Lecture notes| Lecture summary | Interactive test | Download abstract

» Structural organization of skeletal muscle
» Molecular mechanisms of skeletal muscle contraction
» Coupling of excitation and contraction in skeletal muscle
» Relaxation of skeletal muscle
»
» Skeletal muscle work
» Structural organization and contraction of smooth muscle
» Physiological properties of muscles

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

Structural organization of skeletal muscle

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

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

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

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

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

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

Molecular mechanisms of contractionskeletal muscle

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

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

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

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

The cycle “binding – change in conformation – disconnection – restoration of conformation” occurs many times, as a result of which actin and myosin filaments are displaced relative to each other, the Z-disks of sarcomeres come closer and the myofibril is shortened (Fig. 2, D).

Pairing of excitation and contractionin skeletal muscle

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

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

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

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

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

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

Skeletal muscle relaxation

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

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

Phases and modes of skeletal muscle contraction

Phases of muscle contraction

When skeletal muscle is irritated by a single pulse of electric current of suprathreshold strength, a single muscle contraction occurs, in which 3 phases are distinguished (Fig. 4, A):

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

shortening phase (about 50 ms);

relaxation phase (about 50 ms).

Modes of muscle contraction

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

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

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

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

Optimum and pessimum frequency

The amplitude of tetanic contraction depends on the frequency of impulses irritating the muscle. The optimum frequency is the frequency of irritating impulses at which each subsequent impulse coincides with the phase of increased excitability (Fig. 4, A) and, accordingly, causes tetanus of the greatest amplitude. Frequency pessimum is a higher frequency of stimulation at which each subsequent current pulse falls into the refractory phase (Fig. 4, A), as a result of which the amplitude of the tetanus decreases significantly.

Skeletal muscle work

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

- the number of units involved in the reduction;

frequency of contraction of muscle fibers.

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

Types of skeletal muscle work:

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

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

dynamic yielding work is performed if the muscle functions, but at the same time stretches, since the force it makes is not enough to move or hold parts of the body.

During work, the muscle can contract:

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

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

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

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

Fatigue is a physiological state of a muscle that develops after prolonged work and is manifested by a decrease in the amplitude of contractions, an extension of the latent period of contraction and the relaxation phase. The causes of fatigue are: depletion of ATP reserves, accumulation of metabolic products in the muscle. Muscle fatigue during rhythmic work is less than synapse fatigue. Therefore, when the body performs muscular work, fatigue initially develops at the level of the synapses of the central nervous system and neuromuscular synapses.

Structural organization and reductionsmooth muscles

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

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

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

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

Physiological properties of muscles

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. The physiological properties and characteristics of the cardiac muscle are discussed in the section “Physiological mechanisms of homeostasis”.

Table 7.1. Comparative characteristics of skeletal and smooth muscles

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

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

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

All muscles of the body are divided into smooth and striated.

Mechanisms of skeletal muscle contraction

Striated muscles are divided into two types: skeletal muscles and myocardium.

The structure of muscle fiber

The muscle cell membrane, called the sarcolemma, is electrically excitable and capable of conducting action potentials. These processes in muscle cells occur according to the same principle as in nerve cells. The resting potential of a muscle fiber is approximately -90 mV, that is, lower than that of a nerve fiber (-70 mV); the critical depolarization, upon reaching which an action potential occurs, 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 greater amount.

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

Actin is a long protein thread that consists of individual globular proteins linked together in such a way that the entire structure is an elongated chain. Molecules of globular actin (G-actin) have lateral and terminal binding centers with other similar molecules. As a result, they come together in such a way that they form a structure that is often compared to two strands of beads joined together. The ribbon formed from G-actin molecules is twisted into a spiral. This structure is called fibrillar actin (F-actin). The helix pitch (turn length) is 38 nm; for each turn of the helix there are 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. Association of individual G-actin globules into F-actin

The protein tropomyosin is located along the spiral grooves of actin filaments. Each tropomyosin filament, 41 nm long, consists of two identical α-chains twisted together into a spiral with a turn length of 7 nm. Along one turn of F-actin there are two tropomyosin molecules. Each tropomyosin molecule connects, slightly overlapping, to the next, resulting in a tropomyosin filament extending continuously along actin.

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

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

Thick thread consists of a large number of molecules myosin, collected in a bundle. Each myosin molecule, 155 nm long and 2 nm in diameter, consists of six polypeptide threads: two long and four short. The long chains are twisted together into a spiral with a pitch of 7.5 nm and form the fibrillar part 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.

Myosin has two fragments: light meromyosin (LMM) and heavy meromyosin (HMM), between them there is a hinge. TMM consists of two subfragments: S1 and S2. The LMM and subfragment S2 are embedded in a bundle of threads, and subfragment S1 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 myosin filament bundle. The combination of individual myosin molecules into a bundle occurs due to electrostatic interactions between the 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 examining a longitudinal section of striated muscle through a polarizing microscope, light and dark areas are visible. Dark areas (disks) are anisotropic: in polarized light they appear transparent in the longitudinal direction and opaque in the transverse direction, designated by the letter A. Light areas are isotropic and designated by the letter I. Disc I includes only thin threads, and disc A includes both thick and thin. In the middle of disk A there is a bright stripe called the H-zone. It does not have thin threads. Disc I is separated by a thin stripe Z, which is a membrane containing structural elements that hold the ends of thin filaments together. The area between two Z-lines is called sarcomere.

Fig.4. Myofibril structure (cross section)

Fig.5. Structure of 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.

Muscle contraction

When a muscle contracts, the length of actin and myosin filaments does not change. There is only a displacement of them relative to each other: thin threads move into the space between the thick ones. In this case, the length of disk A remains unchanged, but disk I is shortened, and the H strip almost disappears. Such sliding is possible due to the existence of cross bridges (myosin heads) between thick and thin filaments. During contraction, the sarcomere length may change from approximately 2.5 to 1.7 μm.

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

In order for actin and myosin to interact, the presence of calcium ions is necessary. At rest they reside 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. Its inner surface contains proteins capable of binding Ca2+, which somewhat reduces the difference in the concentrations of these ions between the cytoplasm and the reticulum cavity. An action potential propagating along the cell membrane activates the reticulum membrane located close to the cell surface and causes the release of Ca2+ into the cytoplasm.

The troponin molecule has a high affinity for calcium.

Under its influence, it changes the position of the tropomyosin filament on the actin filament in such a way that the active center, previously covered by tropomyosin, opens. A cross bridge is attached to 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 filament by approximately 10 nm. The resulting atin-myosin complex prevents further sliding of the threads relative to each other, so its separation is necessary. This is only possible due to the energy of ATP. Myosin has ATPase activity, that is, it is capable of causing ATP hydrolysis. The energy released in this case breaks the bond between actin and myosin, and the myosin head is able to interact with a 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 bridges of one thread occurs simultaneously. When the muscle relaxes, the calcium pump is activated, which reduces the concentration of Ca2+ in the cytoplasm; consequently, connections between thin and thick threads can no longer be formed. Under these conditions, when the muscle is stretched, the threads slide smoothly relative to each other. However, such extensibility is only possible 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.

Fig.6. Contraction of the sarcomere: 1 – myosin filament; 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 Ca2+, the myosin head binds to the active center on the actin filament;

V) the cross bridges bend and pull the thin thread relative to the 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 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 a 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 model).

Hill's equation, which relates the strength and speed of muscle contraction, has the following form: (P+a)(v+b) = (P0+a)b = a(vmax+b), where v is the speed of muscle shortening; P – muscle force or load applied to it; vmax — maximum speed of muscle shortening; P0 is the force developed by the muscle in the isometric contraction mode; a,b are constants. general power, developed by the muscle, is determined by the formula: Ntotal = (P+a)v = b(P0-P). Efficiency muscles maintains a constant value ( about 40%) in the range of force values ​​from 0.2 P0 to 0.8 P0. During muscle contraction, a certain amount of heat is released. This quantity is called heat production. Heat production depends only on changes in muscle length and does not depend on load. Constants a And b have constant values ​​for a given muscle. Constant A has the dimension of force, and b– speed. Constant b depends largely on temperature. Constant A is in the range of values ​​from 0.25 P0 to 0.4 P0. Based on these data, it is estimated maximum contraction speed for a given muscle: vmax = b (P0 / a).

Characteristics of muscle tissue.

Skeletal muscle contraction and its mechanisms

Types of muscle tissue. Actino-myosin complex and mechanisms of its functioning.

There are 3 types of animal tissues: 1) muscle, 2) nervous, 3) secretory. The first responds to stimulation by contracting and carrying out the work of displacement. The second is the ability to conduct and analyze impulses, the third is to isolate various secrets.

There are 3 types of muscle tissue: 1. striated, 2. smooth, 3. cardiac.

Actino-myosin complex. All muscle cells contain a large number of special contractile proteins - 60-80% of the total muscle proteins. Main contractiles

proteins are fibrillar proteins: - myosin- forms thick threads; — actin- forms thin threads. To regulate contraction, globular proteins are used: troponin-tropomyosin.

Myosin - 2-chain structure 1=180 nm and 0=2.5 ​​nm. Actin is a 2-helix peptide chain.

Reduction mechanism: Actin and myosin are spatially separated in the fibril. The nerve impulse causes the release of acetylcholine into the synaptic cleft of the neuromuscular junction. This

causes depolarization of the postsynaptic membrane after binding of the transmitter and

propagation of the action potential across cell membranes and into the muscle

fibers through T tubes. As a result of the actin-myosin interaction, fibril contraction occurs. This is achieved by the myosin head pushing the actin filament through the formation of a bridge. When the impulse disappears, Ca2+ is restored, the bridge between actin and myosin is destroyed and the muscle returns to its original state.

Troponin is a globular protein with 3 centers:

- T - binds to tropomyosin

- C - binds Ca2+

- 1 - inhibits actin-myosin interaction.

Contraction phases:

1. Latent period - 0.05 seconds.

2. Contraction phase - 0.1 sec

3. Relaxation period - 0.2 seconds.

Biochemistry of muscle function

1. ATP + myosin-actin complex——-ADP + Myosin + actin + F + energy

2. ADP + creatinine phosphate——ATP + creatine

3. Glycogen—Glucose——Glucose + O2—CO2 + H2O + 38 ATP (aerobic process)

4. Glucose—2 lactic acid + 2 ATP (anaerobic process—dissolves nerve endings—

5. Lactic acid + O2—CO2 + H2O (rest) or Molten acid—glucose—glycogen.

Mechanism of skeletal muscle contraction

Muscle shortening is the result of contraction of multiple sarcomeres. During shortening, the actin filaments slide relative to the myosin filaments, as a result of which the length of each sarcomere of the muscle fiber decreases. At the same time, the length of the threads themselves remains unchanged. Myosin filaments have transverse projections (cross bridges) about 20 nm long. Each protrusion consists of a head, which is connected to the myosin filament through a “neck” (Fig. 23).

In a relaxed state, the muscles of the heads of the cross bridges cannot interact with actin filaments, since their active sites (places of mutual contact with the heads) are isolated by tropomyosin. Shortening of the muscle is the result of conformational changes in the cross bridge: its head tilts by bending the “neck”.

Rice. 23. Spatial organization of contractile and regulatory proteins in striated muscle. The position of the myosin bridge is shown (raking effect, the neck is bent) during the interaction of contractile proteins in muscle fibers (fiber contraction)

Sequence of processes , providing muscle fiber contraction(electromechanical interface):

1. After occurrence PD in the muscle fiber near the synapse (due to the electric field of the PKP) excitation spreads across the myocyte membrane, including on transverse membranes T-tubules. The mechanism of conduction of action potentials along a muscle fiber is the same as through an unmyelinated nerve fiber - the emerging action potential near the synapse, through its electric field, ensures the emergence of new action potentials in the adjacent section of the fiber, etc. (continuous conduction of excitation).

2. Potential actions T-tubules due to its electric field, it activates voltage-gated calcium channels on SPR membrane, as a result of which Ca2+ leaves the SPR tanks according to an electrochemical gradient.

3. In the interfibrillar space Ca2+ contacts with troponin, which leads to its conformation and displacement of tropomyosin, resulting in actin filaments active areas are exposed, with which they connect heads of myosin bridges.

4. As a result of interaction with actin ATPase activity of myosin filament heads increases, ensuring the release of ATP energy, which is spent on bending of the myosin bridge, externally resembling the movement of oars when rowing (stroke movement) (see Fig. 23), ensuring the sliding of actin filaments relative to myosin filaments. The energy of one ATP molecule is consumed to complete one rowing movement. In this case, the filaments of contractile proteins are displaced by 20 nm. The attachment of a new ATP molecule to another part of the myosin head leads to the cessation of its engagement, but the energy of ATP is not consumed. In the absence of ATP, myosin heads cannot detach from actin - the muscle is tense; This, in particular, is the mechanism of rigor mortis.

5. After this the heads of the cross bridges, due to their elasticity, return to their original position and establish contact with the next section of actin; then another rowing movement and sliding of actin and myosin filaments occurs again. Similar elementary acts are repeated many times. One rowing movement (one step) causes a decrease in the length of each sarcomere by 1%. When an isolated frog muscle contracts without a load of 50%, sarcomere shortening occurs in 0.1 s. To do this, you need to perform 50 rowing movements.

Mechanism of muscle contraction

Myosin bridges bend asynchronously, but due to the fact that there are many of them and each myosin filament is surrounded by several actin filaments, muscle contraction occurs smoothly.

Relaxation muscle growth occurs due to processes occurring in reverse order. Repolarization of the sarcolemma and T-tubules leads to the closure of voltage-gated calcium channels in the SPR membrane. Ca pumps return Ca2+ to the SPR (the activity of the pumps increases with increasing concentration of free ions).

A decrease in the Ca2+ concentration in the interfibrillar space causes a reverse conformation of troponin, as a result of which tropomyosin filaments isolate the active sites of actin filaments, which makes it impossible for the heads of myosin cross bridges to interact with them. Sliding of actin filaments along myosin filaments in the opposite direction occurs under the influence of gravity and elastic traction of muscle fiber elements, which restores the original dimensions of the sarcomeres.

The source of energy to ensure the work of skeletal muscles is ATP, the costs of which are significant. Even in conditions of the main exchange for the functioning of muscles, the body uses about 25% of all its energy resources. Energy expenditure increases sharply during physical work.

ATP reserves in muscle fiber are insignificant (5 mmol/l) and can provide no more than 10 single contractions.

Energy consumption ATP is necessary for the following processes.

Firstly, ATP energy is spent to ensure the operation of the Na/K pump (it maintains the concentration gradient of Na+ and K+ inside and outside the cell, forming the PP and PD, which ensures electromechanical coupling) and the operation of the Ca pump, which reduces the concentration of Ca2+ in the sarcoplasm after contraction of the muscle fiber, which leads to relaxation.

Secondly, ATP energy is spent on the rowing movement of myosin bridges (bending them).

ATP resynthesis carried out using the three energy systems of the body.

1. The phosphogenic energy system ensures the resynthesis of ATP due to the highly energy-intensive CP present in the muscles and adenosine diphosphoric acid (adenosine diphosphate, ADP) formed during the breakdown of ATP with the formation of creatine (K): ADP + + CP → ATP + K. This is an instant resynthesis of ATP, while the muscle can develop greater power, but for a short time - up to 6 s, since the reserves of CP in the muscle are limited.

2. The anaerobic glycolytic energy system provides ATP resynthesis using the energy of anaerobic breakdown of glucose to lactic acid. This pathway of ATP resynthesis is fast, but also short-lived (1-2 min), since the accumulation of lactic acid inhibits the activity of glycolytic enzymes. However, lactate, causing a local vasodilator effect, improves blood flow in the working muscle and its supply of oxygen and nutrients.

3. The aerobic energy system ensures the resynthesis of ATP using oxidative phosphorylation of carbohydrates and fatty acids, which occurs in the mitochondria of muscle cells. This way can provide energy for muscle function for several hours and is the main way to provide energy for the work of skeletal muscles.

Types of muscle contractions

Depending on the nature of the abbreviations There are three types of muscles: isometric, isotonic and auxotonic.

Auxotonic muscle contraction involves a simultaneous change in muscle length and tension. This type of contraction is characteristic of natural motor acts and comes in two types: eccentric, when muscle tension is accompanied by its lengthening - for example, in the process of squatting (lowering), and concentric, when muscle tension is accompanied by its shortening - for example, when extending the lower limbs after squatting ( climb).

Isometric muscle contraction- when muscle tension increases, but its length does not change. This type of contraction can be observed in experiment, when both ends of the muscle are fixed and there is no possibility of their approach, and in natural conditions - for example, in the process of squatting and fixing the position.

Isotonic muscle contraction consists of shortening the muscle with constant tension. This type of contraction occurs when an unloaded muscle with one tendon attached contracts without lifting (moving) any external load or lifting a load without acceleration.

Depending on duration There are two types of muscle contractions: solitary and tetanic.

Single muscle contraction occurs when a single irritation of the nerve or muscle itself occurs. Typically the muscle shortens by 5-10% of its original length. There are three main periods on the single contraction curve: 1) latent- time from the moment of irritation to the onset of contraction; 2) period shortening (or development of tension); 3) period relaxation. The duration of single contractions of human muscles is variable. For example, in the soleus muscle it is 0.1 s. During the latent period, excitation of muscle fibers occurs and its conduction along the membrane. The relationship between the duration of a single contraction of a muscle fiber, its excitation and phase changes in the excitability of the muscle fiber are shown in Fig. 24.

The duration of muscle fiber contraction is significantly longer than that of the AP because it takes time for the Ca-pumps to work to return Ca2+ to the SPR and the environment and the greater inertia of mechanical processes compared to electrophysiological ones.

Rice. 24. The ratio of the time of occurrence of AP (A) and a single contraction (B) of the slow fiber of the skeletal muscle of a warm-blooded animal. Arrow– moment of application of irritation. The contraction time of fast-twitch fibers is several times shorter

Tetanic contraction- this is a long-term contraction of a muscle that occurs under the influence of rhythmic stimulation, when each subsequent stimulation or nerve impulses arrive at the muscle while it has not yet relaxed. Tetanic contraction is based on the phenomenon of summation of single muscle contractions (Fig. 25) - an increase in the amplitude and duration of contraction when two or more rapidly successive stimuli are applied to a muscle fiber or a whole muscle.

Rice. Fig. 25. Summation of contractions of the frog gastrocnemius muscle: 1 – curve of a single contraction in response to the first stimulation of the relaxed muscle; 2 – curve of single contraction of the same muscle in response to the second stimulus; 3 – curve of the summed contraction obtained as a result of coupled stimulation of the contracting muscle ( indicated by arrows)

In this case, irritations should arrive during the period of the previous contraction. The increase in contraction amplitude is explained by an increase in the concentration of Ca2+ in the hyaloplasm upon repeated excitation of muscle fibers, since the Ca pump does not have time to return it to the SPR. Ca2+ ensures an increase in the number of zones of engagement of myosin bridges with actin filaments.

If repeated impulses or irritations occur during the muscle relaxation phase, serrated tetanus. If repeated stimulation occurs during the shortening phase, smooth tetanus(Fig. 26).

Rice. 26. Contraction of the frog gastrocnemius muscle at different frequencies of sciatic nerve stimulation: 1 – single contraction (frequency 1 Hz); 2.3 – serrated tetanus (15-20 Hz); 4.5 – smooth tetanus (25-60 Hz); 6 – relaxation at pessimal frequency of stimulation (120 Hz)

The amplitude of contraction and the magnitude of tension developed by muscle fibers during smooth tetanus are usually 2-4 times greater than during a single contraction. Tetanic contraction of muscle fibers, in contrast to single contractions, causes them to fatigue more quickly.

As the frequency of nerve or muscle stimulation increases, the amplitude of smooth tetanus increases. Maximum tetanus is called optimum. The increase in tetanus is explained by the accumulation of Ca2+ in the hyaloplasm. With a further increase in the frequency of nerve stimulation (about 100 Hz), the muscle relaxes due to the development of a block of excitation conduction in the neuromuscular synapses - Vvedensky pessimum(irritation frequency pessimal) (see Fig. 26). Vvedensky's pessimum can also be obtained with direct, but more frequent irritation of the muscle (about 200 impulses/s), however, for the purity of the experiment, the neuromuscular synapses should be blocked. If, after the occurrence of a pessimum, the frequency of stimulation is reduced to optimal, then the amplitude of muscle contraction instantly increases - evidence that the pessimum is not the result of muscle fatigue or depletion of energy resources.

Under natural conditions, individual muscle fibers often contract in the serrated tetanus mode, but the contraction of the whole muscle resembles smooth tetanus, due to the asynchrony of their contraction.

And you already have an idea of ​​what a muscle is. But how does muscle contraction occur? What makes our muscles work?

In simple terms, muscle contraction occurs under the influence of nerve impulses that activate nerve cells in the spinal cord - motor neurons, the branches of which are axons brought to the muscle. If you take a closer look, inside the muscle the axon divides and forms a network of branches that, like electrical contacts, are “connected” to the muscle cell. Through such contacts, muscle contraction occurs.

It turns out that each motor neuron controls a group of muscle cells. Such groups were called - neuromotor units, thanks to which a person can use part of the muscle in work. Therefore, we can consciously control the speed and force of muscle contraction.

So, we looked at the process of “launching” muscle contraction. Now let's take a closer look at what happens directly inside the muscle during contraction. This material is somewhat difficult to understand, but very important. You need to understand it, otherwise you will not be able to fully understand how our muscles grow.

Muscle contraction in rough approximation

First of all, it is necessary to understand what consists of numerous strands of two proteins: myosin And actin, which are located along the myofibril. Moreover, myosin is thick filaments, and actin is thin filaments. This explains the light-dark striped structure of the myofibril (dark stripes - myosin, light stripes - actin).

In the literature, the dark areas of the myofibril are called the A-disc, and the light areas are called the I-disc. Actin filaments are attached to the so-called Z-line, which is located in the center of the I-disc. The myofibril segment between the Z-lines, including the myosin A-disc, is called sarcomere, which can be considered a kind of contractile unit of myofibril.

The sarcomere contracts as follows: with the help of lateral branches (bridges), thick myosin filaments draw thin actin filaments along themselves.

That is, the heads of the bridges engage with the actin filament and pull it between the myosin filaments. At the end of the movement, the heads disengage and engage again, continuing to retract. It turns out that muscle contraction is a combination of contractions of many sarcomeres.

If we consider the thin actin filament separately, it is a double helix of actin filaments, between which there is a double chain of tropomyosin.

Tropomyosin is also a protein that blocks the engagement of myosin bridges with actin in a relaxed muscle state. As soon as a nerve impulse is supplied to the muscle through a motor neuron, the charge polarity of the muscle cell membrane changes, as a result of which the cell is saturated with calcium ions (Ca++), which are released from special stores located along each myofibril. The tropomyosin filament, in the presence of calcium ions, instantly deepens between the actin filaments, and myosin bridges are able to engage with actin - muscle contraction becomes possible.

However, after Ca++ enters the cell, it immediately returns to its storage and muscle relaxation occurs. Only with constant impulses emanating from the nervous system can we maintain a prolonged contraction - this condition has been defined tetanic muscle contraction.

Of course, contracting muscles requires energy. Where does it come from, how is the energy that supports the movement of the myosin bridge formed? You will learn about this in the next article.

The materials in this article are protected by copyright law. Copying without providing a link to the source and notifying the author is PROHIBITED!

rice. 2.4. Electrical stimulation and muscle response. Electrical impulses are shown above, muscle response is below.

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

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

Superposition and tetanus

A single electrical stimulation (Fig. 2.4, top) leads to a single muscle contraction (Fig. 2.4, bottom). Two closely following stimuli are superimposed on each other (this is called “superposition”, or the summation of contractions), which leads to a stronger muscle response, close to the maximum. A series of frequently repeated electrical stimulations causes muscle contractions of increasing strength, resulting in the muscle not properly relaxing. If the frequency of electrical impulses is higher than the fusion frequency, then individual irritations merge into one and cause muscle tetanus (tetanic contraction) - a stable, fairly long-term tension of the contracted muscle.

Forms of abbreviations

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

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

• At isotonic contraction the muscle shortens, but its internal tension (tone!) remains unchanged in all phases of the work cycle. A typical example of isotonic muscle contraction is dynamic muscle action of flexors and extensors without significant changes in intramuscular tension, such as a pull-up.

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

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

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

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

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

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

Filament sliding mechanism

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

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

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

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

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

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

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

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

Relationship between sarcomere length and strength of muscle contractions

Rice. 2.7. Dependence of contraction force on sarcomere length

Muscle fibers develop the greatest force of contraction at a length of 2-2.2 microns. With strong stretching or shortening of the sarcomeres, the force of contraction decreases (Fig. 2.7). This dependence can be explained by the mechanism of filament sliding: at a given sarcomere length, the overlap of myosin and actin fibers is optimal; with greater shortening, the myofilaments overlap too much, and with stretching, the overlap of myofilaments is not sufficient to develop sufficient contractile force.

rice. 2.9 The effect of pre-stretching on the force of muscle contraction. Pre-stretching increases muscle tension. The resulting curve describing the relationship between muscle length and force of contraction under active and passive stretching demonstrates a higher isometric tension than at rest.

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

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

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

Introduction

All life activity of animals and humans is inextricably linked with mechanical movement carried out by muscles. All body movements, blood circulation, breathing and other acts are possible due to the presence in the body of muscles that have a special protein contractile complex - actomyosin.

However, the presence of contractile elements is important not only when performing the above macro movements. Currently, more and more data are accumulating on the role of contractile elements in microprocesses, in particular during the active transport of substances through membranes and during the movement of the cytoplasm. It has been established that the cytoplasm of all cells is in constant motion. According to Kamiya, the cytoplasm has oscillatory, circulating, gushing and other types of movement, which undoubtedly plays a large role in the course of metabolic processes in cells. Currently, there is no single point of view on the reasons for the origin of these movements of the cytoplasm, but the most likely hypothesis is the functioning of contractile elements similar to muscle ones.

Skeletal muscle contraction

smooth muscle contraction excitability

The main physiological properties of muscles are their excitability, conductivity and contractility. The latter manifests itself either in muscle shortening or in the development of tension.

Myography To record muscle contraction, the myography technique is used, i.e. graphically recording the contraction using a lever attached to one end of the muscle. The free end of the lever draws a contraction curve - a myogram - on the kymograph tape. This method of recording muscle contraction is simple and does not require complex equipment, but it has the disadvantage that the inertia of the lever and its friction on the surface of the kymograph tape somewhat distorts the recording. To avoid this drawback, a special sensor is now used that converts mechanical changes (linear movements or muscle efforts) into fluctuations in the strength of the electric current. The latter are recorded using a loop or cathode oscilloscope.

An accurate technique is also optical registration, performed using a beam of light reflected from a mirror glued to the belly of the muscle.

According to their own mechanical properties of the muscle belong to elastomers - materials with elasticity (stretchability and elasticity). If a muscle is subjected to external mechanical force, it stretches. The amount of muscle stretch in accordance with Hooke’s law will be proportional to the amount of deforming force (within certain limits):

where Dl is the absolute lengthening of the muscle; l -- initial muscle length; F-- deforming force; S -- cross-sectional area of ​​the muscle; b - elasticity coefficient. Magnitude of ratio F/S is called mechanical stress, and the value l/b is called the elastic modulus; it shows the amount of stress required to elongate a body by 2 times its original length.

In terms of its properties, muscle is close to rubber; the elastic modulus for both of these materials is approximately 10 kgf/cm2. Muscles also have other properties inherent in rubber. As with rubber stretching, when a muscle is strongly stretched, local crystallization is observed (ordering of the macromolecular protein structure of the fibrillar type). This phenomenon was studied by X-ray diffraction analysis. This releases crystallization heat, causing the muscle temperature to increase during stretching.

Once the external force is removed, the muscle regains its length. However, recovery is not complete. The presence of residual deformation characterizes the plasticity of the muscle - the ability to maintain its shape after the cessation of force. Thus, the muscle is not an absolutely elastic body, but has viscoelastic properties. When stretched very strongly, the muscle behaves like a normal elastic body. In this case, when stretched, the temperature of the muscle decreases.

When a muscle contracts, tension develops and work is done. Muscles have contractile and elastic elements. Therefore, the tension that arises and the work done is caused not only by the active contraction of the contractile complex, but also by passive contraction, determined by the elasticity or the so-called sequential elastic component of the muscle. Due to the sequential elastic component, work is performed only if the muscle has been previously stretched, and the amount of this work is proportional to the amount of muscle stretch. This largely explains the fact that the most powerful movements are performed with a large amplitude, which provides preliminary stretching of the muscles.

Muscle contractions are divided into isometric- occurring at a constant muscle length, and isotonic- occurring at constant voltage. Purely isometric or purely isotonic contractions with greater or lesser approximation can only be obtained in laboratory conditions when working on isolated muscles. In the body, muscle contractions are never purely isometric or purely isotonic.

Skeletal muscles are attached to bones by tendons, which form a system of levers. In most cases, muscles are attached to bones in such a way that when they contract, there is a gain in range of motion and an equivalent loss in strength. The lever arm of a muscle is in most cases smaller than the lever arm of the corresponding bone. According to Ackerman, the mechanical gain in range of motion of most human limbs ranges from 2.5 to 20. For the biceps brachii, it is approximately 10. As the bones move, the ratio of the muscle's lever arms to the bones changes, resulting in changes in muscle tension. For this reason, isotonic contractions are not observed under natural conditions. For the same reason, during the contraction process, the above values ​​of the mechanical gain in the amplitude of movements change.

Depending on the amount of force that the muscle overcomes, the speed of contraction (shortening) of the muscle varies. Hill, based on experimental data obtained when working on isolated muscles, derived the so-called basic equation of muscle contraction. According to Hill, the speed of muscle contraction v is hyperbolically dependent on the magnitude of the load F:

(F + a) (v + b) = const,

Where A and b -- constants approximately equal? F and correspondingly? v.

Fig.1. Dependence of the speed of contraction of the frog muscle on the magnitude of the load

Bayer made interesting comments on the equation. The equation is reduced to the form

F" v" = const,

if accepted F" = F + a And v" = v + b. Work F x v" represents the total power developed by a muscle during contraction. Because Fv less F"v", i.e. external power is less than the total power, then it should be assumed that the muscle performs not only external work, but also some internal work, manifested in the fact that the load seems to increase by A, and the speed of contraction by the amount b . This internal work can be interpreted as energy loss due to intramolecular friction in the form of thermal dissipation. Then, taking into account the comments made, it can be noted that the total muscle power within physiological limits is a constant value that does not depend on the magnitude of the load and the speed of contraction.

From a thermodynamic point of view, muscle is a system that converts chemical energy (ATP energy) into mechanical work, i.e. muscle is a chemo-mechanical machine.

As already noted, when a muscle contracts, heat is generated. Hill, using thermoelectric methods, established that with each stimulation, the heat of activation Q, which is constant in value and independent of the load, is first released, and then the heat of contraction kD l, proportional to muscle contraction Dl and load independent (k-proportionality coefficient). If the contraction is isotonic, then the muscle produces work A equal to the product of the load F by the magnitude of the contraction: A = FDl. According to the first law of thermodynamics, the change in internal energy DU of the muscle will be equal to the sum of the heat released and the work done:

-ДU = Q + kДl + FДl = Q + Дl (F + k)

Then the efficiency of muscle contraction will be equal to:

Considering that the values ​​of Q and k do not depend on F, it follows from the last equation that, within certain limits, the efficiency of muscle contraction will increase with increasing load.

Hill, based on the data he obtained in experiments, determined that the efficiency of muscle contraction is approximately 40%. If a muscle worked like a heat engine with an efficiency of 40%, then at an ambient temperature of 20 0 C, the temperature of the muscle should be equal to 215 0 C. The efficiency value of 40% shows the efficiency of converting ATP energy into mechanical energy. If we take into account that the efficiency of oxidative phosphorylation, during which ATP is synthesized, is about 50%, then the total efficiency of converting nutrient energy into mechanical energy will be approximately 20%.

Methods of muscle irritation. In order to cause muscle contraction, it is subjected to irritation. Direct irritation of the muscle itself (for example, by electric current) is called direct irritation; irritation of a motor nerve leading to contraction of a muscle innervated by this nerve is called indirect irritation. Due to the fact that the excitability of muscle tissue is less than that of nervous tissue, the application of irritating current electrodes directly to the muscle does not yet provide direct irritation: the current, spreading through the muscle tissue, acts primarily on the endings of the motor nerves located in it and excites them, which leads to muscle contraction. To obtain muscle contraction under the influence of direct stimulation, it is necessary to either turn off the motor nerve endings in it with curare poison, or apply a stimulus through a microelectrode inserted into the muscle fiber.

The processes of muscle work represent a multi-level complex of physiological and biochemical functions that are vital for the full functioning of the human body. Externally, similar processes can be observed in the examples of voluntary movements when walking, running, changing facial expressions, etc. However, they cover a much wider range of functions, which also include the work of the respiratory apparatus, digestive organs and excretory system. In each case, the mechanism of muscle contraction is supported by the work of millions of cells, which involve chemical elements and physical fibers.

Structural organization of muscle

Muscles are formed by many tissue fibers that have attachment points to the bones of the skeleton. They are located in parallel and interact with each other during muscle work. It is the fibers that provide the mechanism of muscle contraction when impulses arrive. Briefly, the structure of a muscle can be represented as a system consisting of sarcomere and myofibril molecules. It is important to understand that each muscle fiber is formed by many myofibril subunits, located longitudinally in relation to each other. Now it’s worth considering sarcomeres and filaments separately. Because they play an important role in motor processes.

Sarcomeres and filaments

Sarcomeres are segments of fibers that are separated by so-called Z-plates containing beta-actinin. Actin filaments extend from each plate, and the spaces are filled with thick myosin analogues. Actin elements, in turn, look like strings of beads twisted into a double helix. In this structure, each bead is an actin molecule, and in the areas with indentations in the helix there are troponin molecules. Each of these structural units forms a mechanism for contraction and relaxation of muscle fibers by communicating with each other. The cell membrane plays a key role in the excitation of fibers. It contains transverse invagination tubes that activate the function of the sarcoplasmic reticulum - this will be an exciting effect for muscle tissue.

Motor unit

It is now worth moving away from the in-depth structure of the muscle and considering the motor unit in the overall configuration of the skeletal muscle. This will be a collection of muscle fibers innervated by the processes of the motor neuron. The work of muscle tissue, regardless of the nature of the action, will be provided by the fibers included in one motor unit. That is, when a motor neuron is excited, the mechanism of muscle contraction is triggered within the same complex with the innervated processes. This division into motor neurons allows specific muscles to be targeted in a targeted manner without unnecessarily exciting adjacent motor units. In fact, the entire muscle group of one organism is divided into segments of motor neurons, which can unite to work on contraction or relaxation, or can act differently or alternately. The main thing is that they are independent of each other and work only with signals from their own group of fibers.

Molecular mechanisms of muscle work

In accordance with the molecular concept of thread sliding, the work of a muscle group and, in particular, its contraction is realized during the sliding action of myosins and actins. A complex mechanism of interaction between these threads is implemented, in which several processes can be distinguished:

• The central part of the myosin filament is connected to actin bundles.
• The achieved contact of actin with myosin promotes the conformational movement of the molecules of the latter. The heads enter the activity phase and unfold. In this way, the molecular mechanisms of muscle contraction are realized against the background of rearrangement of the threads of active elements in relation to each other.
• Then the mutual divergence of myosins and actins occurs, followed by restoration of the head part of the latter.

The entire cycle is performed several times, as a result of which the above-mentioned threads are displaced, and the Z-segments of the sarcomeres are brought closer together and shortened.

Physiological properties of muscle function

Among the main physiological properties of muscle work are contractility and excitability. These qualities, in turn, are determined by the conductivity of the fibers, plasticity and automatic properties. As for conductivity, it ensures the spread of the excitability process between myocytes along nexuses - these are special electrically conductive circuits responsible for conducting the muscle contraction impulse. However, after contraction or relaxation, fiber work also occurs.

Plasticity in a certain form is responsible for their calm state, which determines the preservation of a constant tone, in which the mechanism of muscle contraction is currently located. The physiology of plasticity can manifest itself both in the form of maintaining the shortened state of the fibers and in their stretched form. The property of automation is also interesting. It determines the ability of muscles to enter the working phase without connecting the nervous system. That is, myocytes independently produce rhythmically repeating impulses for certain fiber actions.

Biochemical mechanisms of muscle work

A whole group of chemical elements is involved in muscle function, including calcium and contractile proteins like troponin and tropomyosin. On the basis of this energy supply, the physiological processes discussed above are carried out. The source of these elements is adenosine triphosphoric acid (ATP), as well as its hydrolysis. At the same time, the ATP reserve in the muscle is capable of providing muscle contraction only for a fraction of a second. Despite this, the fibers can respond to nerve impulses in a constant manner.

The fact is that the biochemical mechanisms of muscle contraction and relaxation with the support of ATP are associated with the process of producing a reserve reserve of macroerg in the form of creatine phosphate. The volume of this reserve is several times greater than the supply of ATP and at the same time contributes to its generation. Also, in addition to ATP, glycogen can be an energy source for muscles. By the way, muscle fibers account for about 75% of the total supply of this substance in the body.

Coupling of excitatory and contractile processes

In a quiet state, the fiber strands do not interact with each other through sliding, since the centers of the ligaments are closed by tropomyosin molecules. Excitation can only take place after electromechanical coupling. This process is also divided into several stages:

• When a neuromuscular synapse is activated, a so-called postsynaptic potential is formed on the myofibril membrane, accumulating energy for action.
• The exciting impulse, thanks to a system of tubes, spreads across the membrane and activates the reticulum. This process ultimately helps to remove barriers from the membrane channels through which troponin-binding ions are released.
• The protein troponin, in turn, opens the centers of actin bundles, after which the mechanism of muscle contraction becomes possible, but it also requires an appropriate impulse to begin.
• The use of the opened centers will begin at the moment when myosin heads join them according to the model described above.

The full cycle of these operations occurs on average in 15 ms. The period from the initial point of fiber excitation to complete contraction is called latent.

The process of relaxation of skeletal muscle

When muscles relax, a reverse transfer of Ca++ ions occurs with the connection of the reticulum and calcium channels. As ions leave the cytoplasm, the number of ligament centers is reduced, resulting in the separation of actin and myosin filaments. In other words, the mechanisms of muscle contraction and relaxation involve the same functional elements, but operate them in different ways. After relaxation, a process of contracture may occur, in which a steady contraction of muscle fibers is noted. This state can persist until the next action of the irritating impulse occurs. There is also short-acting contracture, the prerequisites for which are tetanic contraction in conditions of accumulation of ions with large volumes.

Contraction phases

When the muscles are activated by an irritating impulse of suprathreshold force, a single contraction occurs, in which 3 phases can be distinguished:

• The period of latent type contraction already mentioned above, during which the fibers accumulate energy to perform subsequent actions. At this time, electromechanical coupling processes take place and the centers of the ligaments open. At this stage, the mechanism of muscle fiber contraction is prepared, which is activated after the propagation of the corresponding impulse.
• Shortening phase - lasts 50 ms on average.
• The relaxation phase also lasts approximately 50 ms.

Modes of muscle contraction

Single contraction work has been viewed as an example of “pure” muscle fiber mechanics. However, under natural conditions such work is not performed, since the fibers are in constant response to signals from the motor nerves. Another thing is that, depending on the nature of this response, work can occur in the following modes:

• Contractions occur at a reduced frequency of impulses. If the electrical impulse spreads after completion of relaxation, then a series of single acts of contraction follows.
• High frequency pulse signals may coincide with the relaxing phase of the previous cycle. In this case, the amplitude in which the muscle tissue contraction mechanism worked will be summed up, which will provide a long-term contraction with incomplete acts of relaxation.
• Under conditions of increasing impulse frequency, new signals will act during periods of shortening, which will provoke a prolonged contraction that will not be interrupted by relaxations.

Optimum and pessimum frequency

The amplitude of contractions is determined by the frequency of impulses that irritate muscle fibers. In this system of interaction of signals and responses, an optimum and a pessimum of frequency can be distinguished. The first indicates the frequency, which at the moment of action will be superimposed on the phase of increased excitability. In this mode, the mechanism of muscle fiber contraction with a large amplitude can be activated. In turn, the pessimum determines a higher frequency, the impulse of which falls on the refractory phase. Accordingly, in this case the amplitude decreases.

Types of skeletal muscle work

Muscle fibers can carry out work dynamically, statically and dynamically inferior. Standard dynamic work is overcoming - that is, the muscle at the moment of contraction moves objects or its components in space. The static action of the muscle is in some way relieved of stress, since in this case there is no change in its state. The dynamic yielding mechanism of muscle contraction in skeletal muscle is activated when the fibers function under conditions of tension. The need for parallel stretching may also be due to the fact that the work of the fibers involves performing operations with third-party bodies.

Finally

The processes of organizing muscle action involve a variety of functional elements and systems. The work involves a complex set of participants, each of whom performs their own task. You can see how in the process of activating the mechanism of muscle contractions, indirect functional blocks are also triggered. For example, this concerns the processes of generating energy potential to perform work or the system of blocking the centers of ligaments through which myosins and actins are connected.

The main load falls directly on the fibers that perform certain actions at the commands of the motor units. Moreover, the nature of performing a certain job may be different. It will be influenced by the parameters of the directed impulse, as well as the current state of the muscle.