Muscle contraction 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 muscles with energy. Consider the types and mechanisms of muscle contraction. We will study what stages they consist of and what is necessary for a cyclic process.
Muscle
Muscles are combined into groups that have the same mechanism of muscle contractions. By the same sign, they are divided into 3 types:
- striated muscles of the body;
- striated muscles of the atria and cardiac ventricles;
- smooth muscles of organs, blood vessels and skin.
The striated muscles enter the musculoskeletal system, being a part of it, since in addition to them it includes tendons, ligaments, bones. When the mechanism of muscle contractions is realized, the following tasks and functions are performed:
- the body moves;
- body parts move relative to each other;
- the body is supported in space;
- heat is generated;
- the cortex is activated through afferentation from receptive muscle fields.
From smooth muscles consists of:
- the motor apparatus of the internal organs, which includes the bronchial tree, lungs and digestive tube;
- lymphatic and circulatory systems;
- genitourinary system.
Physiological properties
Like all vertebrates, in the human body there are three most important properties of skeletal muscle fibers:
- contractility - reduction and change in stress upon excitation;
- conductivity - the movement of potential throughout the fiber;
- excitability - response to the stimulus by changing the membrane potential and ion permeability.
Muscles are excited and begin to contract from nerve impulses coming from the centers. But in artificial conditions they use electrical stimulation. The muscle can then be irritated directly (direct irritation) or through the nerve innervating the muscle (indirect irritation).
Types of abbreviations
The mechanism of muscle contractions involves the conversion of chemical energy into mechanical work. This process can be measured in an experiment with a frog: its calf muscle is loaded with a small weight, and then irritated by light electrical impulses. A contraction in which the muscle becomes shorter is called isotonic. With an isometric reduction, shortening does not occur. Tendons do not allow shortening during muscle development. Another auxotonic mechanism of muscle contractions involves conditions of intense stress, when the muscle is shortened in a minimal way, and the maximum strength develops.
Skeletal muscle structure and innervation
The striated skeletal muscles include many fibers located in the connective tissue and attached to the tendons. In some muscles, the fibers are parallel to the long axis, while in others they have an oblique appearance, attaching to the central tendon cord and to the cirrus type.
The main feature of the fiber is the sarcoplasm of the mass of thin filaments - myofibrils. They include light and dark areas, alternating with each other, and in neighboring, the striated fibers are at the same level - at the cross section. Due to this, transverse striping is obtained throughout the muscle fiber.
Sarcomere is a complex of dark and two light discs, and it is delimited by Z-shaped lines. Sarcomeres are the contractile apparatus of the muscle. It turns out that the contractile muscle fiber consists of:
- contractile apparatus (myofibril system);
- trophic apparatus with mitochondria, Golgi complex and weak endoplasmic reticulum ;
- membrane apparatus;
- supporting apparatus;
- nerve apparatus.
Muscle fiber is divided into 5 parts with its structures and functions and is an integral part of muscle tissue.
Innervation
This process in striated muscle fibers is realized through nerve fibers, namely, axons of motor neurons of the spinal cord and brain stem. One motor neuron innervates several muscle fibers. The complex with motor neuron and innervated muscle fibers is called neuromotor (NME), or motor unit (DE). The average number of fibers that one motor neuron innervates characterizes the DE of the muscle, and the inverse is called the density of innervation. The latter is large in those muscles where movements are small and "thin" (eyes, fingers, tongue). On the contrary, its small value will be in muscles with “gross” movements (for example, the trunk).
Innervation can be single and multiple. In the first case, it is implemented by compact motor endings. This is usually characteristic of large motor neurons. Muscle fibers (called physical or fast in this case) generate AP (action potentials), which extend to them.
Multiple innervation occurs, for example, in the external ocular muscles. No action potential is generated here, since there are no electrically excitable sodium channels in the membrane. They spread depolarization throughout the fiber from synaptic endings. This is necessary in order to activate the mechanism of muscle contraction. The process here is not as fast as in the first case. Therefore, it is called slow.
The structure of myofibrils
Studies of muscle fiber today are carried out on the basis of x-ray diffraction analysis, electron microscopy, as well as histochemical methods.
It is estimated that approximately 2,500 protofibrils, that is, elongated polymerized protein molecules (actin and myosin), are included in each myofibril, whose diameter is 1 μm. Actin protofibrils are two times thinner than myosin. At rest, these muscles are so that the actin filaments penetrate the tips between the myosin protofibrils with their tips.
The narrow light band in disk A is free of actin filaments. And the Z membrane holds them together.
On myosin filaments there are transverse protrusions up to 20 nm in length, in the heads of which there are about 150 myosin molecules. They depart biopolarly, and each head connects the myosin with the actin filament. When there is an actin center force on the myosin filaments, the actin filament approaches the center of the sarcomere. At the end, myosin filaments reach the Z line. Then they occupy the entire sarcomere, and the actin are between them. In this case, the length of disk I is reduced, and in the end it disappears completely, with which the line Z becomes thicker.
So, according to the theory of sliding threads, the reduction in muscle fiber length is explained. The theory, called the "gear", was developed by Huxley and Hanson in the mid-twentieth century.
The mechanism of muscle contraction of fiber
The main thing in theory is that not the threads (myosin and actin) are shortened. Their length remains unchanged when the muscles are stretched. But the tufts of thin filaments, slipping, go out between the thick filaments, the degree of their overlap decreases, thus reducing.
The molecular mechanism of muscle contraction by gliding actin filaments is as follows. Myosin heads connect protofibrils with actin. When they are tilted, gliding occurs, moving the actin thread toward the center of the sarcomere. Due to the bipolar organization of myosin molecules on both sides of the threads, conditions are created for the actin filaments to slip in different directions.
With muscle relaxation, the myosin head moves away from the actin filaments. Thanks to easy gliding, relaxed muscles stretch much less. Therefore, they passively lengthen.
Reduction stages
The mechanism of muscle contraction can be briefly divided into the following stages:
- Muscle fiber is stimulated when the action potential comes from motor neurons from synapses.
- A potential action is created on the membrane of the muscle fiber, and then spreads to myofibrils.
- Electromechanical conjugation is performed, which is the conversion of an electric PD into mechanical sliding. Calcium ions are necessarily involved in this.
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 about 1 μm, thickness - from 5 to 7 nm. This is a pair of twisted threads that resemble actin monomer. Every 40 nm, there are spherical troponin molecules, and tropomyosin between the chains.
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 sink deeper, and the sections open.
Then the myosin bridges are attached to actin filaments, and ATP is cleaved, and muscle strength develops. This is made possible by the effects of calcium on troponin. In this case, the molecule of the latter is deformed, thereby pushing tropomyosin.
When a muscle is relaxed, it contains more than 1 μmol of calcium per 1 gram of wet weight. Calcium salts are isolated and located in special storage facilities. Otherwise, the muscles would contract all the time.
Storage of calcium is as follows. On different parts of the membrane of the muscle cell inside the fiber there are tubes through which a connection to the environment outside the cells takes place. This is a transverse duct system. And perpendicular to it is a longitudinal system, at the ends of which are 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 PD is distributed inside the cell, and electromechanical conjugation occurs. Excitation penetrates into the fiber, passes into the longitudinal system, releases calcium. Thus, the mechanism of contraction of muscle fiber is implemented.
3 processes with ATP
In the interaction of both strands in the presence of calcium ions, ATP plays a significant role. When the mechanism of muscle contraction of the skeletal muscle is realized, ATP energy is used to:
- the operation of the sodium and potassium pump, which maintains a constant concentration of ions;
- these substances on opposite sides of the membrane;
- gliding filaments shortening myofibrils;
- Calcium pump works to relax.
ATP is located in the cell membrane, myosin filaments, and sarcoplasmic reticulum membranes. The enzyme is cleaved and utilized by myosin.
ATP consumption
Myosin heads are known to interact with actin and contain elements for ATP cleavage. 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. Moreover, the more transverse bridges, the higher the splitting speed.
ATP mechanism
After the movement is completed, 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. Such is the mechanism of muscle contraction and relaxation at the molecular level.
The activity of the transverse bridges will continue only until ATP hydrolysis occurs. When the enzyme is blocked, the bridges will not reattach.
With the onset of death of the body, the level of ATP in the cells decreases, and the bridges remain stably attached to the actin filament. This is the stage of rigor mortis.
ATP resynthesis
Resynthesis can be implemented in two ways.
By enzymatic transfer from phosphate 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 disrupted by poisons. Then the calcium pump will stop working, as a result of which the muscle will contract irreversibly (that is, contracture will come). Thus, the mechanism of muscle contraction is disrupted.
Process physiology
Summing up the above, we note that the contraction of muscle fiber consists in the shortening of 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 reticulum.