Molecular mechanisms of muscle contraction. Mechanism of skeletal muscle contraction

12.10.2019

Consider what the ideas about the mechanism of alternating muscle contraction and relaxation come down to. It is currently accepted that the biochemical cycle of muscle contraction consists of 5 stages (Fig. 20.8):

1) the myosin "head" can hydrolyze to and H 3 PO 4 (P i), but does not provide the release of products. Therefore, this process is more stoichiometric than catalytic in nature (see Fig. 20.8, a);

According to modern concepts, in a resting muscle (in myofibrils and interfibrillar space) Ca 2+ is maintained below the threshold value as a result of their binding by structures (tubules and vesicles) of the sarcoplasmic reticulum and the so-called T-system with the participation of a special Ca 2+ -binding, called calsequestrin, which is part of these structures.

The ability of a living muscle to stay in a relaxed state if it contains a sufficiently high level is explained by the decrease in Ca 2+ in the environment surrounding myofibrils as a result of the action of the calcium pump, below the limit at which the manifestation of ATPase and contractility of acto-myosin fiber structures are still possible. Rapid contraction of a muscle fiber when irritated by a nerve (or electric shock) is the result of a sudden change in permeability and, as a consequence, the release of a certain amount of Ca 2+ from the cisterns and tubules of the sarcoplasmic reticulum and the T-system into the sarcoplasm.

As noted, the “sensitivity” of the actomyosin system to Ca 2+ (i.e., the loss of the ability to split and contract in the presence of a decrease in Ca 2+ to 10–7 M) is due to the presence in the contractile system (on the filaments of F-actin)

The control of a muscle, consisting of a significant number of motor units (MU), is carried out by a set of motor neurons innervating the muscle, called the motor neuron pool (MP). It is known that motor neurons that send their axons to a particular muscle can be located not only within one segment of the spinal cord, but also occupy neighboring ones. Thus, the MP can be structurally separated over a fairly large distance within several segments of the anterior horns of the spinal cord. Functionally, the MT is the final instance, where the structure of command signals to the muscle is formed, ensuring its inclusion in the motor act. It is on the MP that the integration of input actions on motoneurons from suprasegmental structures and from receptors of the motor periphery is carried out.

The main function of the MP - dosing the force of muscle contraction - is provided in two ways - by the frequency of impulses of the motor neurons included in it and the number of activated motor neurons of this pool.

Rice. 34. Structure of a thin filament.

50 ms after an arbitrary internal command, skeletal muscle contraction begins. During this time, the command is transmitted from the cortex to the motor neurons of the spinal cord and along the motor fibers to the muscle. The mediator at the neuromuscular junction is acetylcholine, which is contained in the synaptic vesicles of the presynapse. The nerve impulse causes the emptying of the synaptic vesicles and the release of acetylcholine into the synaptic cleft. There, the neurotransmitter acts on the postsynaptic receptor, after which it is destroyed. As the reserves of acetylcholine are consumed, they are replenished by synthesis in the presynaptic membrane, but if the impulses are frequent and long, then the consumption exceeds the replenishment and the conduction of excitation through the neuromuscular synapse is disturbed. As a result comes fatigue.

Muscle contraction is the result of contraction of its constituent muscle cells (fibers). The contraction of a muscle fiber is the result of the shortening of each of its sarcomeres due to the interaction of thin and thick filaments.

V resting muscle thin filaments are in contact with the Z-lines and do not reach the center of the sarcomere, while thick filaments are in the center, but do not reach the Z-line. Only on the sides of the A-disk, thin filaments slightly enter the space between the thick filaments (Fig. 33, B).

At moderate effort going on moderate reduction due to the fact that thin filaments move towards each other, so the distance between the Z - lines decreases and the length of the muscle decreases.

At the maximum reduction thick filaments touch the Z-lines.

Physiological and biochemical mechanism of contraction consists in the fact that after the interaction of the mediator with the receptor, with a sufficient frequency of nerve impulses, the muscle membrane develops muscular action potential , which quickly spreads along the muscle fiber, causing the release of Ca 2+ from the sarcoplasmic reticulum. Then Ca 2+ penetrates into the myofibrils to the binding sites on the troponin molecule. in resting muscle Tropomyosin prevents the myosin head from attaching to the nearest actin monomer. Binding of Ca to troponin changes its spatial structure, which weakens the connection between its tropomyosin-binding subunit and actin. As a result, the tropomyosin molecule begins to move along the groove of the thin filament, releasing the previously hidden myosin-binding center on the surface of the actin molecule. The interaction of actin with myosin begins, which brings together molecules belonging to thin and thick filaments. As a result, the distance between Z-lines decreases. When binding actin and myosin, the ATP molecule breaks down into ADP and inorganic phosphorus. The breakdown of ATP leads to relaxation muscle due to changes in myosin conformation. To restore the ability to contract, the next ATP molecule must attach to the myosin head. The whole process from the appearance of muscle potential to the contraction of the muscle fiber is called electromechanical connection (coupling). The contraction of smooth muscles has a feature: Ca 2+ binds to a specific protein - calmodulin.

Russian State University Physical Education Sports and Tourism

Physiology

On the topic: "The mechanism of muscle contraction."

Work completed:

Student of the 2nd year of the 1st group

Institute of Recreation and Tourism

Sankova Irina

Moscow, 2008

Structural organization muscle fiber .................................................................. ......................... 3

Mechanism of muscle contraction .................................................................. .............................................. 4

Modes of muscle contraction .................................................................. ................................................. 5

Muscle work and power .............................................................. ................................................. .......... 7

Energy of muscle contraction .............................................................. ........................................... eight

Heat generation during muscle contraction .............................................................. .......................... 9

Musculoskeletal interaction ............................................................... ........................................... 9

Ergometric methods .................................................................. ................................................. ......... eleven

Electromyographic methods .................................................................. .............................................. eleven

Physiological properties of muscles .............................................................. ............................................. 14

Relaxation of the skeletal muscle .............................................................. ................................................ 14

Conjugation of excitation and contraction in skeletal muscle .............................................................. .... 15

Functions and types of muscle tissue .............................................................. .............................................. sixteen

Bibliography:............................................... ................................................. .................. twenty

A muscle fiber is a multinuclear structure surrounded by a membrane and containing a specialized contractile apparatus - myofibrils. In addition, the most important components of the muscle fiber are mitochondria, a system of longitudinal tubules - the sarcoplasmic reticulum (reticulum) and a system of transverse tubules - the T-system. The functional unit of the contractile apparatus of the muscle cell is the sarcomere (Fig. 2.20, A); The myofibril is made up of sarcomeres. Sarcomeres are separated from each other by Z-plates. Sarcomeres in the myofibril are arranged in series, so the contraction of sarcomeres causes contraction of the myofibril and an overall shortening of the muscle fiber.

The study of the structure of muscle fibers in a light microscope made it possible to reveal their transverse striation. Electron microscopic studies have shown that the transverse striation is due to the special organization of the contractile proteins of myofibrils - actin (molecular weight 42,000) and myosin (molecular weight about 500,000). 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. In the longitudinal grooves of the actin helix are filamentous molecules of the protein tropomyosin. With a step of 40 nm, a molecule of another protein, troponin, is attached to the tropomyosin molecule. Troponin and tropomyosin play an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this region is visible as a dark stripe (due to birefringence) - an anisotropic A-disk. A lighter H-stripe is visible in its center. There are no actin filaments in it at rest. On both sides of the A-disk, light isotropic stripes are visible - 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. Electron microscopy revealed an M-line in the center of the H-band, a structure that holds the myosin filaments. On a cross section of a muscle fiber, you can see the hexagonal organization of the myofilament: each myosin filament is surrounded by six actin filaments (Fig. 2.20, B).

Electron microscopy shows that protrusions called transverse bridges are found on the sides of the myosin filament. They are oriented with respect to the axis of the myosin filament at an angle of 120°. 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 microelectrode technique in combination with interference microscopy made it possible to establish that the application of electrical stimulation to the area of the Z-plate leads to a contraction of the sarcomere, while the size of the disk A zone does not change, and the size of the H and I bands decreases. These observations indicated that the length of myosin filaments does not change. Similar results were obtained during muscle stretching - the proper length of actin and myosin filaments did not change. As a result of these experiments, it turned out that the region of mutual overlap of actin and myosin filaments changed. These facts allowed N. Huxley and A. Huxley to independently propose the theory of filament sliding to explain the mechanism of muscle contraction. According to this theory, during contraction, a decrease in the size of the sarcomere occurs due to the active movement of thin actin filaments relative to thick myosin filaments. At present, many details of this mechanism have been clarified, and the theory has received experimental confirmation.

1) the myosin "head" can hydrolyze ATP to ADP and H3PO4 (Pi), but does not ensure the release of hydrolysis products. Therefore, this process is more stoichiometric than catalytic in nature (see Fig. 3, a);

3) this interaction ensures the release of ADP and H3PO4 from the actin-myosin complex. The actomyosin bond has the lowest energy at an angle of 45°; therefore, the angle of myosin with the fibril axis changes from 90° to 45° (approximately) and actin advances (by 10–15 nm) towards the center of the sarcomere (see Fig. 3, c );

4) a new ATP molecule binds to the myosin–F-actin complex (see Fig. 3d);

5) the myosin-ATP complex has a low affinity for actin, and therefore the separation of the myosin (ATP) “head” from F-actin occurs. The last stage is actually relaxation, which clearly depends on the binding of ATP to the actin-myosin complex (see Fig. 3e). Then the cycle resumes.

These methods for studying human skeletal muscles have found wide application in physiological and clinical practice. Depending on the objectives of the study, registration and analysis of the total electromyogram (EMG) or the potentials of individual muscle fibers are carried out. When registering the total EMG, skin electrodes are more often used, when registering the potentials of individual muscle fibers, multichannel needle electrodes are used.

The advantage of total voluntary effort electromyography is the non-invasiveness of the study and, as a rule, the absence of electrical stimulation of muscles and nerves. On fig. 2.28 shows the EMG of the muscle at rest and with an arbitrary effort. Quantitative analysis of EMG consists in determining the frequencies of EMG waves, conducting spectral analysis, estimating the average amplitude of EMG waves. One of the common methods for analyzing EMG is its integration, since it is known that the value of the integrated EMG is proportional to the value of the developed muscle effort.

Using needle electrodes, it is possible to record both the total EMG and the electrical activity of individual muscle fibers. The electrical activity recorded in this case is largely determined by the distance between the discharge electrode and the muscle fiber. Criteria for assessing the parameters of individual potentials of a healthy and sick person have been developed. On fig. 2.29 shows a record of the potential of a human motor unit.

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

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

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

Sarcoplasmic reticulum

The sarcoplasmic reticulum [SR (SR)] is a branched endoplasmic reticulum-like organelle that surrounds individual myofibrils like a grid (the SR of a heart muscle cell is shown as an example at the top of the diagram). In resting cells, the concentration of Ca2+ is very low (less than 10-5 M). However, in the sarcoplasmic reticulum, the level of Ca2+ ions is significantly higher (about 10-3 M). The high concentration of Ca2+ in SR is maintained by Ca2+-ATPases. In addition, SR contains a special protein calsequestrin (55 kDa), which, due to the high content of acidic amino acids, is able to firmly bind Ca2+ ions.

The transfer of the action potential to the SR of an individual myofibril is facilitated by the transverse tubules of the T-system, which are tubular invaginations of the cell membrane and are in close contact with individual myofibrils. Depolarization of the plasma membrane is transmitted via T-tubules to a voltage-gated membrane protein (the so-called "SR-foot") of the adjacent SR membrane, which opens Ca2+ channels. The result is the release of Ca2+ ions from the SR into the space between the actin and myosin filaments to a level of ≥10-5 M. Ultimately, the release of Ca2+ ions is the trigger for the process of myofibril contraction.

V . Regulation by calcium ions

In relaxed skeletal muscle, the complex of troponin (subunits = T, C, I) with tropomyosin prevents the interaction of myosin heads with actin.

A rapid increase in the concentration of calcium ions in the cytoplasm as a result of the opening of SR channels leads to the binding of Ca2+ to the C-subunit of troponin. The latter is close in properties to calmodulin (see Fig. 375). The binding of Ca2+ ions causes a conformational rearrangement in troponin, the troponin tropomyosin complex is destroyed and releases the myosin binding site on the actin molecule (highlighted in red in the diagram). This initiates a cycle of muscle contraction (see p. 324)

In the absence of subsequent stimulation, ATP-dependent calcium pumps in the SR membrane rapidly reduce the concentration of Ca2+ ions to the initial level. As a result, the Ca2+ complex with troponin C dissociates, troponin restores its original conformation, the myosin binding site on actin is blocked, and the muscle relaxes.

Thus, during the contraction of the muscle fiber of the skeletal muscles of vertebrates, the following sequence of events occurs. When a signal is received from a motor neuron, the muscle cell membrane depolarizes, the signal is transmitted to the Ca2+ channels of the SR. Ca2+ channels open, the intracellular level of Ca2+ ions increases. The Ca2+ ion binds to troponin C, causing a conformational rearrangement in troponin, which leads to the destruction of the troponin-tropomyosin complex and allows myosin heads to bind to actin. The actin-myosin cycle is initiated.

Upon completion of the contraction, the level of Ca2+ ions decreases due to the active reverse transport of Ca2+ to the SR, troponin C gives up Ca2+, the troponin-tropomyosin complex takes its original position on the actin molecule, blocking the actin-myosin cycle. The result is muscle relaxation.

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 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		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. A contracture is a persistent prolonged contraction of a muscle that 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. At rest, the filaments do not slide in the myofibril, since the binding centers on the actin surface are closed by tropomyosin protein molecules (Fig. 1 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. The excitation (action potential) propagates along the myofibril membrane and reaches the sarcoplasmic reticulum through a 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. 1, C). Ca++ ions bind to the protein troponin. Troponin changes its conformation and displaces tropomyosin protein molecules that closed the actin binding centers (Fig. 1D). Myosin heads attach to the opened binding sites, and the contraction process begins (Fig. 1e). 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 the latent period of contraction. Muscle tissue makes up 40% of a person's body weight. Biochemical processes in muscles big influence to the entire human body. Muscle function- and constant temperature. No artificial mechanism is capable of this. Mechanical motion in which chemical energy is converted into mechanical energy at constant pressure. Cross-striped muscles. The functional unit is the sarcomere. Thick thread. Consists of myosin protein molecules. Myosin is a large oligomeric protein, molecular weight 500 kDa, consists of 6 subunits, identical in pairs. Heavy chain: at the C-terminus - a helix, at the N-terminus - a globule. When connecting two heavy chains with C-terminal sections, a supercoil is formed. Two light chains are part of the globule (head). The core section of the supercoil has 2 sections, where the spirals are bare - these places are open to the action of proteolytic enzymes and have increased mobility. properties of myosin. Under physiological conditions (optimal pH, temperature, salt concentrations), myosin molecules spontaneously interact with each other with their rod sections ("end to end", "side to side") using weak types of bonds. Only the rods interact, the heads remain free. The myosin molecule has enzymatic activity (ATPase activity: ATP + H2O -> ADP + F). The active centers are located on the heads of myosin. Stages of an enzymatic reaction. 1st stage Sorption of the substrate. During this stage, ATP is fixed on the adsorption site of the active center of the myosin head. 2nd stage hydrolysis of ATP. Occurs on the catalytic site of the active center of the head. The hydrolysis products (ADP and F) remain fixed, and the released energy accumulates in the head. Note: Pure myosin in vitro has ATPase activity, but it is very low. 3rd stage Myosin is able to interact with the actin of thin filaments. Attachment of actin to myosin increases the ATPase activity of myosin, as a result, the rate of ATP hydrolysis increases by 200 times. It is the 3rd stage of catalysis that is accelerated. Release of reaction products (ADP and P) from the active site of the myosin head. Note: Pure myosin has enzymatic activity, but it is very low. Myosin, with its heads, is able to interact with actin (actin-contractile protein), which is part of thin filaments. Attachment of actin to myosin instantly increases the ATP-ase activity of myosin (more than 200 times). Actin is an allosteric activator of myosin. Thin threads. Thin filaments are composed of three proteins: contractile protein actin; regulatory protein tropomyosin; troponin regulatory protein. Actin is a small globular protein with a molecular weight of 42 kDa. G-actin is a globule. Under physiological conditions, its molecules are capable of spontaneous aggregation, forming F-actin. The thin filament contains two F-actin filaments, forming a superhelix (2 twisted filaments). In the area of Z-lines, actin is attached to a-actinin. mechanism of muscle contraction. The affinity of the myosin-ATP complex for actin is very low. The affinity of the myosin-ADP complex for actin is very high. Actin accelerates the cleavage of ADP and F from myosin and, in this case, a conformational rearrangement occurs - the rotation of the myosin head. 1st stage ATP fixation on the myosin head. 2nd stage hydrolysis of ATP. The hydrolysis products (ADP and F) remain fixed, and the released energy accumulates in the head. The muscle is ready to contract. 3rd stage Formation of the actin-myosin complex. He is very durable. It can be destroyed only by the sorption of a new ATP molecule. 4th stage Conformational changes in the myosin molecule that result in rotation of the myosin head. Release of reaction products (ADP and P) from the active site of the myosin head. The heads of myosin "work" in cycles, like the fins of a fish or the oars of a boat, which is why this process is called the "oar mechanism" of muscle contraction. Researcher Györgyi was the first to isolate pure actin and myosin. In vitro, the necessary physiological conditions were created, under which spontaneous formation of thick and thin filaments was observed, then ATP was added - muscle contraction occurred in the test tube. Regulation of muscle contraction. Tropomyosin. Fibrillar protein, molecular weight - 70 kDa. It has the form of an a-helix. In a thin filament, there are 7 G-actin molecules per tropomyosin molecule. Tropomyosin is located in the groove between the two helices of G-actin. Connects tropomyosin "end to end", the chain is continuous. The tropomyosin molecule closes the active binding sites of actin on the surface of actin globules. Troponin. Globular protein, molecular weight 80 kDa, has 3 subunits: troponin "T", troponin "C" and troponin "I". It is located on tropomyosin with equal intervals, the length of which is equal to the length of the tropomyosin molecule. Troponin T (TnT) - is responsible for the binding of troponin to tropomyosin, through troponin "T" conformational changes in troponin are transmitted to tropomyosin. Troponin C (TnC) - Ca2+-binding subunit, contains 4 sites for calcium binding, similar in structure to calmodulin protein. Troponin I (TnI) - inhibitory subunit - is not a true inhibitor - it only creates a spatial obstacle that interferes with the interaction of actin 1) http://www.bibliotekar.ru/447/index.htm 2) http://www.bio.bsu.by/phha/index.htm 3) www.xumuk.ru/ biologhim/306.html 4) www.scienceandapologetics.org/text/202_2.htm 5) http://yanko.lib.ru/books/biolog/nagl_biochem/326.htm 6) http://www.4medic.ru/page.php?id=116 7) http://physiolog.spb.ru/tema6.html 8) http://www.hameleon.su/2008_034_136_med.shtml

All muscles are divided into 2 types:

Smooth muscles that are found in the internal organs and walls of blood vessels.
Striated - a) cardiac, b) skeletal

Skeletal (striated) muscles perform the following functions:

movement of the body in space
movement of body parts relative to each other
posture maintenance

The structural and functional unit of the striated muscle is the neuromotor unit (NME). It is represented by the axon of the motor neuron, its branches and muscle fibers, which are innervated by them.

The structure of the muscle fiber

Each muscle consists of muscle fibers located longitudinally, which are multinucleated cells. Outside, they are covered with a basement membrane and plasmalemma, between which cambial cells (myosatelliocytes) are located. On the plasmalemma in many places there are finger-shaped impressions - T-tubules. They connect the sarcolemma to the sarcoplasmic reticulum (SR). Inside there is the usual set of organelles: numerous nuclei occupying a peripheral position, mitochondria, etc. SPR is a system of interconnected tubules with a high content of Ca +

The central part of the cytoplasm is occupied by specific organelles - myofibrils - contractile elements located longitudinally.

Fig.10. The structure of the sarcomere

The structural unit of myofibrils is the sarcomere. This is a constantly repeating part of the myofibril enclosed between two Z-membranes (telophragms). In the middle of the sarcomere there is a line M - mesophragm. The filaments of myosin, a contractile protein, are attached to the mesophragm, and actin (also a contractile protein) to the telophragm.

The alternation of these contractile proteins constitutes the transverse striation (Fig. 10). In the sarcomere, an anisotropic disk (A) is distinguished - a disk with birefringence (myosin + actin ends), an H-zone - only myosin filaments (part of disk A) and an I-disk - only actin filaments.

With the contraction of the sarcomere, there is a shortening of disc I and a decrease in the light zone H.

The contraction of the entire muscle is determined by the shortening of the sarcomere, and its length is reduced due to the formation of actomyosin complexes.

Myosin is a thick protein molecule that is located in the center of the sarcomere and consists of two chains - light and heavy meromyosin. On the cross section, myosin has the form of a camomile - the central part and drooping heads. The head of the lung meromyosin has ATPase activity, which is manifested only at the moment of contact with the active site of actin.

Actin is a globular protein consisting of two chains intertwined in the form of beads. Each globule has active sites that are covered by tropomyosin, and its position is regulated by troponin. At rest, the active sites of actin do not interact with the myosin head, since they are covered in the form of a lid by tropomyosin (Fig. 11).

mechanism of muscle contraction.

When the motor neuron is excited, the impulses approach the myoneural plate (the place of contact between the axon and the plasmolemma). Acetylcholine (ACh) is released from the presynaptic membrane, which passes through the synaptic cleft and acts on the plasma membrane (in this place it can be called postsynaptic), finds receptors for ACh, and interaction with them affects the permeability of the membrane for sodium ions. The permeability of the membrane to sodium increases, depolarization occurs, which leads to the occurrence of AP. It spreads along the membrane and is transmitted to the T-tubules, which are closely associated with the SBP. PD in the area of T-tubules causes an increase in the permeability of the SPR membrane for calcium, and calcium is released into the cytoplasm in quanta (portions) depending on the pulse frequency.

Calcium triggers the sarcomere shortening mechanism. The concentration of calcium determines how much the sarcomere (and the muscle as a whole) is reduced.

Calcium released into the cytoplasm finds the troponin protein, interacts with it and causes its conformational changes (that is, it changes the spatial arrangement of the protein).

Conformational changes in troponin shift tropomyosin from its place, thus opening the active (reactive) site of actin.

In that open area the myosin head is embedded. This contact activates enzymatic systems located in series. And this contact of two proteins, like a gear, mechanically moves the actin filament to the center of the sarcomere. An actin step occurs.

The more actin steps occur, the more the sarcomere shortens.

At the moment of contact of the myosin head and the reactive site of actin, the head acquires ATPase activity.

What is ATP energy used for?

- on the comb-like movement and breaking the bonds between actin and myosin;

- for the operation of the calcium pump;

- for the operation of the sodium-potassium pump.

Thus, the more calcium is released, the more acto-myosin complexes are formed, the more strokes myosin makes, the more the sarcomere shortens.

As soon as the motor neuron ceases to send impulses to the muscle fiber membrane, and PD from T-tubules ceases to enter the SR, the release of calcium from the SR stops, and the work of the calcium pump increases, the actomyosin bridges break, the Z-membrane returns to its place and the sarcomere relaxes. (and muscles in general).

Phases of muscle contraction.

Muscle contraction can be registered on a kymograph. To do this, the muscle is attached to a tripod, and to the other end - a scribe, which records the muscle contraction (Fig. 12).

In muscle contraction, the following phases are distinguished:

- latent (0.01 sec) - from the onset of the stimulus to a visible response;

— contraction phase (0.04 sec);

- relaxation phase (0.05 sec).

Thus, a single muscle contraction takes 0.1 sec. During the period of muscle contraction, the excitability of the tissue changes, that is, its ability to re-response under the action of high-frequency stimuli.

At relatively low frequencies, the response will look like a series of single muscle contractions (up to 10 pulses per second).

Tetanuses. Optimum and pessimum frequency.

If you increase the frequency of applied stimuli, then you can choose a frequency at which each subsequent stimulus will act in the relaxation phase. In this case, the muscle will contract from an incompletely relaxed state, and the response will be a dentate tetanus. For the frog gastrocnemius muscle, dentate tetanus occurs at a frequency of more than 10, but less than 20 impulses (each subsequent impulse comes in 0.09 - 0.06 sec)

With a further increase in the frequency of more than 20 pulses per second (up to 50), a smooth tetanus is recorded, since each pulse falls into the period of contraction, and the muscle contracts from the contracted state (each subsequent pulse comes in 0.02 - 0.05 sec).

The serrated tetanus is higher than a single muscle contraction, and the smooth one is even higher. Tetanus is based on the summation (superposition) of contractions and a high concentration of calcium ejected from the SPR. With an increase in the frequency of the stimulus, the release of calcium from the SPR increases, which is released in quanta and does not have time to return back.

But not all high-frequency stimuli cause optimal contraction. Most often, the optimal contraction causes a smooth tetanus.

Optimum frequency - the maximum response to the action of high-frequency stimuli.

Very high frequency stimuli can reduce the response, and then a frequency pessimum occurs. At a frequency of 100 pulses per second, the stimulus reaches the end of the latent phase (each subsequent pulse arrives after 0.01 sec), and a single muscle contraction occurs in response. At a frequency of 200 pulses / sec (each subsequent pulse comes after 0.005 sec), either a single muscle contraction occurs, or there is no reaction.

Reducing the response during the period pessimism associated with the action of the stimulus in the period of either absolute or relative refractoriness. Absolute refractoriness takes 0.005 sec. Then, during the period of relative refractoriness, excitability is below 100%. Excitability is restored after 0.01 sec. (Fig.13).

muscle contraction is a complex mechano-chemical process during which the chemical energy of the hydrolytic breakdown of ATP is converted into mechanical work performed by the muscle.

At present, this mechanism has not yet been fully elucidated. But the following is known for certain:

1. The source of energy needed for muscle work is ATP;

2. Hydrolysis of ATP, accompanied by the release of energy, is catalyzed by myosin, which, as already noted, has enzymatic activity;

3. The trigger mechanism of muscle contraction is an increase in the concentration of Ca 2+ ions in the sarcoplasm of myocytes, caused by a motor nerve impulse;

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

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

There are many hypotheses attempting to explain the molecular mechanism of muscle contraction. The most reasonable at present is hypothesis « rowing boat » or « rowing hypothesis » H. Huxley. In a simplified form, its essence is as follows.

In a muscle at rest, thick and thin filaments of myofibrils are not connected to each other, since the binding sites on actin molecules are closed by tropomyosin molecules.

Muscle contraction occurs under the influence of a motor nerve impulse, which is a wave of increased membrane permeability propagating along the nerve fiber. This wave of increased permeability is transmitted through the neuromuscular junction to the T-system of the sarcoplasmic reticulum and eventually reaches the cisterns containing high concentrations of calcium ions. As a result of a significant increase in the permeability of the tank wall ( It's also a membrane! calcium ions leave the tanks and their concentration in the sarcoplasm for a very a short time (about 3ms) increases by about 1000 times. Calcium ions, being in high concentration, attach to the protein of thin filaments - troponin and change its spatial shape ( conformation). The change in the conformation of troponin, in turn, leads to the fact that the tropomyosin molecules are displaced along the fibrillar actin groove, which forms the basis of thin filaments, and releases the area of actin molecules that is intended for binding to myosin heads. As a result, between myosin and actin ( those. between thick and thin threads) a transverse bridge appears, located at an angle of 90 º . Since thick and thin filaments contain a large number of myosin and actin molecules (about 300 each). then between the muscle filaments a rather a large number of cross bridges or adhesions. On an electron micrograph ( rice. 15) It is clearly seen that there are a large number of transverse bridges between thick and thin filaments.

Rice. 15. An electron micrograph of a longitudinal cut

myofibril site(Magnification 300,000 times)(L. Streiner, 1985)

The formation of a bond between actin and myosin is accompanied by an increase in the ATPase activity of the latter ( those. actin acts like an allosteric enzyme activator). resulting in the hydrolysis of ATP:

Chapter 1. EXCITABLE TISSUES

PHYSIOLOGY OF MUSCLE TISSUE

Skeletal muscles

The mechanism of muscle contraction

Skeletal muscle is a complex system that converts chemical energy into mechanical work and heat. At present, the molecular mechanisms of this transformation are well studied.

Structural organization of the muscle fiber. A muscle fiber is a multinuclear structure surrounded by a membrane and containing a specialized contractile apparatus - myofibrils. In addition, the most important components of the muscle fiber are mitochondria, a system of longitudinal tubules - the sarcoplasmic reticulum (reticulum) and a system of transverse tubules - the T-system. The functional unit of the contractile apparatus of the muscle cell is the sarcomere (Fig. 2.20, A); The myofibril is made up of sarcomeres. Sarcomeres are separated from each other by Z-plates. Sarcomeres in the myofibril are arranged in series, so the contraction of sarcomeres causes contraction of the myofibril and an overall shortening of the muscle fiber.

The study of the structure of muscle fibers in a light microscope made it possible to reveal their transverse striation. Electron microscopic studies have shown that the transverse striation is due to the special organization of the contractile proteins of myofibrils - actin (molecular weight 42,000) and myosin (molecular weight about 500,000). 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. In the longitudinal grooves of the actin helix are filamentous molecules of the protein tropomyosin. With a step of 40 nm, a molecule of another protein, troponin, is attached to the tropomyosin molecule. Troponin and tropomyosin play an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this region is visible as a dark stripe (due to birefringence) - an anisotropic A-disk. A lighter H-stripe is visible in its center. There are no actin filaments in it at rest. On both sides of the A-disk, light isotropic stripes are visible - 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. Electron microscopy in the center of the H-band revealed an M-line, a structure that holds myosin filaments. On a cross section of a muscle fiber, you can see the hexagonal organization of the myofilament: each myosin filament is surrounded by six actin filaments (Fig. 2.20, B).

mechanism of muscle contraction. In the process of muscle fiber contraction, the following transformations occur in it:

A. Electrochemical conversion:

2. Propagation of PD along the T-system.

3. 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 Ca2+ ions.

B. Chemomechanical transformation:

4. Interaction of Ca2+ ions with troponin, release of active centers on actin filaments.

5. Interaction of the myosin head with actin, head rotation and development of elastic traction.

6. 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 oC. Thus, AP generation is the first stage of muscle contraction.

The second stage is the propagation of AP into 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 T-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 site of contact and the formation of inositol triphosphate. Inositol triphosphate activates calcium channels in the membranes of the terminal cisterns, which leads to the release of Ca2+ ions from the cisterns and an increase in the intracellular concentration of Ca2+ from 107 to 105 M. The totality of processes leading to an increase in the intracellular concentration of Ca2+ is the essence of the third stage of muscle contraction. Thus, at the first stages, the electrical signal of AP is converted into a chemical signal - an increase in the intracellular Ca2+ concentration, i.e., an electrochemical transformation.

With an increase in the intracellular concentration of Ca2+ ions, tropomyosin shifts into the groove between the actin filaments, while the actin filaments open areas with which myosin cross-bridges can interact. This displacement of tropomyosin is due to a change in the conformation of the troponin protein molecule upon Ca2+ binding. Therefore, the participation of Ca2+ ions in the mechanism of interaction between actin and myosin is mediated through troponin and tropomyosin.

The essential role of calcium in the mechanism of muscle contraction was proved in experiments with the use of the protein aequorin, which, when interacting with calcium, emits light. After injection of aequorin, the muscle fiber was subjected to electrical stimulation and simultaneously measured muscle tension in the isometric mode and aequorin luminescence. Both curves were fully correlated with each other (Fig. 2.21). Thus, the fourth stage of electromechanical coupling is the interaction of calcium with troponin.

The next, fifth, stage of electromechanical coupling is the attachment of the head of the cross bridge to the actin filament to the first of several sequentially located stable centers. In this case, the myosin head rotates around its axis, since it has several active centers that sequentially interact with the corresponding centers on the actin filament. 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.

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The successive reaction of connecting and disconnecting the heads of the transverse bridges with the actin filament leads to the sliding of thin and thick filaments relative to each other and a decrease in the size of the sarcomere and the total length of the muscle, which is the sixth stage. The totality of the described processes is the essence of the theory of sliding threads

It was originally believed that Ca2+ 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.

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 106M.

The described mechanisms of shortening of the muscle fiber suggest that relaxation primarily requires a decrease in the concentration of Ca2+ ions. It has been experimentally proven that the sarcoplasmic reticulum has special mechanism- a calcium pump that 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 (see Chapter 4). Then the 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 mechanism of muscle contraction

All muscles of the body are divided into smooth and striated. Striated muscles are divided into two types: skeletal muscles and myocardium.

The structure of the muscle fiber

The membrane of muscle cells, called the sarcolemma, is electrically excitable and capable of conducting an action potential. These processes in muscle cells occur according to the same principle as in nerve cells. The resting potential of the muscle fiber is approximately -90 mV, i.e. lower than that of the nerve fiber (-70 mV); the critical depolarization, upon reaching which an action potential arises, is the same as that of a nerve fiber. Hence: the excitability of the muscle fiber is somewhat lower than the excitability of the nerve fiber, since the muscle cell needs to be depolarized by a large amount.

The response of a muscle fiber to stimulation is reduction. which the contractile apparatus of the cell performs - myofibrils. They are strands consisting of two types of threads: thick - myosin. and thin - actin. Thick filaments (15 nm in diameter and 1.5 µm long) contain only one protein, myosin. Thin filaments (7 nm in diameter and 1 µm long) contain three types of proteins: actin, tropomyosin, and troponin.

Actin is a long protein filament, which consists of individual globular proteins linked together in such a way that the whole structure is an elongated chain. Molecules of globular actin (G-actin) have lateral and terminal binding sites with other similar molecules. As a result, they are combined in such a way that they form a structure that is often compared to two strands of beads connected together. The ribbon formed from G-actin molecules is twisted into a spiral. This structure is called fibrillar actin (F-actin). The helix pitch (coil length) is 38 nm; each helix coil has 7 pairs of G-actin. The polymerization of G-actin, that is, the formation of F-actin, occurs due to the energy of ATP, and, conversely, when F-actin is destroyed, energy is released.

Fig.1. Fusion of individual G-actin globules into F-actin

Along the spiral grooves of the actin filaments is the protein tropomyosin. Each strand of tropomyosin, which is 41 nm long, consists of two identical α-chains, twisted together into a spiral with a turn length of 7 nm. Two molecules of tropomyosin are located along one turn of F-actin. Each tropomyosin molecule overlaps slightly with the next, resulting in a tropomyosin filament that extends continuously along the actin.

Fig.2. The structure of a thin filament of myofibril

In striated muscle cells, in addition to actin and tropomyosin, the composition of thin filaments also includes the protein troponin. This globular protein has a complex structure. It consists of three subunits, each of which performs its function in the contraction process.

thick thread made up of many molecules myosin. collected in a bundle. Each myosin molecule 155 nm long and 2 nm in diameter consists of six polypeptide filaments: two long and four short. The long chains are coiled together in a 7.5 nm pitch helix and form the fibrillar portion of the myosin molecule. At one end of the molecule, these chains unwind and form a forked end. Each of these ends forms a complex with two short chains, that is, there are two heads on each molecule. This is the globular part of the myosin molecule.

Fig.3. The structure of the myosin molecule.

Two fragments are distinguished in myosin: light meromyosin (LMM) and heavy meromyosin (HMM), between them there is a hinge. TMM consists of two subfragments: S 1 and S 2 . LMM and subfragment S 2 are nested in a bundle of threads, and subfragment S 1 protrudes above the surface. This protruding end (myosin head) is able to bind to the active site on the actin filament and change the angle of inclination to the bundle of myosin filaments. The combination of individual myosin molecules into a bundle occurs due to electrostatic interactions between LMMs. The central part of the thread has no heads. The entire complex of myosin molecules extends over 1.5 µm. It is one of the largest biological molecular structures known in nature.

When viewing a longitudinal section of a striated muscle through a polarizing microscope, light and dark areas are visible. Dark areas (disks) are anisotropic: in polarized light, they look transparent in the longitudinal direction and opaque in the transverse direction, denoted by the letter A. Light areas are isotropic and are denoted by the letter I. Disk I includes only thin filaments, and disk A - and thick and thin. In the middle of disk A, a light strip is visible, called the H-zone. It does not have thin threads. Disk I is divided by a thin strip Z, which is a membrane containing structural elements that fasten the ends of thin threads together. The area between two Z-lines is called sarcomere .

Fig.4. Myofibril structure (cross section)

Fig.5. The structure of the striated muscle (longitudinal section)

Each thick thread is surrounded by six thin ones, and each thin thread is surrounded by three thick ones. Thus, in a cross section, the muscle fiber has a regular hexagonal structure.

During muscle contraction, the length of actin and myosin filaments does not change. There is only their displacement relative to each other: thin threads move into the gap between the thick ones. In this case, the length of disk A remains unchanged, and disk I shortens, the strip H almost disappears. Such sliding is possible due to the existence of transverse bridges (myosin heads) between thick and thin filaments. With contraction, a change in the length of the sarcomere from approximately 2.5 to 1.7 microns is possible.

The myosin filament has many heads with which it can bind to actin. The actin filament, in turn, has sites (active centers) to which myosin heads can attach. In a resting muscle cell, these binding sites are covered by tropomyosin molecules, which prevents the formation of a bond between thin and thick filaments.

For actin and myosin to interact, calcium ions must be present. At rest, they are located in the sarcoplasmic reticulum. This organelle is a membrane cavity containing a calcium pump, which, using the energy of ATP, transports calcium ions into the sarcoplasmic reticulum. His inner surface contains proteins capable of binding Ca 2+. which somewhat reduces the difference in the concentrations of these ions between the cytoplasm and the cavity of the reticulum. spreading across cell membrane The action potential activates the reticulum membrane close to the cell surface and causes the release of Ca 2+ into the cytoplasm.

The troponin molecule has a high affinity for calcium. Under its influence, it changes the position of the tropomyosin filament to the actin filament in such a way that the active center, previously covered by tropomyosin, opens. A transverse bridge joins the opened active center. This leads to the interaction of actin with myosin. After bond formation, the myosin head, previously located at right angles to the filaments, tilts and pulls the actin filament relative to the myosin head by about 10 nm. The formed atin-myosin complex prevents the further sliding of the threads relative to each other, so its separation is necessary. This is possible only due to the energy of ATP. Myosin has ATPase activity, that is, it is able to cause ATP hydrolysis. The energy released in this process breaks the bond between actin and myosin, and the myosin head is able to interact with the new part of the actin molecule. The work of the bridges is synchronized in such a way that the binding, tilting and breaking of all the bridges of one thread occurs simultaneously. When the muscle relaxes, the work of the calcium pump is activated, which lowers the concentration of Ca 2+ in the cytoplasm; therefore, bonds between thin and thick threads can no longer be formed. Under these conditions, when stretched, the muscles of the thread slide freely relative to each other. However, such extensibility is possible only in the presence of ATP. If there is no ATP in the cell, then the actin-myosin complex cannot break. The threads remain rigidly linked to each other. This phenomenon is observed in rigor mortis.

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Fig.6. Reduction of the sarcomere: 1 - myosin thread; 2 - active center; 3 - actin filament; 4 - myosin head; 5 - Z-line.

a) there is no interaction between thin and thick threads;

b) in the presence of Ca 2+, the myosin head binds to the active site on the actin filament;

v) the transverse bridges bend and pull a thin thread through a relatively thick one, as a result of which the length of the sarcomere decreases;

G) the bonds between the threads are broken due to the energy of ATP, the myosin heads are ready to interact with new active centers.

There are two modes of muscle contraction: isotonic(the length of the fiber changes, but the voltage remains unchanged) and isometric(the ends of the muscle are fixedly fixed, as a result of which it is not the length that changes, but the tension).

Power and speed of muscle contraction

Important characteristics of the muscle are the strength and speed of contraction. The equations expressing these characteristics were empirically obtained by A. Hill and subsequently confirmed by the kinetic theory of muscle contraction (Deshcherevsky's model).

Hill equation. which relates the force and speed of muscle contraction, has the following form: (P+a)(v+b) = (P+a)b = a(vmax +b). where v is the speed of muscle shortening; P - muscle force or the load applied to it; v max is the maximum speed of muscle shortening; P is the force developed by the muscle in the isometric mode of contraction; a,b are constants. general power. developed by the muscle is determined by the formula: N total \u003d (P + a) v \u003d b (P - P). efficiency muscle remains constant ( about 40%) in the range of force values from 0.2 P to 0.8 P . During muscle contraction, a certain amount of heat is released. This value is called heat production. Heat production depends only on the change in muscle length and does not depend on the load. Constants a and b have constant values for a given muscle. Constant a has the dimension of force, and b- speed. Constant b largely dependent on temperature. Constant a is in the range of values from 0.25 P to 0.4 P . Based on these data, it is estimated maximum contraction speed for this muscle: v max = b (P / a) .

14. Fine structure of myofibrils. Proteins of thick and thin filaments - structure and functions + (muscle contraction and composition 15 question)

mechanism of muscle contraction. Functions and properties of skeletal muscles

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

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

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

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

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

Smooth muscle consists of:

the motor apparatus of the internal organs, which includes the bronchial tree, lungs and digestive tube;
lymphatic and circulatory systems;
urinary system.

Physiological properties

Like all vertebrates, the human body is divided into three most important properties skeletal muscle fibers:

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

Muscles are excited and begin to contract from nerve impulses coming from the centers. But in artificial conditions, electrical stimulation is used. The muscle can then be stimulated directly (direct irritation) or through the nerve innervating the muscle (indirect stimulation).

Types of abbreviations

The mechanism of muscle contraction involves the conversion of chemical energy into mechanical work. This process can be measured in an experiment with a frog: it calf muscle loaded with a small weight, and then irritated with light electrical impulses. A contraction in which the muscle becomes shorter is called isotonic. With isometric contraction, shortening does not occur. Tendons do not allow shortening when the muscle develops strength. Another auxotonic mechanism of muscle contractions involves conditions of intense loads, when the muscle is shortened in a minimal way, and the strength is developed to the maximum.

Structure and innervation of skeletal muscles

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

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

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

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

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

innervation

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

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

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

Structure of myofibrils

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

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

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

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

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

Mechanism of muscle fiber contraction

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

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

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

Stages of reduction

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

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