvantage of this route is the lower ATP output per unit of time compared to the previous one. In addition, during glycolysis, under-oxidized products (lactic, pyruvic acids) are formed, which, in the case of intensive formation, do not have time to leave the muscle, leading to a disruption of homeostasis in it (pH shift to the acidic side).

Aerobic oxidation has the greatest potential for ATP resynthesis (almost unlimited time with an adequate supply of oxygen and oxidation products). But this is the most inert way, since its enzyme system is activated slowly. It reaches its maximum level of activity 2-3 minutes after the start of muscle work. In addition to the mitochondrial enzymes of the muscle fiber itself, ensuring the specified mechanism of ATP resynthesis requires an adequate supply of muscles with oxygen and raw materials. In addition, the productivity (the amount of ATP synthesized per unit time) of aerobic oxidation is not the same depending on the compound being oxidized: when oxidizing energy carbohydrates.

Naturally, the indicated capabilities of the ATP resynthesis pathways determine muscle performance.

Efficiency and heat generation during muscle work

According to the first law of thermodynamics (the law of conservation of energy), the chemical energy converted into muscle is equal to the sum of mechanical energy (work) and calorific value. Hydrolysis of one mole of ATP provides 48 kJ of energy. Only 40-45% of it is converted into mechanical energy, and the remaining 55-60 % turn into to the initial heat. However, in natural conditions the mechanical efficiency of muscle activity, or efficiency, does not exceed 20-30%. This is due to the fact that not all the ATP energy in the muscle goes to muscle contraction itself: part of it is spent on recovery processes. Consequently, the higher the intensity of muscle work, the more active the processes of heat formation.

Types and modes of muscle contractions

Under natural conditions, both ends of the muscle are attached to the bones by tendons and, when contracted, attract them to each other. If one end of the muscle (joint) is fixed, then the other is pulled towards it (Fig. 23). When a load is attached to this end of the muscle, which the muscle is not ABLE to lift, it only tenses, in which case its length does not change. Conditions also occur when the muscle gradually increases in length (the load is heavier than the lifting force of the muscle, or it is necessary to slowly lower the load).

Under experimental conditions, one muscle, one fiber, and even one actomyosin filament can be isolated with or without a nerve innervating it. If you fix one end motionless in a tripod and hang a weight or recording device on the other, you can record a muscle contraction - a myogram.

As a result, the following types of muscle contractions are distinguished:

o isotonic(concentric) - muscle contraction with shortening while maintaining constant tension;

o isometric, when the length of the muscle does not change (tension);

o eccentric(plyometric) when the muscle lengthens.

As a rule, most natural muscle contractions are mixed, that is, of the anisotonic type, when the muscle shortens when tension increases.

In Fig. 24, A The curve of a single contraction is shown. On it you can distinguish contraction phases And relaxation. The second phase is longer. The time of one contraction of even a single fiber significantly exceeds the lifetime of the AP.

Rice. 24. Various muscle contraction modes:

A- single contractions; V- incomplete tetanus; G g - complete tetanus

Rice. 23. Interaction between flexor (a) and extensor muscles(b)

The amplitude of a single contraction of an isolated muscle fiber does not depend on the strength of stimulation, but obeys the “all or nothing” law. In contrast, on a solid muscle you can get a “ladder” (Bowditch ladder): that the greater the strength (up to a certain value) of irritation, the stronger the contraction. A further increase in the strength of stimulation does not affect the amplitude of muscle contraction. This pattern can be observed both in the case of irritation through the nerve, and in the case of irritation of the muscle itself. This is due to the fact that almost all muscles (and nerves) are mixed, that is, they consist of many motor units (MOs) with different excitability.

Motor unit

A single motor neuron nerve fiber and the muscle fibers that it innervates constitute one motor unit (Fig. 25). In most skeletal muscles, a motor unit contains several hundred (even thousands) of muscle fibers. Even in very small muscles that require high precision movements (eyes, hands), a motor unit may contain 10-20 muscle fibers. From a functional point of view, there are several types of motor unit, which can be grouped as follows: fast And slow. Their functional differences are due to the corresponding structural features, moreover, both at the level of relatively rough morphology and fine biochemical differentiation. Different types of motor units distinguish muscle parts and nerve fibers. These differences ensure the appropriate functional manifestation of each type of motor unit. Fast and slow ones are distinguished by excitability, the speed of impulses carried out by the axon, the optimal frequency of impulses and resistance to fatigue after performing work. In addition, in each type, the motor neuron and muscle fibers as partners are connected to each other, which provides their functional characteristics.

Motor neurons. The excitability or sensitivity to the strength of the current stimulus of motor neurons of the same muscle is inversely related

Rice. 25.

1 - motor neuron body; 2 - core; WITH- dendrites; 4 - axon; 5 - axon myelin sheath; 6 - terminal branches of the axon; 7 - neuromuscular synapses

on the size of their body: the smaller the motor neuron, the higher its excitability, that is, with less stimulus strength, an action potential appears in them. Small motor neurons innervate a relatively small number of slow muscle fibers, large motor neurons innervate fast muscle fibers, of which there are usually many in one motor unit.

The diameter of the axon and the speed of excitation along it depend on the size of the neuron: it is higher in large motor neurons. In addition, nerve impulses with high frequency can occur in such motor neurons. Consequently, by changing the frequency of motor neuron impulses, the muscle fibers that are part of the corresponding motor unit can receive a high frequency range of action potential, and this will determine a greater force of their contraction.

Each motor neuron also corresponds to the structure of the muscle fibers of the motor unit. Thus, the speed of muscle fiber contraction is directly dependent on the activity of actomyosin Atphase (the number of actin and myosin filaments): the higher its activity, the faster actomyosin bridges are formed, and therefore, the higher the contraction speed. The density of “packing” of actomyosis of new filaments in fast muscle fibers is higher than in slow ones. In addition, the sarcoplasmic reticulum (calcium depot) is more pronounced in the fast fiber. Therefore, during the receipt of PD:

o hidden time before the start of contraction is shorter;

o calcium pump density is higher.

So, the muscle contracts and relaxes faster. In fast muscle fiber, the activity of glycolytic enzymes is increased, which ensures rapid restoration of ATP, which is consumed during intense muscle contractions.

In contrast, in slow muscle fiber the activity of oxidation enzymes is higher, due to which ATP recovery occurs, although slower, but more economically. So, if from 1 mole of glucose as a result of glycolysis only 2-3 moles of ATP are formed, then in the case of aerobic oxidation - 36-38 moles of ATP. In addition, during glycolysis, under-oxidized substrates (for example, lactic acid) are formed, which “acidify” the muscle and reduce its performance. Two more structural differences in slow muscle fibers contribute to increased performance and improved oxidation conditions:

1) slow fibers are better provided with oxygen than fast fibers due to the greater density of blood capillaries surrounding them;

2) inside the slow fibers there is a large amount of myoglobin, which gives them a red color and are oxygen depots, which can be used for oxidation at the time of muscle contraction, when the supply of oxygen with the blood is difficult due to compression of the blood vessels by the muscle that is contracting.

Fast muscle fibers have a short contraction period - up to 7.5 ms, and slow muscle fibers have a long contraction period - up to 100 ms.

Thus, summing up the functional differences of motor units, it can be noted: slow motor units are characterized by easy excitability, lower force and speed of contraction with low fatigue and high endurance. Fast motor units have the opposite properties.

Research in recent years has proven that each person has innate differences in the percentage of fast and slow fibers in their skeletal muscles. For example, in the outer thigh muscle the range of fluctuations in the number of slow fibers is from 13 to 96%. The advantage of slow fibers provides the “stayer”, and a small percentage of them provides the “sprint” capabilities of the athlete. In addition, the composition of different muscles in one person also differs. Thus, on average, the content of slow fibers in the triceps muscle of the shoulder is 33%, in the biceps - 49, in the anterior magnum - 46, in the soleus - 84%.

Summation contraction and tetanus

Under natural conditions of human life, single muscle contractions do not occur. Typically, nerve impulses travel to the muscles via motor neurons in “bundles”, that is, several in a row with relatively short time intervals. This leads to the formation of not one, but several PDs in the skeletal muscle itself. If the muscle is not affected by single impulses (IP), but by impulses that quickly follow one after another, then the contractile effects are summed up, and as a result, the muscle contracts for a long time (see Fig. 24). In addition, if further stimuli arrive at the initial moment of relaxation, the myographic curve will be jagged, and if before the onset of relaxation, it will be without jaggedness. This type of abbreviation is called tetanus.

Distinguish serrated And Tetanusi's incompetence. During tetanus, not only does the contraction time lengthen, but its strength also increases. This is due to the fact that only minor “steps” will have time to occur in response to the first PD. The final reserve creates the opportunity to increase the force of contraction during the arrival of further PD. In this case, the calcium concentration (the number of actomyosin bridges) in such a muscle fiber can be the same as during a single contraction.

Tetanic contraction is likely primarily because the muscle fiber membrane is capable of conducting fairly frequent PD (more than 100 per 1 s), since the refractory period in skeletal muscles is much shorter than a single contraction itself. Consequently, when the next PD arrives at the muscle, it again becomes sensitive to them.

The frequency and strength of the stimulus required to excrete the muscle fiber into the tetanus are not the same for all muscles, but depend on the characteristics of their motor unit. The duration of one contraction of a slow muscle fiber can reach 100 ms, and a fast one - 10-30 ms. Therefore, to obtain undamaged tetanus in slow fibers, 10-15 impulses per s are enough, and fast fibers need up to 50 impulses per s and above.

Under natural conditions, it almost never happens that all muscle fibers are in a contracted state. Therefore, with voluntary contraction, muscle strength is less than in the case of artificial stimulation. The mechanism of a sharp increase in the force of muscle contraction in an extreme situation is based on this principle: the synchrony of nerve impulses arriving at different motor units increases. One of the mechanisms that ensures an increase in muscle strength, for example in an athlete during training, is an increase in the synchrony of contraction of individual motor units.

Maximum rhythm of excitation. The limiting rhythm of excitation determined by the concept lability, of all excitable tissues depends on the duration of the period required to restore the sensitivity of sodium channels after the previous stimulation, that is, on the refractory period. The lability of a motor unit, consisting of three structures (nerve, synapse, muscle), is determined by the most “narrow” link, the synapse, since it is this that has the minimum frequency of excitation transmission. Motor neurons, even the smallest ones, are capable of conducting more than 200 impulses per second, muscle fibers - more than 100 impulses per second, and the neuromuscular synapse - less than 100 impulses per second.

Functional characteristics of skeletal muscles

Muscle strength determined by the traction force at its ends. The maximum traction force develops during isometric contraction of a muscle under the following conditions: a) activation of all motor units that make up this muscle; b) the beginning of muscle contraction at rest length; c) complete tetanus mode in all motor units.

Rice. 26.(according to A.A. Ukhtomsky)

To measure muscle strength, either the maximum load it can lift or the maximum tension it can develop under isometric contraction is determined. (A single muscle fiber can develop a tension of 100-200 mg.) The human body contains about 30 million muscle fibers and theoretically, if they all pulled in one direction, they would create a tension of up to 30 tons. In addition, it is necessary to take into account such circumstances. First, the strength of different muscle fibers is somewhat different: fast motor units are stronger than slow ones. Secondly, the strength of a muscle depends on its cross-section: the larger the volume of the muscle, the stronger it is. In addition, depending on the course of the fibers, oblique and rectus muscles are distinguished. The oblique course of the fibers provides a large number of muscle fibers passing through its cross section, as a result of which the strength of such a muscle is greater. Therefore, they distinguish physiological And anatomical diameter of the muscle: the physiological diameter is perpendicular to the direction of the muscle fibers, and the anatomical diameter is to the length of the muscle (Fig. 26). Naturally, in muscles with the longitudinal direction of the fibers, both named diameters coincide, and in the cirrus, the physiological diameter is larger than the anatomical one, therefore, for the same anatomical diameter, the latter are stronger. For example, the relative strength of human muscles (per 1 cm2 cross-sectional area):

o ankle muscle - 5.9 kg;

o shoulder flexor muscle - 8.1 kg;

o chewing muscle - 10.0 kg;

o biceps brachii muscle - 11.4 kg;

o triceps brachii muscle - 16.7 kg.

Under natural conditions, the expression of muscle force is influenced not only by the above three conditions, but also by the angle at which the muscle approaches the bone. The greater the angle of attachment, the better conditions for the manifestation of force. If the muscle approaches the bone at a right angle, almost all of its force is spent on providing movement, and if it is at an acute angle, part of the force is used to ensure movement, the rest is used to compress the lever.

Fatigue

During prolonged or intense muscular work, fatigue develops, which is expressed first in a decrease in performance, and then in cessation of work. Fatigue is characterized by corresponding changes that occur not only in the muscles, but also in the systems that serve them.

Fatigue They call a condition that develops as a result of work and is manifested by a deterioration in the motor and autonomic functions of the body and their coordination. In this case, performance decreases, a feeling appears tired (psychological condition). Fatigue is a holistic reaction of the whole organism. Therefore, when fatigue of the nerve, synapse, and muscles is discussed below, it is necessary to remember the conventions of these concepts. It would be more correct to talk about some mechanisms that determine the “performance” of the main parts of the motor unit - nerves, muscle fibers, synapses.

Nerve fiber fatigue. Under natural conditions, the nerve fiber practically does not get tired. Conducting a nerve impulse requires energy expenditure only for the operation of the K+ pump, which is quite energy-efficient. ATP resynthesis systems are quite capable of providing energy to the nerve fiber.

Fatigue of the neuromuscular junction. The efficiency, that is, the ability to conduct excitation, is significantly lower for the synapse than for the nerve fiber. This may be a consequence of two phenomena. Depression of excitation transmission in the synapse can be caused by the depletion of a significant part of the transmitter or the weakening of its restoration when the frequency of APs supplied by the nerve fiber is too high. In addition, during intense muscular activity, underoxidized products (actively formed during glycolysis) reduce the sensitivity of the postsynaptic membrane to the ACh mediator. This leads to a decrease in the amplitude of each EPP and, with an excessive reduction, the occurrence of AP becomes impossible.

Muscle fiber fatigue. Violation of the excitability and contractility of muscle fiber is primarily due to a violation of its energy, that is, the mechanisms of ATP resynthesis. In this case, the intensity of muscle work becomes the decisive factor. Its ultra-high activity is associated with a deficiency of the creatine phosphokinase pathway or the accumulation of under-oxidized products during glycolysis. The latter, on the one hand, reduces the sensitivity of the postsynaptic membrane, on the other, it shifts the pH of the sarcoplasm to the acidic side, which in itself inhibits the activity of glycolytic enzymes. All this causes the rapid development of fatigue during intense muscular work. Fatigue during long-term low-intensity work develops slowly, which is associated with a violation of regulatory mechanisms from the central parts of the nervous system.

Muscle tissue is recognized as the dominant tissue of the human body, the proportion of which in the total weight of a person is up to 45% in men and up to 30% in women. Musculature includes a variety of muscles. There are more than six hundred types of muscles.

The importance of muscles in the body

Muscles play an extremely important role in any living organism. With their help, the musculoskeletal system is set in motion. Thanks to the work of muscles, a person, like other living organisms, can not only walk, stand, run, make any movement, but also breathe, chew and process food, and even the most important organ - the heart - also consists of muscle tissue.

How do muscles work?

The functioning of muscles occurs due to their following properties:

• Excitability is a process of activation, manifested in the form of a response to a stimulus (usually an external factor). The property manifests itself in the form of changes in metabolism in the muscle and its membrane.
• Conductivity is a property that means the ability of muscle tissue to transmit a nerve impulse formed as a result of exposure to a stimulus from the muscle organ to the spinal cord and brain, and also in the opposite direction.
• Contractility is the final action of the muscles in response to a stimulating factor, manifested in the form of shortening of the muscle fiber; muscle tone also changes, that is, the degree of their tension. At the same time, the speed of contraction and maximum muscle tension may be different as a result of different influences of the stimulus.

It should be noted that muscle work is possible due to the alternation of the above-described properties, most often in the following order: excitability-conductivity-contractility. If we are talking about voluntary muscle work and the impulse comes from the central nervous system, then the algorithm will have the form conductivity-excitability-contractility.

Muscle structure

Any human muscle consists of a collection of elongated cells acting in the same direction, called a muscle bundle. The bundles, in turn, contain muscle cells up to 20 cm long, also called fibers. The shape of the cells of striated muscles is oblong, while that of smooth muscles is fusiform.

A muscle fiber is an elongated cell bounded by an outer membrane. Under the shell, contractile protein fibers are located parallel to each other: actin (light and thin) and myosin (dark, thick). In the peripheral part of the cell (in striated muscles) there are several nuclei. Smooth muscles have only one nucleus; it is located in the center of the cell.

Classification of muscles according to various criteria

The presence of various characteristics that are different from certain muscles allows them to be conditionally grouped according to a unifying characteristic. Today, anatomy does not have a single classification by which human muscles could be grouped. Types of muscles, however, can be classified according to various criteria, namely:

1. By shape and length.
2. According to the functions performed.
3. In relation to the joints.
4. By location in the body.
5. By belonging to certain parts of the body.
6. According to the location of muscle bundles.

Along with the types of muscles, three main muscle groups are distinguished depending on the physiological characteristics of the structure:

1. Cross-striated skeletal muscles.
2. Smooth muscles that make up the structure of internal organs and blood vessels.
3. Cardiac fibers.

The same muscle can simultaneously belong to several groups and types listed above, since it can contain several cross characteristics at once: shape, function, relation to a part of the body, etc.

Shape and size of muscle bundles

Despite the relatively identical structure of all muscle fibers, they can be of different sizes and shapes. Thus, the classification of muscles according to this criterion identifies:

1. Short muscles move small areas of the human musculoskeletal system and, as a rule, are located in the deep layers of the muscles. An example is the intervertebral spinal muscles.
2. Long ones, on the contrary, are localized on those parts of the body that perform large amplitudes of movement, for example, limbs (arms, legs).
3. Wide ones cover the main body (on the stomach, back, sternum). They can have different directions of muscle fibers, thereby providing a variety of contractile movements.

Various forms of muscles are also found in the human body: round (sphincter), straight, square, diamond-shaped, fusiform, trapezoidal, deltoid, serrated, single- and double-pinnate and other shapes of muscle fibers.

Types of muscles according to functions performed

Human skeletal muscles can perform various functions: flexion, extension, adduction, abduction, rotation. Based on this feature, muscles can be conditionally grouped as follows:

1. Extensors.
2. Flexors.
3. Leading.
4. Abductors.
5. Rotational.

The first two groups are always on the same part of the body, but in opposite directions in such a way that when the first ones contract, the second ones relax, and vice versa. The flexor and extensor muscles move the limbs and are antagonistic muscles. For example, the biceps brachii muscle flexes the arm, and the triceps brachii extends it. If, as a result of the work of muscles, a part of the body or organ makes a movement towards the body, these muscles are adductor, if in the opposite direction - abductor. Rotators provide circular movements of the neck, lower back, and head, while rotators are divided into two subtypes: pronators, which provide inward movement, and instep supports, which provide outward movement.

In relation to the joints

Muscles are attached to the joints by tendons, causing them to move. Depending on the type of attachment and the number of joints on which the muscles act, they can be single-joint or multi-joint. Thus, if the muscle is attached to only one joint, then it is a single-joint muscle, if it is attached to two, it is a two-joint muscle, and if there are more joints, it is a multi-joint muscle (finger flexors/extensors).

As a rule, single-joint muscle bundles are longer than multi-joint ones. They provide a more complete range of motion of the joint relative to its axis, since they spend their contractility on only one joint, while multi-joint muscles distribute their contractility over two joints. The latter types of muscles are shorter and can provide much less mobility while simultaneously moving the joints to which they are attached. Another property of multi-joint muscles is called passive insufficiency. It can be observed when, under the influence of external factors, the muscle is completely stretched, after which it does not continue to move, but, on the contrary, slows down.

Localization of muscles

Muscle bundles can be located in the subcutaneous layer, forming superficial muscle groups, or in deeper layers - these include deep muscle fibers. For example, the muscles of the neck consist of superficial and deep fibers, some of which are responsible for movements cervical region, while others pull back the skin of the neck, the adjacent area of ​​the skin of the chest, and also participate in turning and tilting the head. Depending on the location in relation to a particular organ, there may be internal and external muscles (external and internal muscles of the neck, abdomen).

Types of muscles by body part

In relation to body parts, muscles are divided into the following types:

1. The muscles of the head are divided into two groups: chewing muscles, responsible for the mechanical grinding of food, and facial muscles - types of muscles thanks to which a person expresses his emotions and mood.
2. The muscles of the body are divided into anatomical sections: cervical, pectoral (sternal major, trapezius, sternoclavicular), dorsal (rhomboid, latissimus dorsal, teres major), abdominal (internal and external abdominal, including the abs and diaphragm).
3. Muscles of the upper and lower extremities: brachialis (deltoid, triceps, biceps brachialis), elbow flexors and extensors, gastrocnemius (soleus), tibia, foot muscles.

Types of muscles according to the location of muscle bundles

The anatomy of muscles in different species may differ in the location of muscle bundles. In this regard, muscle fibers such as:

1. The feathery ones resemble the structure of a bird's feather; in them, bundles of muscles are attached to the tendons on only one side, and diverge on the other. The feathery shape of the arrangement of muscle bundles is characteristic of the so-called strong muscles. The place of their attachment to the periosteum is quite extensive. As a rule, they are short and can develop great strength and endurance, while the muscle tone will not differ greatly.
2. Muscles with parallel fascicles are also called dexterous. Compared to feathery ones, they are longer and less hardy, but can perform more delicate work. When contracting, the tension in them increases significantly, which significantly reduces their endurance.

Muscle groups by structural features

Clusters of muscle fibers form entire tissues, structural features which determines their conditional division into three groups:

Skeletal muscles - the active part of the musculoskeletal system, which also includes bones, ligaments, tendons and their joints. From a functional point of view, motor neurons that cause excitation of muscle fibers can also be classified as the motor system. The axon of a motor neuron branches at the entrance to the skeletal muscle, and each branch participates in the formation of the neuromuscular synapse on a separate muscle fiber.

A motor neuron, together with the muscle fibers it innervates, is called a neuromotor (or motor) unit (MU). In the eye muscles, one motor unit contains 13-20 muscle fibers, in the muscles of the trunk - 1 tons of fibers, in the soleus muscle - 1500-2500 fibers. Muscle fibers of one motor unit have the same morphofunctional properties.

Functions of skeletal muscles are: 1) movement of the body in space; 2) movement of body parts relative to each other, including the implementation of respiratory movements that provide ventilation of the lungs; 3) maintaining body position and posture. In addition, striated muscles are important in the production of heat, which maintains temperature homeostasis, and in the storage of certain nutrients.

Physiological properties of skeletal muscles highlight:

1)excitability. Due to the high polarization of the membranes of striated muscle fibers (90 mV), their excitability is lower than that of nerve fibers. Their action potential amplitude (130 mV) is greater than that of other excitable cells. This makes it quite easy to record the bioelectrical activity of skeletal muscles in practice. The duration of the action potential is 3-5 ms. This determines the short period of absolute refractoriness of muscle fibers;

conductivity. The speed of excitation along the muscle fiber membrane is 3-5 m/s;

contractility. Represents the specific property of muscle fibers to change their length and tension with the development of excitation.

Skeletal muscles also have elasticity and viscosity.

Modes and types of muscle contractions. Isotonic regime - the muscle shortens in the absence of an increase in its tension. Such a contraction is possible only for an isolated (removed from the body) muscle.

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

Auxotonic mode the muscle shortens and its tension increases. This reduction is most often observed when implementing labor activity person. Instead of the term "auxotonic mode" the name is often used concentric mode.

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

Single muscle contraction manifests itself as a result of the development of a single wave of excitation in muscle fibers. This can be achieved by applying a very short (about 1 ms) stimulus to the muscle. The development of a single muscle contraction is divided into a latent period, a shortening phase and a relaxation phase. Muscle contraction begins to appear 10 ms from the beginning of the stimulus. This time interval is called the latent period (Fig. 5.1). This will be followed by the development of shortening (duration about 50 ms) and relaxation (50-60 ms). It is believed that an average of 0.1 s is spent on the entire cycle of a single muscle contraction. But it should be borne in mind that the duration of a single contraction in different muscles can vary greatly. It also 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 period of single contraction include the muscles of the tongue and the muscles that close the eyelid.

Rice. 5.1. Temporal relationships between different manifestations of skeletal muscle fiber excitation: a - ratio of action potential, release of Ca 2+ into the sarcoplasm and contraction: / - latent period; 2 - shortening; 3 - relaxation; b - ratio of action potential, contraction and level of excitability

Under the influence of a single stimulus, an action potential first arises and only then does a period of shortening begin to develop. It continues after the end of repolarization. The restoration of the initial polarization of the sarcolemma also indicates the restoration of excitability. Consequently, against the background of developing contraction in muscle fibers, new waves of excitation can be caused, the contractile effect of which will be cumulative.

Tetanic contraction or tetanus called a muscle contraction that appears as a result of the occurrence of numerous waves of excitation in motor units, the contractile effect of which is summarized in amplitude and time.

There are serrated and smooth tetanus. To obtain dentate tetanus, it is necessary to stimulate the muscle with such a frequency that each subsequent impact is applied after the shortening phase, but before the end of relaxation. Smooth tetanus occurs with more frequent stimulation, when subsequent impacts are applied during the development of muscle shortening. For example, if the shortening phase of a muscle is 50 ms, and 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.

Despite

Amplitude abbreviations

relaxed

Pessimum

for ongoing irritation, muscle

30 Hz

1 Hz 7 Hz

200 Hz

50 Hz

Frequency of irritation

Rice. 5.2. Dependence of the contraction amplitude on the frequency of stimulation (the strength and duration of the stimuli are unchanged)

To demonstrate various types of tetanus, recording contractions of the isolated frog gastrocnemius muscle on a kymograph is usually used. An example of such a kymogram is shown in Fig. 5.2. The amplitude of a single contraction is minimal, increases with serrated tetanus and becomes maximum with smooth tetanus. One of the reasons for this increase in amplitude is that when frequent waves of excitation occur, Ca 2+ accumulates in the sarcoplasm of muscle fibers, stimulating the interaction of contractile proteins.

With a gradual increase in the frequency of stimulation, the strength and amplitude of muscle contraction increases only to a certain limit - optimal 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 pessimism of the response, and irritation frequencies exceeding the optimal value are pessimal. The phenomena of optimum and pessimum were discovered by N.E. Vvedensky.

When assessing the functional activity of muscles, they talk about their tone and phasic contractions. Muscle tone called a state of prolonged continuous tension. In this case, visible shortening of the muscle may be absent due to the fact that excitation does not occur in all, but only in some motor units of the muscle and they are not excited synchronously. Phasic muscle contraction called short-term shortening of the muscle, followed by its relaxation.

Structurally-functional characteristics of muscle fiber. The structural and functional unit of skeletal muscle is the muscle fiber, which is an elongated (0.5-40 cm long) multinucleated cell. The thickness of muscle fibers is 10-100 microns. Their diameter can increase with intense training loads, but the number of muscle fibers can increase only until 3-4 months of age.

The muscle fiber membrane is called sarcolemma, cytoplasm - sarcoplasm. The sarcoplasm contains nuclei, numerous organelles, the sarcoplasmic reticulum, which includes longitudinal tubules and their thickenings - cisterns that contain Ca 2+ reserves. The cisterns are adjacent to transverse tubules that penetrate the fiber in the transverse direction (Fig. 5.3).

In the sarcoplasm, about 2000 myofibrils (about 1 µm thick) run along the muscle fiber, which include filaments formed by the interweaving of contractile protein molecules: actin and myosin. Actin molecules form thin filaments (myofilaments) that lie parallel to each other and penetrate a kind of membrane called the Z-line or stripe. Z-lines are located perpendicular to the long axis of the myofibril and divide the myofibril into sections 2-3 µm long. These areas are called sarcomeres.

Sarcolemma Cistern

Transverse tube

Sarcomere

Tube s-p. ret^|

Jj3H ssss s_ z zzzz tccc ;

; zzzz sssss

z zzzz ssss s

j3333 CCSS£

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J3333 ss s s_

The sarcomere is shortened

3 3333 ssss s

The sarcomere is relaxed

Rice. 5.3. The structure of the muscle fiber sarcomere: Z-lines - limit the sarcomere,/! - anisotropic (dark) disk, / - isotropic (light) disk, H - zone (less dark)

The sarcomere is the contractile unit of the myofibril. In the center of the sarcomere, thick filaments formed by myosin molecules lie in a strictly ordered manner one above the other, and thin filaments of actin are similarly located at the edges of the sarcomere. The ends of the actin filaments extend between the ends of the myosin filaments.

The central part of the sarcomere (width 1.6 µm), in which the myosin filaments lie, appears dark under a microscope. This dark area can be traced across the entire muscle fiber, since the sarcomeres of neighboring myofibrils are located strictly symmetrically above each other. The dark areas of sarcomeres are called A-disks from the word “anisotropic.” These areas are birefringent in polarized light. The areas at the edges of the A-disc, where the actin and myosin filaments overlap, appear darker than in the center, where only the myosin filaments are located. This central area is called the H strip.

The areas of the myofibril in which only actin filaments are located do not exhibit birefringence; they are isotropic. Hence their name - I-discs. In the center of the I-disc there is a narrow dark line formed by the Z-membrane. This membrane keeps the actin filaments of two neighboring sarcomeres in an ordered state.

In addition to actin molecules, the actin filament also includes the proteins tropomyosin and troponin, which influence the interaction of actin and myosin filaments. The myosin molecule has sections called the head, neck and tail. Each such molecule has one tail and two heads with necks. Each head has a chemical center that can bind ATP and a site that allows it to bind to the actin filament.

During the formation of the myosin filament, myosin molecules are intertwined with their long tails, located in the center of this filament, and the heads are located closer to its ends (Fig. 5.4). The neck and head form a protrusion protruding from the myosin filaments. These projections are called cross bridges. They are mobile, and thanks to such bridges, myosin filaments can establish connections with actin filaments.

When ATP attaches to the head of the myosin molecule, the bridge is briefly positioned at an obtuse angle relative to the tail. At the next moment, partial cleavage of ATP occurs and due to this, the head rises and moves into an energized position, in which it can bind to the actin filament.

Actin molecules form a double helix Trolonin

ATF Communications Center

A section of a thin filament (tropomyosin molecules are located along the actin chains, trolonine is located at the nodes of the helix)

Neck

Tail

Tropomyoein ti

Myosin molecule at high magnification

Section of a thick filament (the heads of myosin molecules are visible)

Actin filament

Head

+Ca 2+

Sa 2+ "*Sa 2+

ADF-F

Sa 2+ N

Relaxation

Cycle of myosin head movements during muscle contraction

myosin 0 +ATP

Rice. 5.4. The structure of actin and myosin filaments, the movement of myosin heads during muscle contraction and relaxation. Explanation in the text: 1-4 - stages of the cycle

The mechanism of muscle fiber contraction. Excitation of skeletal muscle fibers under physiological conditions is caused only by impulses coming from motor neurons. The nerve impulse activates the neuromuscular synapse, causes the occurrence of PC.P, and the end plate potential ensures the generation of an action potential at the sarcolemma.

The action potential propagates both along the surface membrane of the muscle fiber and deeper along the transverse tubules. In this case, the cisterns of the sarcoplasmic reticulum are depolarized and Ca 2+ channels open. Since in the sarcoplasm the concentration of Ca 2+ is 1(G 7 -1(G b M, and in the tanks it is approximately 10,000 times greater), then when the Ca 2+ channels open, calcium along the concentration gradient leaves the tanks into the sarcoplasm and diffuses to myofilaments and triggers processes that ensure contraction. Thus, the release of Ca 2+ ions.

into the sarcoplasm is a factor that couples electrical skies and mechanical phenomena in muscle fiber. Ca 2+ ions bind to troponin and this, with the participation of tropomyo- zina, leads to the opening (unblocking) of actino sites howl filaments that can bind to myosin. After this, the energized myosin heads form bridges with actin, and the final breakdown of ATP previously captured and held by the myosin heads occurs. The energy obtained from the breakdown of ATP is used to rotate the myosin heads towards the center of the sarcomere. With this rotation, the myosin heads pull the actin filaments along with them, moving them between the myosin filaments. In one stroke, the head can advance the actin filament by -1% of the sarcomere length. For maximum contraction, repeated rowing movements of the heads are required. This occurs when there is a sufficient concentration of ATP and Sa 2+ in the sarcoplasm. For the myosin head to move again, a new ATP molecule must be attached to it. The addition of ATP causes a break in the connection between the myosin head and actin, and it momentarily takes its original position, from which it can move on to interact with a new section of the actin filament and make a new rowing movement.

This theory of the mechanism of muscle contraction was called theory of "sliding threads"

To relax the muscle fiber, it is necessary that the concentration of Ca 2+ ions in the sarcoplasm becomes less than 10 -7 M/l. This occurs due to the functioning of the calcium pump, which drives Ca 2+ from the sarcoplasm into the reticulum. In addition, for muscle relaxation, the bridges between the myosin heads and actin must be broken. This rupture occurs when ATP molecules are present in the sarcoplasm and bind to myosin heads. After the heads detach, elastic forces stretch the sarcomere and move the actin filaments to their original position. Elastic forces are formed due to: 1) elastic traction of spiral-shaped cellular proteins included in the structure of the sarcomere; 2) elastic properties of the membranes of the sarcoplasmic reticulum and sarcolemma; 3) elasticity of connective tissue of muscles, tendons and the effects of gravity.

Muscle strength. The strength of a muscle is determined by the maximum value of the load that it can lift, or by the maximum force (tension) that it can develop under conditions of isometric contraction.

A single muscle fiber is capable of developing a tension of 100-200 mg. There are approximately 15-30 million fibers in the body. If they acted in parallel in the same direction and at the same time, they could create a voltage of 20-30 tons.

Muscle strength depends on a number of morphofunctional, physiological and physical factors.

Muscle strength increases with increasing geometric and physiological cross-sectional area. To determine the physiological cross-section of a muscle, find the sum of the cross-sections of all muscle fibers along a line drawn perpendicular to the course of each muscle fiber.

In a muscle with parallel fibers (sartorius), the geometric and physiological cross sections are equal. In muscles with oblique fibers (intercostal) the physiological cross-section is larger than the geometric one and this helps to increase muscle strength. The physiological cross-section and strength of muscles with a pennate arrangement (most muscles of the body) of muscle fibers increases even more.

To be able to compare the strength of muscle fibers in muscles with different histological structures, the concept of absolute muscle strength was introduced.

Absolute muscle strength- the maximum force developed by the muscle, calculated per 1 cm 2 of physiological cross-section. Absolute strength of biceps - 11.9 kg/cm2, triceps brachii - 16.8 kg/cm2, gastrocnemius 5.9 kg/cm2, smooth muscle - 1 kg/cm2

The strength of a muscle depends on the percentage of different types of motor units that make up that muscle. The ratio of different types of motor units in the same muscle varies among people.

The following types of motor units are distinguished: a) slow, non-fatiguing (have a red color) - they have low strength, but can be in a state of tonic contraction for a long time without signs of fatigue; b) fast, easily fatigued (white in color) - their fibers have a great contraction force; c) fast, resistant to fatigue - have a relatively large force of contraction and fatigue develops slowly in them.

In different people, the ratio of the number of slow and fast motor units in the same muscle is determined genetically and can vary significantly. Thus, in the human quadriceps muscle, the relative content of copper fibers can vary from 40 to 98%. The greater the percentage of slow fibers in a person’s muscles, the more they are adapted to long-term, but low-power work. People with a high content of fast strong motor units are able to develop greater strength, but are prone to fatigue quickly. However, we must keep in mind that fatigue depends on many other factors.

The strength of a muscle increases with moderate stretching. This is due to the fact that with moderate stretching of the sarcomere (up to 2.2 μm), the number of bridges that can form between actin and myosin increases. When a muscle is stretched, elastic traction also develops in it, aimed at shortening. This thrust is added to the force developed by the movement of the myosin heads.

Muscle strength is regulated by the nervous system by changing the frequency of impulses sent to the muscle, synchronizing excitation large number motor units, selection of motor unit types. The strength of contractions increases: a) with an increase in the number of excited motor units involved in the response; b) with an increase in the frequency of excitation waves in each of the activated fibers; c) when synchronizing excitation waves in muscle fibers; d) upon activation of strong (white) motor units.

First (if it is necessary to develop a small effort), slow, non-fatiguing motor units are activated, then fast, resistant to fatigue. And if it is necessary to develop a force of more than 20-25% of the maximum, then fast, easily fatigued motor units are involved in the contraction.

At a voltage of up to 75% of the maximum possible, almost all motor units are activated and a further increase in strength occurs due to an increase in the frequency of impulses arriving at the muscle fibers.

With weak contractions, the frequency of impulses in the axons of motor neurons is 5-10 impulses/s, and with a strong contraction force it can reach up to 50 impulses/s.

IN childhood the increase in strength occurs mainly due to an increase in the thickness of muscle fibers, and this is associated with an increase in the number of myofibrils. The increase in the number of fibers is insignificant.

When training adult muscles, an increase in their strength is associated with an increase in the number of myofibrils, while an increase in endurance is due to an increase in the number of mitochondria and the intensity of ATP synthesis due to aerobic processes.

There is a relationship between force and speed of shortening. The greater the length of a muscle, the higher the speed of muscle contraction (due to the summation of the contractile effects of sarcomeres) and depends on the load on the muscle. As the load increases, the contraction speed decreases. A heavy load can only be lifted by moving slowly. The maximum contraction speed achieved during human muscle contraction is about 8 m/s.

The force of muscle contraction decreases as fatigue develops.

Fatigue and its physiological basis.Fatigue called a temporary decrease in performance, caused by previous work and disappearing after a period of rest.

Fatigue is manifested by a decrease in muscle strength, speed and accuracy of movements, changes in the performance of the cardiorespiratory system and autonomic regulation, and a deterioration in the functions of the central nervous system. The latter is evidenced by a decrease in the speed of simple mental reactions, weakening of attention, memory, deterioration of thinking indicators, and an increase in the number of erroneous actions.

Subjectively, fatigue can be manifested by a feeling of tiredness, muscle pain, palpitations, symptoms of shortness of breath, a desire to reduce the load or stop working. Symptoms of fatigue may vary depending on the type of work, the intensity of the work, and the degree of fatigue. If fatigue is caused by mental work, then, as a rule, symptoms of decreased functionality are more pronounced mental activity. With very heavy muscular work, symptoms of disorders at the level of the neuromuscular system may come to the fore.

Fatigue, which develops under conditions of normal work activity, both during muscular and mental work, has largely similar development mechanisms. In both cases, the processes of fatigue develop first in the nervous centers One indicator of this is a decrease in intelligence national performance with physical fatigue, and with mental fatigue - a decrease in efficiency we cervical activities.

Rest called a state of rest or performing a new activity, in which fatigue is eliminated and performance is restored. THEM. Sechenov showed that restoration of performance occurs faster if, when resting after fatigue of one muscle group (for example, the left arm), work is performed by another muscle group ( right hand). He called this phenomenon "active recreation"

Recovery are processes that ensure the elimination of shortages of energy and plastic substances, the reproduction of structures spent or damaged during work, the elimination of excess metabolites and deviations of homeostasis indicators from the optimal level.

The length of the period required to restore the body depends on the intensity and duration of the work. The greater the intensity of work, the shorter the period of rest required.

Various indicators of physiological and biochemical processes are restored after different times from the end of physical activity. One important test of recovery rate is to determine the time it takes for your heart rate to return to resting levels. The recovery time for heart rate after a moderate exercise test in a healthy person should not exceed 5 minutes.

With very intense physical activity fatigue phenomena develop not only in the central nervous system, but also in neuromuscular synapses, as well as muscles. In the system of the neuromuscular preparation, the nerve fibers have the least fatigue, the neuromuscular synapse has the greatest fatigue, and the muscle occupies an intermediate position. Nerve fibers can conduct high frequency action potentials for hours without signs of fatigue. With frequent activation of the synapse, the efficiency of excitation transmission first decreases, and then a blockade of its conduction occurs. This occurs due to a decrease in the supply of transmitter and ATP in the presynaptic terminal and a decrease in the sensitivity of the postsynaptic membrane to acetylcholine.

A number of theories have been proposed for the mechanism of development of fatigue in a very intensely working muscle: a) the theory of “exhaustion” - the consumption of ATP reserves and the sources of its formation (creatine phosphate, glycogen, fatty acids), b) the theory of “suffocation” - the lack of oxygen delivery comes first into the fibers of the working muscle; c) the “clogging” theory, which explains fatigue by the accumulation of lactic acid and toxic metabolic products in the muscle. It is currently believed that all these phenomena occur during very intense muscle work.

It has been established that maximum physical work before the development of fatigue is performed at an average level of difficulty and pace of work (the rule of average loads). In the prevention of fatigue, the following are also important: the correct ratio of periods of work and rest, alternation of mental and physical work, taking into account circadian, annual and individual biological rhythms.

Muscle power is equal to the product of muscle force and the rate of shortening. Maximum power develops at an average speed of muscle shortening. For the arm muscle, maximum power (200 W) is achieved at a contraction speed of 2.5 m/s.

5.2. Smooth muscle

Physiological properties and characteristics of smooth muscles.

Smooth muscles are an integral part of some internal organs and are involved in providing the functions performed by these organs. In particular, they regulate the patency of the bronchi for air, blood flow in various organs and tissues, the movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gall bladders), expel the fetus from the uterus, dilate or constrict the pupils (by contracting the radial or circular muscles of the iris), change the position of hair and skin relief. Smooth muscle cells are spindle-shaped, 50-400 µm long, 2-10 µm thick.

Smooth muscles, like skeletal muscles, have excitability, conductivity and contractility. Unlike skeletal muscles, which have elasticity, smooth muscles are plastic (able to maintain the length given to them by stretching for a long time without increasing tension). This property is important for performing the function of depositing food in the stomach or liquids in the gall and bladder.

Peculiarities excitability smooth muscle fibers are to a certain extent associated with their low transmembrane potential (E 0 = 30-70 mV). Many of these fibers are automatic. The duration of their action potential can reach tens of milliseconds. This happens because the action potential in these fibers develops mainly due to the entry of calcium into the sarcoplasm from the intercellular fluid through the so-called slow Ca 2+ channels.

Speed carrying out the initiation in smooth muscle cells small - 2-10 cm/s. Unlike skeletal muscles, excitation in smooth muscle can be transmitted from one fiber to another nearby. This transmission occurs due to the presence of nexuses between smooth muscle fibers, which have low resistance to electric current and ensure the exchange between cells of Ca 2+ and other molecules. As a result, smooth muscle has the properties of functional syncytium.

Contractility smooth muscle fibers are distinguished by a long latent period (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles have a low contractile force, but are able to remain in tonic contraction for a long time without developing fatigue. This is due to the fact that smooth muscle spends 100-500 times less energy to maintain tetanic contraction than skeletal muscle. Therefore, the ATP reserves consumed by smooth muscle have time to be restored even during contraction, and the smooth muscles of some body structures are in a state of tonic contraction throughout their lives.

Conditions for smooth muscle contraction. The most important feature of smooth muscle fibers is that they are excited under the influence of numerous stimuli. Normal skeletal muscle contraction is initiated only by a nerve impulse arriving at the neuromuscular junction. Contraction of smooth muscle can be caused by both nerve impulses and biologically active substances (hormones, many neurotransmitters, prostaglandins, some metabolites), as well as the influence of physical factors, such as stretching. In addition, excitation of smooth muscle can occur spontaneously - due to automation.

The very high reactivity of smooth muscles and their ability to respond with contraction to the action of various factors create significant difficulties for correcting disturbances in the tone of these muscles in medical practice. This can be seen in the examples of the treatment of bronchial asthma, arterial hypertension, spastic colitis and other diseases that require correction of the contractile activity of smooth muscles.

The molecular mechanism of smooth muscle contraction also has a number of differences from the mechanism of skeletal muscle contraction. The filaments of actin and myosin in smooth muscle fibers are located less orderly than in skeletal fibers, and therefore smooth muscle does not have cross-striations. Smooth muscle actin filaments do not contain the protein troponin, and actin molecular centers are always open to interact with myosin heads. For this interaction to occur, ATP molecules must be broken down and phosphate transferred to the myosin heads. Then the myosin molecules are woven into filaments and bind with their heads to the myosin. This is followed by the rotation of the myosin heads, during which the actin filaments are pulled between the myosin filaments and contraction occurs.

Phosphorylation of myosin heads is carried out using the enzyme myosin light chain kinase, and dephosphorylation is carried out by myosin light chain phosphatase. If myosin phosphatase activity prevails over kinase activity, the myosin heads are dephosphorylated, the myosin-actin bond is broken, and the muscle relaxes.

Therefore, for smooth muscle contraction to occur, an increase in the activity of myosin light chain kinase is necessary. Its activity is regulated by the level of Ca 2+ in the sarcoplasm. When the smooth muscle fiber is excited, the calcium content in its sarcoplasm increases. This increase is due to the intake of Ca^+ from two sources: 1) intercellular space; 2) sarcoplasmic reticulum (Fig. 5.5). Next, Ca 2+ ions form a complex with the protein calmodulin, which converts myosin kinase into an active state.

The sequence of processes leading to the development of smooth muscle contraction: Ca 2 entry into the sarcoplasm - acti

calmodulin activation (by formation of the 4Ca 2+ - calmodulin complex) - activation of myosin light chain kinase - phosphorylation of myosin heads - binding of myosin heads to actin and rotation of the heads, in which actin filaments are pulled between myosin filaments.

Conditions necessary for smooth muscle relaxation: 1) decrease (to 10 M/l or less) Ca 2+ content in the sarcoplasm; 2) disintegration of the 4Ca 2+ -calmodulin complex, leading to a decrease in the activity of myosin light chain kinase - dephosphorylation of myosin heads, leading to the rupture of bonds between actin and myosin filaments. After this, elastic forces cause a relatively slow restoration of the original length of the smooth muscle fiber and its relaxation.

Test questions and assignments

Cell membrane

Rice. 5.5. Scheme of the pathways of Ca 2+ entry into the sarcoplasm of smooth muscle-

of the cell and its removal from the plasma: a - mechanisms that ensure the entry of Ca 2+ into the sarcoplasm and the initiation of contraction (Ca 2+ comes from the extracellular environment and the sarcoplasmic reticulum); b - ways to remove Ca 2+ from the sarcoplasm and ensure relaxation

The influence of norepinephrine through α-adrenergic receptors

Ligand-dependent Ca 2+ channel

Leakage channels

Potential dependent Ca 2+ channel

Smooth muscle cell

a-adreno! receptorfNorepinephrineG

Name the types of human muscles. What are the functions of skeletal muscles?

Describe the physiological properties of skeletal muscles.

What is the relationship between action potential, contraction and excitability of a muscle fiber?

What modes and types of muscle contractions exist?

Give the structural and functional characteristics of muscle fiber.

What are motor units? List their types and features.

What is the mechanism of muscle fiber contraction and relaxation?

What is muscle strength and what factors influence it?

What is the relationship between the force of contraction, its speed and work?

Define fatigue and recovery. What are their physiological basis?

What are the physiological properties and characteristics of smooth muscles?

List the conditions for contraction and relaxation of smooth muscle.

Muscles are one of the main components of the body. They are based on tissue whose fibers contract under the influence of nerve impulses, allowing the body to move and stay in its environment.

Muscles are located in every part of our body. And even if we don’t know about their existence, they still exist. It’s enough, for example, to go to the gym or do aerobics for the first time - the next day even those muscles that you didn’t even know you had will start to ache.

They are responsible not only for movement. At rest, muscles also require energy to maintain their tone. This is necessary so that at any moment a certain one can respond to a nerve impulse with the appropriate movement, and does not waste time on preparation.

To understand how muscles are structured, we suggest remembering the basics, repeating the classification and looking into the cellular. We will also learn about diseases that can worsen their function, and how to strengthen skeletal muscles.

General concepts

According to their filling and the reactions that occur, muscle fibers are divided into:

• striated;
• smooth.

Skeletal muscles are elongated tubular structures, the number of nuclei in one cell can reach several hundred. They consist of muscle tissue that is attached to various parts bone skeleton. Contractions of striated muscles contribute to human movements.

Varieties of forms

How are muscles different? The photos presented in our article will help us figure this out.

Skeletal muscles are one of the main components of the musculoskeletal system. They allow you to move and maintain balance, and are also involved in the process of breathing, voice production and other functions.

There are more than 600 muscles in the human body. As a percentage, their total mass is 40% of the total body mass. Muscles are classified by shape and structure:

• thick fusiform;
• thin lamellar.

Classification makes learning easier

The division of skeletal muscles into groups is carried out depending on their location and significance in the activity of various organs of the body. Main groups:

Muscles of the head and neck:

• facial expressions - are used when smiling, communicating and creating various grimaces, while ensuring the movement of the constituent parts of the face;
• chewing - promote a change in the position of the maxillofacial region;
• voluntary muscles of the internal organs of the head (soft palate, tongue, eyes, middle ear).

Skeletal muscle groups of the cervical spine:

• superficial - promote inclined and rotational movements of the head;
• middle ones - create the lower wall of the oral cavity and promote downward movement of the jaw and laryngeal cartilages;
• deep ones tilt and turn the head, create elevation of the first and second ribs.

The muscles, photos of which you see here, are responsible for the torso and are divided into muscle bundles of the following sections:

• thoracic - activates top part torso and arms, and also helps to change the position of the ribs during breathing;
• abdominal section - allows blood to move through the veins, changes the position of the chest during breathing, affects the functioning of the intestinal tract, promotes flexion of the torso;
• dorsal - creates the motor system of the upper limbs.

Muscles of the limbs:

• upper - consist of muscle tissue shoulder girdle and free upper limb, help move the arm in the shoulder joint capsule and create movements of the wrist and fingers;
• lower - play the main role in a person’s movement in space, are divided into the muscles of the pelvic girdle and the free part.

Structure of skeletal muscle

In its structure, it has a huge number of oblong shapes with a diameter of 10 to 100 microns, their length ranges from 1 to 12 cm. Fibers (microfibrils) are thin - actin, and thick - myosin.

The former consist of a protein that has a fibrillar structure. It's called actin. Thick fibers are composed of different types of myosin. They differ in the time it takes to decompose the ATP molecule, which causes different contraction rates.

Myosin in smooth muscle cells is dispersed, although there is a large amount of protein, which, in turn, is significant in prolonged tonic contraction.

The structure of skeletal muscle is similar to a rope or stranded wire woven from fibers. It is surrounded on top by a thin sheath of connective tissue called the epimysium. From its inner surface, deeper into the muscle, thinner branches of connective tissue extend, creating septa. They are “wrapped” with individual bundles of muscle tissue, which contain up to 100 fibrils in each. Narrower branches extend from them even deeper.

Blood and nervous system. The arterial vein runs along the perimysium - this is the connective tissue covering the bundles of muscle fibers. Arterial and venous capillaries are located nearby.

Development process

Skeletal muscles develop from the mesoderm. Somites are formed on the side of the neural groove. After time, myotomes are released into them. Their cells, taking on a spindle shape, evolve into myoblasts, which divide. Some of them progress, while others remain unchanged and form myosatellite cells.

A small part of myoblasts, due to the contact of the poles, creates contact with each other, then the plasma membranes disintegrate in the contact zone. Thanks to the fusion of cells, symplasts are created. Undifferentiated young muscle cells move to them, being in the same environment with the myosymplast of the basement membrane.

Functions of skeletal muscles

This muscle is the basis of the musculoskeletal system. If it is strong, it is easier to maintain the body in the desired position, and the likelihood of stooping or scoliosis is minimized. Everyone knows about the benefits of playing sports, so let’s look at the role that muscles play in this.

The contractile tissue of skeletal muscles performs many functions in the human body. various functions, which are needed for the correct positioning of the body and its interaction individual parts together.

Muscles perform the following functions:

• create body mobility;
• protect the thermal energy created inside the body;
• promote movement and vertical retention in space;
• promote contraction of the airways and help with swallowing;
• form facial expressions;
• promote heat production.

Ongoing support

When muscle tissue is at rest, there is always a slight tension in it, called muscle tone. It is formed due to minor impulse frequencies that enter the muscles from the spinal cord. Their action is determined by signals penetrating from the head to the spinal motor neurons. Muscle tone also depends on their general condition:

• sprains;
• level of filling of muscle cases;
• blood enrichment;
• general water and salt balance.

A person has the ability to regulate the level of muscle load. As a result of prolonged physical exercise or severe emotional and nervous stress, muscle tone involuntarily increases.

Skeletal muscle contractions and their types

This function is the main one. But even it, despite its apparent simplicity, can be divided into several types.

Types of contractile muscles:

• isotonic - the ability of muscle tissue to shorten without changes in muscle fibers;
• isometric - during the reaction, the fiber contracts, but its length remains the same;
• auxotonic - the process of contraction of muscle tissue, where the length and tension of the muscles are subject to changes.

Let's look at this process in more detail.

First, the brain sends an impulse through a system of neurons, which reaches the motor neuron adjacent to the muscle bundle. Next, the efferent neuron is innervated from the synoptic vesicle, and a neurotransmitter is released. It binds to receptors on the sarcolemma of the muscle fiber and opens a sodium channel, which leads to depolarization of the membrane causing, in sufficient quantities, the neurotransmitter to stimulate the production of calcium ions. It then binds to troponin and stimulates its contraction. This, in turn, pulls back tropomeasesin, allowing actin to combine with myosin.

Next, the process of actin filament sliding relative to the myosin filament begins, resulting in skeletal muscle contraction. A schematic diagram will help you understand the process of compression of striated muscle bundles.

How skeletal muscles work

The interaction of a large number of muscle bundles contributes to various movements torso.

The work of skeletal muscles can occur in the following ways:

• synergistic muscles work in one direction;
• Antagonist muscles promote opposite movements to produce tension.

The antagonistic action of muscles is one of the main factors in the activity of the musculoskeletal system. When performing any action, not only the muscle fibers that perform it, but also their antagonists are included in the work. They promote resistance and give the movement concreteness and grace.

When acting on a joint, striated skeletal muscle performs difficult work. Its character is determined by the location of the joint axis and the relative position of the muscle.

Some functions of skeletal muscle are poorly understood and often not discussed. For example, some of the bundles act as a lever for the operation of the bones of the skeleton.

Muscle work at the cellular level

The action of skeletal muscles is carried out by two proteins: actin and myosin. These components have the ability to move relative to each other.

For muscle tissue to work, it is necessary to expend energy contained in chemical bonds organic compounds. The breakdown and oxidation of such substances occurs in the muscles. There is always air present here, and energy is released, 33% of all this is spent on the performance of muscle tissue, and 67% is transferred to other tissues and spent on maintaining a constant body temperature.

Diseases of the skeletal muscles

In most cases, deviations from the norm in muscle functioning are due to the pathological state of the responsible parts of the nervous system.

The most common pathologies of skeletal muscles:

• Muscle cramps are an imbalance of electrolyte in the extracellular fluid surrounding muscle and nerve fibers, as well as changes in osmotic pressure in it, especially its increase.
• Hypocalcemic tetany is an involuntary tetanic contraction of skeletal muscle observed when the extracellular Ca2+ concentration falls to approximately 40% of normal levels.
• characterized by progressive degeneration of skeletal muscle fibers and myocardium, as well as muscle disability, which can lead to death due to respiratory or cardiac failure.
• Myasthenia gravis is a chronic autoimmune disease in which antibodies to the nicotinic ACh receptor are formed in the body.

Relaxation and restoration of skeletal muscles

Proper nutrition, lifestyle and regular exercise will help you become the owner of healthy and beautiful skeletal muscles. It is not necessary to exercise and build up muscle mass. Regular cardio training and yoga are enough.

Do not forget about the mandatory intake of essential vitamins and minerals, as well as regular visits to saunas and baths with brooms, which allow you to enrich muscle tissue and blood vessels with oxygen.

Systematic relaxing massages will increase the elasticity and reproduction of muscle bundles. Visiting a cryosauna also has a positive effect on the structure and functioning of skeletal muscles.