Mobility is a characteristic property of all forms of life. Directional movement occurs when chromosomes diverge during cell division, active transport of molecules, movement of ribosomes during protein synthesis, muscle contraction and relaxation. Muscle contraction is the most advanced form of biological mobility. Any movement, including muscle movement, is based on common molecular mechanisms.
In humans, several types of muscle tissue are distinguished. Striated muscle tissue makes up the skeletal muscles (skeletal muscles that we can contract at will). Smooth muscle tissue is part of the muscles of internal organs: the gastrointestinal tract, bronchi, urinary tract, blood vessels. These muscles contract involuntarily, regardless of our consciousness.
In this lecture, we will consider the structure and processes of contraction and relaxation of skeletal muscles, since they are of the greatest interest for the biochemistry of sports.
Mechanism muscle contraction so far not fully disclosed.
The following is reliably known.
1. The source of energy for muscle contraction are ATP molecules.
2. Hydrolysis of ATP is catalyzed during muscle contraction by myosin, which has enzymatic activity.
3. The triggering mechanism of muscle contraction is an increase in the concentration of calcium ions in the sarcoplasm of myocytes, caused by a nerve motor impulse.
4. During muscle contraction, transverse bridges or adhesions arise between the thin and thick filaments of myofibrils.
5. During muscle contraction, thin threads slide along thick ones, which leads to shortening of myofibrils and all muscle fiber generally.
There are many hypotheses explaining the mechanism of muscle contraction, but the most reasonable is the so-called hypothesis (theory) of "sliding threads" or "rowing hypothesis".
In a muscle at rest, thin and thick threads are in a disconnected state.
Under the influence of a nerve impulse, calcium ions leave the cisterns of the sarcoplasmic reticulum and attach to the protein of thin filaments - troponin. This protein changes its configuration and changes the configuration of actin. As a result, a transverse bridge is formed between the actin of thin filaments and myosin of thick filaments. This increases the ATPase activity of myosin. Myosin breaks down ATP and, due to the energy released during this, the myosin head turns like a hinge or a boat's oar, which leads to the sliding of muscle filaments towards each other.
Having made a turn, the bridges between the threads are torn. The ATPase activity of myosin sharply decreases, and the hydrolysis of ATP stops. However, with the further arrival of the nerve impulse, the transverse bridges are formed again, since the process described above is repeated again.
Each contraction cycle consumes 1 ATP molecule.
Muscle contraction is based on two processes:
Spiral twisting of contractile proteins;
Cyclically repeating formation and dissociation of the complex between the myosin chain and actin.
Muscle contraction is initiated by the arrival of an action potential at the end plate of the motor nerve, where the neurohormone acetylcholine is released, the function of which is to transmit impulses. First, acetylcholine interacts with acetylcholine receptors, which leads to the propagation of the action potential along the sarcolemma. All this causes an increase in the permeability of the sarcolemma for Na + cations, which rush into the muscle fiber, neutralizing the negative charge on the inner surface of the sarcolemma. The transverse tubules of the sarcoplasmic reticulum are associated with the sarcolemma, along which the excitation wave propagates. From the tubules, the excitation wave is transmitted to the membranes of the vesicles and cisterns, which braid the myofibrils in the areas where the actin and myosin filaments interact. When a signal is transmitted to the cisterns of the sarcoplasmic reticulum, the latter begin to release Ca 2+ in them. The released Ca 2+ binds to Tn-C, which causes conformational shifts that are transferred to tropomyosin and then to actin. Actin is, as it were, released from the complex with components of thin filaments in which it was located. Then actin interacts with myosin, and the result of this interaction is the formation of adhesions, which makes possible the movement of thin filaments along thick ones.
The generation of force (shortening) is due to the nature of the interaction between myosin and actin. There is a movable hinge on the myosin rod, in the area of which a rotation occurs when the globular head of myosin binds to a specific area of actin. It is these turns, occurring simultaneously in numerous areas of interaction between myosin and actin, that are the reason for the retraction of actin filaments (thin filaments) into the H-zone. Here they are in contact (at maximum shortening) or even overlap with each other, as shown in the figure.
v
Drawing. Reduction mechanism: a- a state of rest; b- moderate reduction; v- maximum reduction
The energy for this process is supplied by the hydrolysis of ATP. When ATP is attached to the head of the myosin molecule, where the active center of myosin ATPase is located, no connection is formed between the thin and thick filaments. The emerging calcium cation neutralizes the negative charge of ATP, facilitating rapprochement with the active center of myosin ATPase. As a result, phosphorylation of myosin occurs, i.e., myosin is charged with energy, which is used to form adhesion with actin and to advance the thin filament. After the thin filament has advanced one "step", ADP and phosphoric acid are cleaved from the actomyosin complex. Then a new ATP molecule is attached to the myosin head, and the whole process is repeated with the next head of the myosin molecule.
ATP expenditure is also necessary for muscle relaxation. After the cessation of the action of the motor impulse, Ca 2+ passes into the cisterns of the sarcoplasmic reticulum. Tn-C loses the calcium bound to it, which results in conformational shifts in the troponin-tropomyosin complex, and Tn-I again closes the active centers of actin, making them unable to interact with myosin. The concentration of Ca 2+ in the area of contractile proteins becomes below the threshold, and muscle fibers lose their ability to form actomyosin.
Under these conditions, the elastic forces of the stroma, deformed at the moment of contraction, take over, and the muscle relaxes. In this case, thin threads are removed from the space between the thick threads of disk A, zone H and disk I acquire their original length, lines Z move away from each other at the same distance. The muscle becomes thinner and longer.
Hydrolysis rate ATF during muscular work it is enormous: up to 10 micromolar mol per 1 g of muscle in 1 min. General stocks ATF are small, therefore, to ensure normal muscle function ATF must recover at the same rate as it is consumed.
Muscle relaxation occurs after the cessation of the receipt of a prolonged nerve impulse. In this case, the permeability of the wall of the cisterns of the sarcoplasmic reticulum decreases, and calcium ions under the action of a calcium pump, using the energy of ATP, go into the cisterns. Removal of calcium ions into the cisterns of the reticulum after the cessation of the motor impulse requires significant energy consumption. Since the removal of calcium ions occurs towards a higher concentration, i.e. against the osmotic gradient, then two ATP molecules are spent to remove each calcium ion. The concentration of calcium ions in the sarcoplasm rapidly decreases to the initial level. Proteins again acquire the conformation characteristic of the resting state.
Thus, both the process of muscle contraction and the process muscle relaxation- these are active processes with energy expenditure in the form of ATP molecules,
Smooth muscle lacks myofibrils, which consist of several hundred sarcomeres. Thin filaments are attached to the sarcolemma, while thick filaments are inside the filaments. Calcium ions also play a role in contraction, but they enter the muscle not from the cisterns, but from the extracellular substance, since there are no cisterns with calcium ions in smooth muscles. This process is slow and therefore smooth muscles work slowly.
on subject
"Biochemistry"
"Biochemistry of muscle contraction"
Completed: 3rd year student of the EHF
department "Valeology", gr. 1A
Litvichenko E.M.
Checked by: Saykovich E.G.
Novosibirsk 2000
The interest of biochemistry in the processes occurring in contracting muscles is based not only on the elucidation of the mechanisms of muscle diseases, but what may be even more important is the disclosure of the mechanism for converting electrical energy into mechanical energy, bypassing the complex mechanisms of traction and transmission.
In order to understand the mechanism and biochemical processes occurring in the contracting muscles, it is necessary to look into the structure of the muscle fiber. The structural unit of muscle fiber is myofibrils - specially organized bundles of proteins located along the cell. Myofibrils, in turn, are built of two types of protein filaments (filaments) - thick and thin. The main protein of thick filaments is myosin, and thin - actin... Myosin and actin filaments are the main component of all contractile systems in the body. Electron microscopic examination showed a strictly ordered arrangement of myosin and actin filaments in the myofibril. The functional unit of the myofibril is the sarcomere - a section of the myofibril between two Z-plates. The sarcomere includes a bundle of myosin filaments, interconnected in the middle along the so-called M-plate, and actin filaments passing between them, which in turn are attached to the Z-plates.
Contraction occurs by sliding thin actin and thick myosin filaments towards each other or pushing actin filaments between myosin filaments in the direction of the M-line. The maximum shortening is achieved when the Z-plates, to which the actin filaments are attached, approach the ends of the myosin filaments. With a reduction, the sarcomere is shortened by 25-50%.
The sarcoplasm containing myofibrils is penetrated between them by a network of cisterns and tubules of the endoplasmic reticulum, as well as a system of transverse tubules that are in close contact with it, but do not communicate.
The structure of myosin filaments.
Myosin filaments are formed by the protein myosin, the molecule of which contains two identical heavy polypeptide chains with a molecular weight of about 200,000 and four light chains (about 20,000). Each heavy chain has an a-helix conformation for most of its length, and both heavy chains are twisted together, forming part of the molecule in the shape of a rod. At opposite ends of each chain, two light chains are attached; together with the globular shape of these ends of the chain, they form the "heads" of molecules. Rod-shaped ends of molecules can connect to each other longitudinally, forming bundles, while the heads of the molecules are located outside of the bundle in a spiral. In addition, in the region of the M-line, the beams are connected to each other "tail to tail". Each myosin filament contains about 400 myosin molecules.
actin molecules
troponin molecules tropomyosin molecules
Another protein that is part of the actin filaments - tropomyosin - has the shape of rods, it is located near the grooves of the spiral tape of fibrillar actin, along it. Its length is 8 times more size globular actin, because one tropomyosin molecule contacts seven actin molecules at once and ends are connected to each other, forming a third longitudinal spirally twisted chain.
The third protein of actin filaments - troponin - consists of three different subunits and has a globular shape. It is non-covalently bound to actin and tropomyosin in such a way that there is one tropomyosin molecule per troponin molecule, in addition, one of its subunits contains Ca- connecting centers. Thin actin filaments are attached to Z-plates, also protein structures.
The mechanism of muscle contraction.
Muscle contraction is the result of shortening of each sarcomere; the maximum shortening of the sarcomere is achieved when the Z-plates, to which the actin filaments are attached, come close to the ends of the myosin filaments.
Actin and myosin filaments have their own roles in muscle contraction: myosin filaments contain an active center for hydrolysis of ATP, a device for converting ATP energy into mechanical energy, a device for adhesion to actin filaments and devices for perceiving regulatory signals from the actin filaments, actin filaments have a mechanism adhesion to myosin filaments and a mechanism for regulating contraction and relaxation.
Muscle contraction is triggered by the action potential of the nerve fiber, which is transformed through the neuromuscular synapse through a mediator into the action potential of the sarcolemma and T-system tubules. The branches of the tubules surround each myofibril and contact the cisterns of the sarcoplasmic reticulum. The tanks contain a significant concentration of Ca... The action potential flowing through the ducts causes the release of ions Ca 2+ from the cisterns of the sarcoplasmic reticulum. Jonah Ca 2+ attach to the Ca-binding subunit of troponin. In the presence of ions Ca 2+ on the monomers of actin filaments, the centers of binding of myosin heads are opened, and throughout the troponin-tropomyosin-actin system. As a result of these changes, the myosin head attaches to the nearest actin monomer.
Myosin heads have a high affinity for ATP, so that most of the heads in muscle contain bound ATP. The attachment of the myosin head to actin activates the ATPase center, ATP is hydrolyzed, ADP and phosphate leave the active center, which leads to a change in the myosin conformation: additional stress arises, tending to reduce the angle between the head and tail of the myosin molecule, i.e. tilt the head in the direction of the M-line. Since the myosin head is connected to the actin filament, bending towards the M-line, it displaces the actin filament in the same direction.
ADP released from multiple heads undergoes the following transformation:
2 ADP ® ATP + AMP
The heads released from ATP again attract ATP due to its high affinity, which was already mentioned above, the addition of ATP reduces the affinity of the myosin head with actin filaments and myosin returns to its original state. Then the whole cycle is repeated from the very beginning, but since in the previous cycle the actin filament brought the Z-plate closer due to its movement, the same myosin head is attached to another actin monomer closer to the Z-plate.
Hundreds of myosin heads of each myosin filament work simultaneously, thus pulling in the actin filament.
Sources of energy for muscle contraction.
Skeletal muscle, working at maximum intensity, consumes hundreds of times more energy than at rest, and the transition from a state of rest to a state of maximum work occurs in a fraction of a second. In this regard, in muscles, the mechanism for changing the rate of ATP synthesis in a very wide range is structured in a completely different way.
As mentioned with muscle contraction great importance has a process for the synthesis of ATP from ADP released from myosin heads. This happens with the help of a high-energy substance in the muscles. creatine phosphate which is formed from creatine and ATP on action creatine kinase :
C-NH 2 C-NH-PO 3 H 2
N-CH 3 + ATP- N-CH 3 + ADP
Creatine Creatine Phosphate
This reaction is easily reversible and anaerobic, which allows the muscles to be quickly recruited in the early stages. As the load continues, the role of such energy supply decreases, and glycogen mechanisms of providing a large amount of ATP come to replace it.
Bibliography:
G. Duga, K. Penny "Bioorganic chemistry", M., 1983
D. Metzler "Biochemistry", M., 1980
A. Leinger "Fundamentals of Biochemistry", M., 1985
Lecture notes| Summary of the lecture | 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 muscles
» Physiological properties muscle
Muscle contraction is a vital body function associated with defensive, respiratory, food, sexual, excretory and other physiological processes. All types of voluntary movements - walking, facial expressions, eyeball movements, swallowing, breathing, etc. are carried out at the expense of skeletal muscles. Involuntary movements (except for the contraction of the heart) - peristalsis of the stomach and intestines, changes in the tone of blood vessels, maintenance of tone Bladder- due to the contraction of smooth muscles. The work of the heart is provided by the contraction of the heart muscles.
Structural organization of skeletal muscle
Muscle fiber and myofibril (Fig. 1). Skeletal muscle is made up of many muscle fibers that have points of attachment to bones and are parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from blocks repeating in the longitudinal direction (sarcomeres). The sarcomere is a functional unit of the contractile apparatus of the skeletal muscle. Myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a cross-striation pattern.
Sarcomere and phylaments. The sarcomeres in the myofibril are separated from each other by Z-plates, which contain the beta-actinin protein. In both directions, thin actin filaments extend from the Z-plate. In between, thicker myosin filaments are located.
Actin filament looks like two strands of beads twisted into a double helix, where each bead is an actin protein molecule. In the depressions of the actin helices, at an equal distance from each other, there are troponin protein molecules connected to the filamentous molecules of the tropomyosin protein.
Myosin phylaments are formed by repeating molecules of the myosin protein. Each myosin molecule has a head and tail. The myosin head can bind to the 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 tubules) is an intracellular network of closed tubules and performs the function of depositing Ca ++ ions.
Motor unit. The functional unit of skeletal muscle is the motor unit (MU). DE - a set of muscle fibers that are innervated by the processes of one motor neuron. Excitation and contraction of the fibers that make up one DE occurs simultaneously (when the corresponding motoneuron is excited). Individual MUs can be excited and contracted independently of each other.
Molecular mechanisms of contractionskeletal muscle
According to the theory of slipping filaments, 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 the binding sites of actin phylaments (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, C).
• The cycle "binding - change of conformation - detachment - restoration of conformation" occurs many times, as a result of which actin and myosin phylaments are displaced relative to each other, Z-disks of sarcomeres come closer and myofibril is shortened (Fig. 2, D).
Conjugation of excitement and contractionin skeletal muscle
At rest, the filaments do not slip in the myofibril, since the binding sites on the actin surface are closed by the tropomyosin protein molecules (Fig. 3, A, B). Excitation (depolarization) of the myofibril and muscle contraction proper are associated with the process of electromechanical coupling, which includes a number of sequential events.
• As a result of the triggering of the neuromuscular synapse on the postsynaptic membrane, 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, due to the system of transverse tubules, reaches the sarcoplasmic reticulum. Depolarization of the membrane of the sarcoplasmic reticulum leads to the opening of Ca ++ channels in it, through which Ca ++ ions escape into the sarcoplasm (Fig. 3, C).
• Ca ++ ions bind to the troponin protein. Troponin changes its conformation and displaces the tropomyosin protein molecules that closed the actin binding sites (Fig. 3d).
• The myosin heads are attached 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.
Relaxation of skeletal muscle
Muscle relaxation is caused by the reverse transport of Ca ++ ions through a calcium pump into the channels of the sarcoplasmic reticulum. As Ca ++ is removed from the cytoplasm open centers binding becomes less and less and in the end the actin and myosin phylaments are completely disconnected; muscle relaxation occurs.
A contracture is called a persistent long-term muscle contraction that persists after the termination of the stimulus. Short-term contracture can develop after 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 with poisons, metabolic disorders.
Phases and modes of skeletal muscle contraction
Muscle contraction phases
In case of irritation of skeletal muscle with a single impulse electric current of suprathreshold strength, a single muscle contraction occurs, in which 3 phases are distinguished (Fig. 4, A):
• latent (latent) period of contraction (about 10 ms), during which the action potential develops and the processes of electromechanical coupling take place; muscle excitability during a single contraction changes in accordance with the phases of the action potential;
• shortening phase (about 50 ms);
• relaxation phase (about 50 ms).
Muscle contraction modes
Under natural conditions, in the body, a single muscle contraction is not observed, since a series of action potentials are running along the motor nerves that innervate the muscle. Depending on the frequency of nerve impulses coming to the muscle, the muscle can contract in one of three modes (Fig. 4, B).
• Single muscle contractions occur at a low frequency of electrical impulses. If the next impulse enters the muscle after the end of the relaxation phase, a series of successive single contractions occurs.
• At a higher frequency of impulses, the next impulse may coincide with the relaxation phase of the previous contraction cycle. The amplitude of contractions will be summed up, serrated tetanus will appear - a prolonged contraction interrupted by periods of incomplete muscle relaxation.
• 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 prolonged contraction, not interrupted by periods of relaxation.
Optimum and pessimum of frequency
The amplitude of tetanic contraction depends on the frequency of impulses that irritate the muscle. The frequency optimum is called 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 enters the refractory phase (Fig. 4, A), as a result of which the tetanus amplitude decreases significantly.
Skeletal muscle work
The strength of skeletal muscle contraction is determined by 2 factors:
• the number of DEs participating in the reduction;
• the frequency of contraction of muscle fibers.
The work of the skeletal muscle is accomplished by a consistent change in the tone (tension) and length of the muscle during contraction.
Types of work of skeletal muscle:
• 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 kept in a certain position;
• dynamic yielding work is performed if the muscle is functioning, but at the same time stretching, since the effort it makes is not enough to move or hold parts of the body.
While doing the work, the muscle can contract:
• isotonic - the muscle is shortened with constant tension (external load); isotonic contraction is reproduced only in 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 with dynamic overcoming work.
Average load rule- the muscle can perform maximum work at medium loads.
Fatigue is a physiological condition of a muscle that develops after long-term work and is manifested by a decrease in the amplitude of contractions, lengthening the latent period of contraction and the relaxation phase. The causes of fatigue are: depletion of the ATP reserve, the accumulation of metabolic products in the muscle. Muscle fatigue during rhythmic work is less than synaptic fatigue. Therefore, when the body performs muscular work, fatigue initially develops at the level of synapses of the central nervous system and neuro-muscular synapses.
Structural organization and reductionsmooth muscles
Structural organization. Smooth muscle consists of single spindle-shaped cells (myocytes), which are located in the muscle more or less randomly. The contractile filaments are located irregularly, as a result of which there is no transverse striation of the muscle.
The contraction mechanism is similar to that in skeletal muscle, but the sliding rate of filaments and the rate of ATP hydrolysis is 100–1000 times lower than in skeletal muscle.
The mechanism of conjugation 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 an enzyme (myosin kinase) that transfers the phosphate group from ATP to myosin. The phosphorylated myosin heads acquire the ability to bind to actin phylaments.
Contraction and relaxation of smooth muscles. The rate of removal of Ca ++ ions from the sarcoplasm is much lower than in the skeletal muscle, as a result of which relaxation occurs very slowly. Smooth muscles make long tonic contractions and slow rhythmic movements. Due to the low intensity of ATP hydrolysis, smooth muscles are optimally adapted for prolonged contraction, which does not lead to fatigue and high energy consumption.
Physiological properties of muscles
The general physiological properties of skeletal and smooth muscle are excitability and contractility. Comparative characteristics of skeletal and smooth muscles are given in table. 6.1. Physiological properties and features of cardiac muscles are discussed in the section "Physiological mechanisms of homeostasis".
Table 7.1. Comparative characteristics of skeletal and smooth muscles
|
Property |
Skeletal muscle |
Smooth muscles |
|
Depolarization rate |
slow |
|
|
Refractory period |
short |
long |
|
The nature of the reduction |
fast phasic |
slow tonic |
|
Energy consumption |
||
|
Plastic |
||
|
Automation |
||
|
Conductivity |
||
|
Innervation |
motor neurons of somatic NS |
postganglionic neurons of the autonomic NS |
|
Movements performed |
arbitrary |
involuntary |
|
Chemical sensitivity |
||
|
The ability to divide and differentiate |
The plasticity of smooth muscles is manifested in the fact that they can maintain a constant tone both in a shortened and in a stretched 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 automatic property of smooth muscles 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
The striated muscles are classified into two types: skeletal muscle and myocardium.
Muscle fiber structure
The muscle cell membrane, 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, that is, lower than that of the nerve fiber (-70 mV); 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, since the muscle cell needs to be depolarized by a large amount.
The muscle fiber response to excitation is reduction, which is performed by the contractile apparatus of the cell - 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 thread, which consists of individual globular proteins, linked together in such a way that the entire structure is an elongated chain. Globular actin (G-actin) molecules have lateral and terminal binding sites with other molecules of the same kind. As a result, they combine 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 (turn length) is 38 nm; there are 7 G-actin pairs for each helix turn. 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. Combining individual globules of G-actin into F-actin
The protein tropomyosin is located along the spiral grooves of actin filaments.Each tropomyosin strand, which is 41 nm long, consists of two identical α-chains, together twisted into a helix with a turn length of 7 nm. Two tropomyosin molecules are located along one loop of F-actin. Each tropomyosin molecule combines, slightly overlapping, with the next, as a result of which the tropomyosin filament extends continuously along the actin.
Fig. 2. The structure of a thin filament of myofibril
In the cells of striated muscles, the composition of thin filaments, in addition to actin and tropomyosin, also includes the protein troponin. This globular protein has a complex structure. It consists of three subunits, each of which performs its own function in 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 strands: two long and four short. The long chains are coiled together 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.
Two fragments are distinguished in myosin: light meromyosin (LMM) and heavy meromyosin (TMM), between them there is a hinge. TMM consists of two sub-fragments: S1 and S2. LMM and sub-fragment S2 are embedded in the bundle of filaments, and sub-fragment S1 protrudes above the surface. This protruding end (myosin head) is able to bind to the active center 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 microns. It is one of the largest biological molecular structures known in nature.
When viewed in a polarizing microscope a longitudinal section of the striated muscle, light and dark areas are visible. Dark areas (discs) are anisotropic: in polarized light they look transparent in the longitudinal direction and opaque in the transverse direction, they are denoted by the letter A. Light areas are isotropic and are denoted by the letter I. Disc I includes only thin filaments, and disc A - and thick and thin. In the middle of disk A, a light stripe is visible, called the H-zone. It has no fine threads. Disc I is divided by a thin strip Z, which is a membrane containing structural elements that hold together the ends of thin filaments. 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 the cross section, the muscle fiber has the correct hexagonal structure.
Muscle contraction
With muscle contraction, the length of the actin and myosin filaments does not change. There is only their displacement relative to each other: thin threads are pushed into the gap between thick ones. In this case, the length of the disc A remains unchanged, and the disc I is shortened, 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, it is possible to change the length of the sarcomere from approximately 2.5 to 1.7 microns.
The myosin filament has many heads on it, with which it can bind to actin. The actin filament, in turn, has sections (active centers) to which the myosin heads can attach. In a resting muscle cell, these binding sites are covered by tropomyosin molecules, which prevents the formation of bonds between thin and thick filaments.
For actin and myosin to interact, calcium ions must be present. At rest, they are in the sarcoplasmic reticulum. This organelle is a membrane cavity containing a calcium pump, which, due to 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. The action potential spreading 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 one 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 the bond is formed, the myosin head, previously located at a right angle to the filaments, tilts and pulls the actin filament relative to the myosin one by about 10 nm. The formed atin-myosin complex prevents further sliding of the filaments relative to each other, therefore, its separation is necessary. This is possible only due to the energy of ATP. Myosin has ATP-ase activity, that is, it is capable of causing ATP hydrolysis. The energy released during this breaks the bond between actin and myosin, and the myosin head is able to interact with a new region 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 muscles are relaxed, the work of the calcium pump is activated, which lowers the concentration of Ca2 + in the cytoplasm; therefore, bonds between thin and thick threads can no longer form. Under these conditions, when the muscles are stretched, the threads slide freely 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 rupture. The threads remain rigidly interconnected. This phenomenon is observed with rigor mortis.
Fig. 6. Contraction of the sarcomere: 1 - myosin thread; 2 - active center; 3 - actin thread; 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) transverse bridges tilt and pull a thin thread relatively thick, as a result of which the length of the sarcomere decreases;
G) the bonds between the filaments 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 motionlessly fixed, as a result of which it is not the length that changes, but the tension).
Power and speed of muscle contraction
The strength and speed of contraction are important characteristics of a muscle. The equations expressing these characteristics were empirically obtained by A. Hill and subsequently confirmed by the kinetic theory of muscle contraction (Descherevsky's model).
Hill's equation linking the strength and speed of muscle contraction is as follows: (P + a) (v + b) = (P0 + a) b = a (vmax + b), where v is the speed of muscle shortening; P - muscle strength or the load applied to it; vmax - maximum speed shortening of the muscle; P0 is the force developed by the muscle in the isometric contraction mode; a, b - constants. general power developed by the muscle is determined by the formula: Ntot = (P + a) v = b (P0-P). Efficiency muscle remains constant ( about 40%) in the range of force values from 0.2 P0 to 0.8 P0. In the process of muscle contraction, a certain amount of heat is released. This quantity is called heat products... 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 highly dependent 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 rate for a given muscle: vmax = b (P0 / a).
Characteristics of muscle tissue.
Skeletal muscle contraction and its mechanisms
Types of muscle tissue. Actin-myosin complex and mechanisms of its functioning.
There are 3 types of animal tissues 1) muscular, 2) nervous, 3) secretory. The first responds to arousal by contracting and carrying out the work of moving. The second - the ability to conduct and analyze impulses, the third - to highlight various secrets.
There are 3 types of muscle tissue: 1. striated, 2. smooth, 3. cardiac.
| Specifications | cross-striped | smooth | heart |
| specialization | very high | least specialized | secondary specialized |
| structure | fibers up to 10 cm long, divided into subunits - sarcomeres. The fibers are interconnected by connective tissue, blood vessels. The nerve endings that form neuromuscular connections | Consists of individual spindle-like. cl., connected in bundles. | Cells branch out at the ends, connect with others with the help of processes. |
| core | Several cores at the periphery | 1 core per cent | several cores in the center |
| cytoplasm | contains mitochondria, sarcoplasm. reticulum, T tube, glycogen, fat drops | sod. mitochondr., sarcoplasm. reticulum, Tubes, | sod. mitochondr., sarcoplasm. reticulum, T tube, |
| sarcolemma | there is | No | there is |
| regulation | neurogenic | neurogenic | neurog. and humoral |
| transverse stripes | there is | No | there is |
| Connection activity. | powerful, fast contractions. The period of refractoriness is short; the rest time is short; rapid fatigue. | slow rhythm | fast rhythm, long refractory time - no fatigue. |
Actin-myosin complex. All muscle cells. contain a large amount of special contractile proteins - 60-80% of the total amount of muscle proteins. The main contractile
proteins are fibrillar proteins: - myosin- forms thick threads; - actin- forms fine threads. Globular proteins are used to regulate contraction: troponin-tropomyosin.
Myosin is a 2-chain structure 1 = 180 nm and 0 = 2.5 nm. Actin is a 2-helical 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 mediator and
propagation of the action potential along cell membranes and inside the muscle
fibers on T tubes. As a result of the actin-myosin interaction, fibrils are reduced. This is achieved by pushing the actin filament by the myosin head as a result of 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 the actin-myosin interaction.
Reduction phases:
1. Latent period - 0.05 sec.
2. Phase of contraction - 0.1 sec
3. Relaxation period - 0.2 sec.
Biochemistry of muscle work
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 - decomp. Nervous. End -
5. Dairy to - that + O2 - CO2 + H2O (rest) or Mol.k - that - glucose - glycogen.
Skeletal muscle contraction mechanism
Muscle shortening is the result of many sarcomeres contracting. When shortened, the actin filaments slide relative to the myosin filaments, as a result of which the length of each sarcomere of the muscle fiber decreases. In this case, the length of the threads themselves remains unchanged. Myosin filaments have transverse protrusions (transverse bridges) about 20 nm long. Each protrusion consists of a head, which is connected to the myosin filament by means of a "neck" (Fig. 23).
In a relaxed state, the muscles of the head of the transverse bridges cannot interact with the actin filaments, since their active sites (places of mutual contact with the heads) are isolated by tropomyosin. Muscle shortening is the result of conformational changes in the transverse bridge: its head tilts by flexing the "neck".
Rice. 23. Foreign organization of contractile and regulatory proteins in the striated muscle. The position of the myosin bridge is shown (rowing effect, the neck is bent) in the process of interaction of contractile proteins in the muscle fiber (fiber contraction)
Sequence of processes , providing muscle fiber contraction(electromechanical interface):
1. After the occurrence PD in the muscle fiber near the synapse (due to the electric field of the EPP) excitation spreads along the myocyte membrane, including transverse membranes T-tubules... The mechanism of AP conduction along the muscle fiber is the same as for the non-myelinated nerve fiber - the AP that has arisen near the synapse through its electric field ensures the emergence of new AP in the adjacent section of the fiber, etc. (continuous conduction of excitement).
2. Potential actions T-tubules due to its electric field activates voltage-gated calcium channels on membrane SPR, as a result of which Ca2 + leaves the LSS tanks according to the electrochemical gradient.
3. In the interfibrillar space Ca2 + contacts with troponin, which leads to its conformation and displacement of tropomyosin, resulting in active sites are exposed connected to heads of myosin bridges.
4. As a result of interaction with actin ATPase activity of the heads of myosin filaments is enhanced, ensuring the release of ATP energy, which is spent on flexion of the myosin bridge, outwardly resembling the movement of oars when rowing (rowing movement) (see fig. 23), providing the sliding of actin filaments relative to myosin... The energy of one ATP molecule is consumed to complete one stroke movement. In this case, the strands 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 termination of its engagement, but the ATP energy is not consumed. In the absence of ATP, the myosin heads cannot break away from actin - the muscle is tense; such is, in particular, the mechanism of rigor mortis.
5. After that the heads of the transverse bridges, due to their elasticity, return to their original position and establish contact with the next section of actin; then the next rowing movement and sliding of actin and myosin filaments occurs again. Such elementary acts are repeated many times. One stroke movement (one step) causes a decrease in the length of each sarcomere by 1%. When shortening isolated muscle frogs without load 50% shortening of sarcomeres occurs in 0.1 s. This requires 50 stroke movements.
The 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 occurs through processes proceeding in the reverse order. Repolarization of the sarcolemma and T-tubules leads to the closure of the voltage-gated calcium channels of the SPR membrane. Ca-pumps return Ca2 + to the SPR (the activity of the pumps increases with an increase in the concentration of free ions).
A decrease in the concentration of Ca2 + in the interfibrillar space causes the reverse conformation of troponin, as a result of which the tropomyosin filaments isolate the active regions of actin filaments, which makes it impossible for the heads of myosin transverse bridges to interact with them. The sliding of actin filaments along myosin filaments in the opposite direction occurs under the action of gravitational forces and elastic traction elements of muscle fiber, which restores the original size of the sarcomeres.
The source of energy for ensuring the work of skeletal muscles is ATP, the costs of which are significant. Even under conditions of basic metabolism for the functioning of the muscles, the body affects about 25% of all its energy resources. Energy costs rise 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 required for the following processes.
First, the energy of ATP 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, which form PP and AP, providing electromechanical coupling) and the operation of the Ca-pump, which lowers the concentration of Ca2 + in the sarcoplasm. after the contraction of the muscle fiber, which leads to relaxation.
Secondly, ATP energy is spent on the stroke movement of the myosin bridges (bending them).
Resynthesis of ATP carried out with the help of three energy systems of the body.
1. Phosphogenic energy system provides resynthesis of ATP due to the high-energy CP available in the muscles and adenosine diphosphoric acid formed during the breakdown of ATP (adenosine diphosphate, ADP) with the formation of creatine (C): ADP + + CP → ATP + K. This is an instant resynthesis of ATP, while the muscle can develop great power , but for a short time - up to 6 s, since the reserves of CF in the muscle are limited.
2. Anaerobic glycolytic energy system provides resynthesis of ATP due to 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, by causing a local vasodilating effect, improves blood flow in the working muscle and its supply of oxygen and nutrients.
3. The aerobic energy system provides ATP resynthesis through oxidative phosphorylation of carbohydrates and fatty acids flowing in the mitochondria of muscle cells. This way can provide energy for muscle work 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 contractions muscles distinguish three types of them: isometric, isotonic and auxotonic.
Auxotonic muscle contraction consists in a simultaneous change in the length and tension of the muscle. This type of contraction is characteristic of natural motor acts and is of 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 lower limbs after squatting (lifting).
Isometric muscle contraction- when muscle tension increases, but its length does not change. This type of contraction can be observed in the experiment, when both ends of the muscle are fixed and there is no possibility of their convergence, and in natural conditions - for example, in the process of squatting and fixing the position.
Isotonic muscle contraction consists in shortening the muscle with its constant tension. This type of contraction occurs when an unloaded muscle with one fixed tendon contracts without lifting (moving) any external weight, or lifting a weight without acceleration.
Depending on the duration muscle contractions are of two types: solitary and tetanic.
Single muscle contraction occurs with a single irritation of the nerve or the muscle itself. Usually the muscle is shortened by 5-10% of its original length. There are three main periods on the single contraction curve: 1) latent- the time from the moment of irritation to the start 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 soleus muscle it is 0.1 s. In the latency period, muscle fibers are excited and carried along the membrane. The ratios of the duration of a single contraction of the muscle fiber, its excitation and phase changes in the excitability of the muscle fiber are shown in Fig. 24.
The duration of the muscle fiber contraction is much longer than that of the AP, because it takes time for the operation of the Ca-pumps to return Ca2 + to the SPR and the environment and the greater inertia of mechanical processes as compared to electrophysiological ones.
Rice. 24. The ratio of the time of occurrence of PD (A) and a single contraction (B) of the slow fiber of the skeletal muscle of warm-blooded. Arrow- the moment of irritation. Contraction time fast fibers several times shorter
Tetanic contraction- This is a prolonged contraction of a muscle that occurs under the action of rhythmic irritation, when each subsequent irritation or nerve impulses arrive at the muscle while it has not yet relaxed. The 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 following stimuli are applied to a muscle fiber or a whole muscle.
Rice. 25. Sum of abbreviations calf muscle frogs: 1 - single contraction curve in response to the first stimulation of the relaxed muscle; 2 - curve of uniaxial contraction of the same muscle in response to the second irritation; 3 - curve of the summed contraction obtained as a result of paired stimulation of the contracting muscle ( indicated by arrows)
In this case, irritations should come in the period of the previous contraction. An increase in the amplitude of contractions 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 + provides an increase in the number of zones of engagement of myosin bridges with actin filaments.
If repeated impulses or stimuli enter the muscle relaxation phase, scalloped tetanus... If repeated irritations occur in the shortening phase, smooth tetanus(fig. 26).
Rice. 26. Contraction of the frog gastrocnemius muscle at different frequency of sciatic nerve irritation: 1 - single contraction (frequency 1 Hz); 2,3 - dentate tetanus (15-20 Hz); 4.5 - smooth tetanus (25-60 Hz); 6 - relaxation at a pessimal frequency of stimulation (120 Hz)
The amplitude of contraction and the amount 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.
With an increase in the frequency of stimulation of a nerve or muscle, the amplitude of smooth tetanus increases. Maximum tetanus got the name 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 neuromuscular synapses - pessimum Vvedensky(frequency of irritation pessimal) (see Fig. 26). Vvedensky's pessimum can also be obtained with direct, but more frequent stimulation of the muscle (about 200 imp./s), however, for the purity of the experiment, neuromuscular synapses should be blocked. If, after the onset of a pessimum, the frequency of stimulation is reduced to the optimum, 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 dentate tetanus mode, however, the contraction of the whole muscle resembles smooth tetanus, due to the asynchrony of their contraction.
There are two main types of muscles in animals and humans:
- cross-striped (attached to the bones, that is, to the skeleton, and therefore are also called skeletal, they also secrete the heart muscle, which has its own characteristics);
- smooth (musculature of the walls of hollow organs and skin).
Muscle cell structure
The striated muscle is made up of numerous elongated muscle cells. Motor nerves enter the muscle fiber at various points and transmit an electrical impulse to it, causing it to contract. Muscle fiber is usually considered as a giant multinucleated cell covered with an elastic membrane - sarcolemma. The diameter of a functionally mature striated muscle fiber is usually 10 to 100 µm, and the length of the fiber often corresponds to the length of the muscle.
A number of structures are found in the sarcoplasm of muscle fibers: mitochondria, microsomes, ribosomes, tubules and cisterns of the sarcoplasmic reticulum, various vacuoles, glycogen clumps and lipid inclusions that play the role of reserve energy materials, etc.
In each muscle fiber in a semi-liquid sarcoplasm along the length of the fiber, there are many filamentous formations - myofibrils (their thickness is usually less than 1 micron), which, like the entire fiber as a whole, have a transverse striation, often in the form of bundles. The transverse striation of the fiber, depending on the optical inhomogeneity of protein substances localized in all myofibrils at the same level, is easily detected when examining skeletal muscle fibers in a polarization or phase contrast microscope (Fig. 2).
A repeating element of the striated myofibril is the sarcomere - a section of the myofibril, the boundaries of which are narrow 2-lines. Each myofibril consists of several hundred sarcomeres. The average length of the sarcomere is 2.5-3.0 microns. In the middle of the sarcomere there is a zone with a length of 1.5-1.6 microns, dark in a phase contrast microscope. In polarized light, it produces strong birefringence. This zone is usually called disk A (anisotropic disk). In the center of the disk A there is a line M, which can only be observed in an electron microscope. The middle part of the disk A is occupied by the zone H of weaker birefringence. Finally, there are isotropic disks, or disks I, with very weak birefringence. In a phase contrast microscope, they appear lighter than discs A. The length of discs I is about 1 μm. Each of them is divided into two equal halves by a Z-membrane, or Z-line. According to modern concepts, disks A contain thick filaments, consisting mainly of the protein myosin, and thin filaments, consisting, as a rule, of the second component of the actomyosin system, the protein actin. Thin (actin) filaments begin within each sarcomere at the Z-line, stretch through disk I, penetrate into disk A, and break in the area of zone H.
Rice. 2. Photo of a micropreparation of striated muscle tissue
Rice. 3. Scheme of the structure of the sarcomere
When examining thin sections of muscles under an electron microscope, it was found that the protein filaments are arranged in a strictly ordered manner. Thick filaments with a diameter of 12-16 nm and a length of about 1.5 microns are laid in the form of a hexagon with a diameter of 40-50 nm and pass through the entire disk A. Between these thick filaments are thin filaments with a diameter of 8 nm, extending from the 2-line to a distance of about 1 μm (Fig. 3). The study of the muscle in the state of contraction showed that the disks I in it almost disappear, and the area of overlapping of thick and thin filaments increases (in the skeletal muscle in the state of contraction, the sarcomere is shortened to 1.7-1.8 μm).
According to the model proposed by E. Huxley and R. Niedergerke, as well as H. Huxley and J. Henson, during the contraction of myofibrils, one system of filaments penetrates the other, i.e., the filaments begin to slide over each other, as it were, which is the cause of muscle reduction.
An important role in this process is played by calcium ions and sarcoplasmic proteins - calsequestrin and a protein with a high affinity for calcium. The membranes of the sarcoplasmic reticulum are surrounded by muscle filaments. These proteins are located in the SPR cisterns on the inner membrane, where Ca 2+ ions are bound. Calsequestrin - acidic glycoprotein (MW 45,000 Da), capable of attaching 45 Ca 2+ ions, protein with high affinity for calcium (MW 55,000 Da) binds 25 Ca 2+ ions. The transfer of Ca 2+ from the tanks occurs along the concentration gradient by simple diffusion; the transfer of Ca 2+ from the cytoplasm to the cisterns - against the gradient with the participation of Ca 2+ -dependent ATPase and ATP. At rest, the active transport system accumulates calcium in the tanks. Muscle contraction begins with the arrival action potential on the end plate of the motor nerve. Acetylcholine is released into the synapse, which binds to postsynaptic receptors in muscle fibers. Further, the action potential propagates along the sarcolemma to the transverse tubules of the T-system. In the region of the Z-lines, a signal is transmitted from the transverse tubules to the cisterns of the sarcoplasmic reticulum.
Depolarization of the cistern membranes leads to the release of calcium and the onset of muscle contraction. Calcium binds to the C subunit of troponin. This changes the conformation of the entire troponin molecule - subunit I ceases to interfere with the interaction of actin with myosin; a change in the conformation of the T subunit is transferred to tropomyosin. Then tropomyosin turns by 20 ° and opens previously closed centers in actin for binding with myosin. The myosin head, which at rest is an ADP + F n + myosin complex, attaches to actin perpendicularly, and actin has a great affinity for this complex (the formation of transverse bridges). The attachment of actin causes a rapid release of ADP and Pn from myosin. This leads to a change in conformation, and the myosin head rotates 45 ° (stroke). Turning the actin-associated head causes the fine filament to move relative to myosin. Instead of the departed ADP and Pn, ATP rejoins the myosin head, forming the M + ATP complex. Actin has a low affinity for it, which causes detachment of the myosin head (rupture of transverse bridges). It again becomes perpendicular to the thin thread. ATP hydrolysis occurs in the myosin head, which is not associated with actin. The ADP + F n + myosin complex is formed again, and everything is repeated. The addition of ATP to myosin and hydrolysis of ATP occur very quickly, however, the hydrolysis products of ADP and Pn are slowly cleaved from myosin.
After the cessation of the action of the motor impulse, Ca 2+ with the help of Ca 2+ -dependent ATPase passes into the sarcoplasmic reticulum. The departure of calcium from the troponin complex leads to a displacement of tropomyosin and the closure of the active centers of actin, making it unable to interact with myosin - the muscle relaxes.
Important for muscle health processes of neuromuscular transmission. one) With myasthenia gravis, antibodies are found in the blood against their own acetylcholine receptors, which is manifested by muscle weakness.
2).
A number of drugs (atropine, succinylcholine, curare poison) inhibit receptor proteins, h They block neuromuscular conduction.
3).
Medications(neostigmine, eserine) inhibit acetylcholine esterase thereby enhancing the action of acetylcholine.
4).
More potent inhibitors of the enzyme are organic fluorophosphates. They form a strong bond with acetylcholinesterase and cause death by respiratory arrest. These are nerve poisons - herd, sarin.