The mechanism of muscle contractions. Functions and properties of skeletal muscles

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

Muscles

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

• Striated muscles of the body;
• Striated muscles of the atria and cardiac ventricles;
• Smooth muscles of organs, vessels and skin.

Cross-striated muscles enter the musculoskeletal system, being a part of it, as well as tendons, ligaments, bones. When the mechanism of muscle contractions is implemented, the following tasks and functions are performed:

• The body moves;
• Parts of the body move relative to each other;
• The body is supported in space;
• Heat is generated;
• The cortex is activated by afferentation from the receptive muscle fields.

From smooth muscles consists of:

• The motor apparatus of internal organs, which includes the bronchial tree, lungs and digestive tube;
• Lymphatic and circulatory system;
• System of genitourinary organs.

Physiological properties

Like all vertebrates, three most important properties of skeletal muscle fibers are distinguished in the human body:

• Contractility - reduction and change in voltage when excited;
• Conductivity - the motion of the potential along the entire fiber;
• Excitability - response to the stimulus by changing the membrane potential and ionic permeability.

The muscles are excited and begin to contract from the nerve impulses coming from the centers. But under artificial conditions, electrostimulation is used . The muscle can then be irritated directly (direct irritation) or through the nerve that innervates the muscle (indirect irritation).

Types of abbreviations

The mechanism of muscle contractions implies the transformation of chemical energy into mechanical work. This process can be measured by experimenting with a frog: its gastrocnemius muscle is loaded with a small weight, and then irritated with light electric pulses. The contraction, in which the muscle becomes shorter, is called isotonic. With isometric contraction, there is no shortening. Tendons do not allow for the development of muscle strength to shorten. Another auxotonic mechanism of muscle contractions assumes the conditions of intense loads, when the muscle is shortened in the minimal way, and the force develops maximum.

Structure and innervation of skeletal muscles

In cross-striated skeletal muscles, there are many fibers that are in connective tissue and attached to the tendons. In some muscles, the fibers are parallel to the long axis, while in others they are oblique, attaching to the central tendon tendon and to the pinnate type.

The main feature of the fiber lies in the sarcoplasm of a mass of thin threads - myofibrils. They include light and dark areas alternating with each other, and adjacent strands are on the same level - on the cross section. Due to this, a transverse banding is obtained along the entire muscle fiber.

A sarcomer is a complex of dark and two light discs, and it is delimited by Z-shaped lines. Sarcomery is a contractile muscle apparatus. It turns out that the contractile muscle fiber consists of:

• Contractile apparatus (myofibril system);
• Trophic apparatus with mitochondria, the Golgi complex, and a weak endoplasmic reticulum ;
• A membrane apparatus;
• The support apparatus;
• Of the nervous apparatus.

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

Innervation

This process in transversely striated muscle fibers is realized by means of nerve fibers, namely the axons of the motoneurons of the spinal cord and the head trunk. One motoneuron innervates several muscle fibers. The complex with motoneuron and innervated muscle fibers is called neuromotor (NME), or motor unit (DE). The average number of fibers that innervates one motoneuron, characterizes the amount of DE muscle, and the reciprocal value is called the innervation density. The latter is large in those muscles where the movements are small and "thin" (eyes, fingers, tongue). Its small value will, on the contrary, be in muscles with "rough" movements (for example, the trunk).

Innervation can be single and multiple. In the first case, it is realized by compact motor endings. Usually this is typical for large motor neurons. Muscle fibers (called in this case physical, or fast) generate PD (action potentials) that propagate to them.

Multiple innervation occurs, for example, in the outer eye muscles. The action potential is not generated here, since there are no electrically excitable sodium channels in the membrane. They include depolarization throughout the fiber from the synaptic ends. This is necessary in order to activate the mechanism of muscle contraction. The process here does not happen as quickly as in the first case. Therefore, it is called slow.

Structure of myofibrils

Muscular fiber research is conducted today on the basis of X-ray diffraction analysis, electron microscopy, and histochemical methods.

It is calculated that approximately 2500 protofibrils, i.e., extended polymerized protein molecules (actin and myosin) enter into each myofibril with a diameter of 1 μm. Actin protofibrils are two times thinner than myosin. In rest, these muscles are located so that the actin filaments penetrate into the gaps between the myosin protofibrils with their tips.

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

On myosin filaments there are transverse protrusions up to 20 nm long, in the heads of which there are about 150 molecules of myosin. They depart biopolarly, and each head connects the myosin with the actin filament. When there is an effort of actin centers on the filaments of myosin, the actin filament approaches the center of the sarcomere. At the end, the myosin filaments reach the Z line. Then they occupy the entire sarcomere, and the actinic filaments lie between them. In this case, the length of the disk I is shortened, and at the end it disappears completely, together with which the Z-line becomes thicker.

Thus, according to the theory of sliding threads, the length of the muscle fiber is explained. The theory, called the cogwheel, was developed by Huxley and Hanson in the middle of the twentieth century.

Mechanism of muscle contraction of the fiber

The main thing in theory is that not threads (myosin and actin) are shortened. Their length remains unchanged even when the muscles are stretched. But the bundles of fine filaments, slipping, exit between the thick threads, the degree of their overlap decreases, so the contraction occurs.

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

With muscle relaxation, the myosin head departs from the actin filaments. Due to the easy sliding, the relaxed muscles of the stretching resist much less. Therefore, they passively extend.

Stages of reduction

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

1. Muscle fiber is stimulated when the action potential comes from motoneurons from synapses.
2. The action potential is created on the membrane of the muscle fiber, and then spreads to the myofibrils.
3. Electromechanical coupling is performed, which is the transformation of electric PD into mechanical sliding. In this, calcium ions are necessarily involved.

Calcium ions

For a better understanding of the process of fiber activation by calcium ions, it is convenient to consider the structure of the actin filament. Its length is of the order of 1 μm, the thickness is from 5 to 7 nm. This is a pair of twisted threads that resemble an actin monomer. Approximately every 40 nm here are spherical troponin molecules, and between the chains - tropomyosin.

When calcium ions are absent, that is, myofibrils relax, long tropomyosin molecules block the attachment of actin chains and myosin bridges. But with the activation of calcium ions, the tropomyosin molecules descend deeper, and the areas open.

Then the myosin bridges attach to the actin threads, and ATP splits, and muscle strength develops. This is made possible by the action of calcium on troponin. In this case, the molecule of the latter is deformed, pushing thereby tropomyosin.

When the muscle is relaxed, 1 gram of wet weight contains more than 1 μmol of calcium per gram. Salts of calcium are isolated and are in special stores. Otherwise, the muscles would be reduced all the time.

The storage of calcium occurs as follows. In different parts of the membrane of the muscle cells inside the fiber there are tubes through which the connection with the environment outside the cells occurs. This is a system of transverse tubules. A perpendicular to it is a system of longitudinal, at the ends of which - bubbles (terminal tanks), located in close proximity to the membranes of the transverse system. Together we get a triad. It is in the bubbles that calcium is stored.

So the PD spreads into the cell, and electromechanical coupling occurs. Excitation penetrates into the fiber, passes into the longitudinal system, releases calcium. Thus, the mechanism of muscle fiber contraction is realized.

3 processes with ATP

In the interaction of both filaments in the presence of calcium ions, ATP plays a significant role. When the mechanism of muscle contraction of the skeletal muscle is realized, the energy of ATP is used for:

• The work of the sodium and potassium pump, which maintains a constant concentration of ions;
• These substances on different sides of the membrane;
• Slip threads shortening myofibrils;
• The work of a calcium pump, acting to relax.

ATP is found in the cell membrane, the filaments of myosin and the membranes of the reticulum of the sarcoplasmic. The enzyme is split and utilized by myosin.

Consumption of ATP

It is known that the myosin heads interact with actin and contain elements for the cleavage of ATP. The latter is activated by actin and myosin in the presence of magnesium ions. Therefore, the cleavage of the enzyme occurs when the myosin head is attached to actin. In this case, the more transverse bridges, the faster the splitting rate will be.

ATP mechanism

After completion of the movement, the AFT molecule provides energy for the separation of myosin and actin involved in the reaction. Myosin heads are separated, ATP is cleaved to phosphate and ADP. At the end, a new ATP molecule is connected, and the cycle resumes. This is the mechanism of muscle contraction and relaxation at the molecular level.

The activity of the transverse bridges will continue only as long as the hydrolysis of ATP occurs. When the enzyme is blocked, the bridges will not become attached again.

With the onset of the death of the body, the level of ATP in the cells falls, and the bridges remain stably attached to the actin filament. This is the stage of rigor mortis.

Resuscitation of ATP

Resynthesis can be realized in two ways.

By enzymatic transfer from phosphate group creatine phosphate to ADP. Since the reserves in the creatine phosphate cell are much larger than ATP, resynthesis is realized very quickly. At the same time, through the oxidation of pyruvic and lactic acids, resynthesis will be slow.

ATP and CF can disappear completely if resynthesis is disturbed by poisons. Then the calcium pump will stop working, as a result of which the muscle will contract irreversibly (that is, a contracture will occur). Thus, the mechanism of muscle contraction is broken.

The physiology of the process

Summarizing the foregoing, we note that a reduction in muscle fiber consists in shortening the myofibrils in each of the sarcomeres. The filaments of myosin (thick) and actin (thin) are connected by the ends in a relaxed state. But they begin sliding movements towards each other when the mechanism of muscle contraction is realized. Physiology (briefly) explains the process when under the influence of myosin, the necessary energy is released for the conversion of ATP to ADP. In this case, the activity of myosin will be realized only with a sufficient content of calcium ions accumulating in the sarcoplasmic network.