All the variety of movements that constantly occur in the body, provides the muscles. Thanks to them, the body is kept in balance and moves in space, the respiratory movements of the chest and diaphragm, eye movements, voice formation, swallowing, the most important functions of the internal organs, including the heart, are carried out.
There are smooth and striated muscles.
Smooth muscles form the muscle membrane of the stomach walls, intestines. bladder, uterus and other hollow organs, as well as blood and lymphatic vessels, ducts of the glands. They consist of small muscle cells, the length of which, as a rule, does not exceed 100 micrometers. Regulates the activity of smooth muscles of the autonomic nervous system. They do not obey volitional orders, so they can not be reduced and relaxed arbitrarily.
Striated muscles make up about 40 percent of body weight. Among them are skeletal muscles, as well as the heart muscle.
Skeletal muscles cover the skeleton so that the bones only in some places lie directly under the skin. The development of muscles largely (especially in men) determines the shape and relief of the body.
Skeletal muscle is formed by muscle fibers (this is its working part) and layers of connective tissue. Each muscle is suited to one or more nerves and blood vessels supplying it; both those and others penetrate into the thickness of the muscles in the area of the so-called neurovascular field.
Muscle fibers are multi-core muscle formations composed of thin fibers, myofibrils. Under the microscope, skeletal muscle fibers appear lined with transverse light and dark stripes (hence the name of the muscles – striated). The number of fibers in the muscles is different: in small ones there may be several hundred, and in large ones there is much more. Accordingly, the strength of the muscles is not the same: it is determined by the thickness, or rather, the cross-sectional area of all its fibers. In an adult, the number of fibers remains constant, and their diameter depends mainly on the training of muscles. The more often and harder the muscle works, the more it becomes thicker and, hence, stronger. Constant training, physical labor can lead to an increase in fiber diameter by half. The layers of connective tissue muscle fibers are combined into bundles: connective – woven sheath surrounds the muscle and the outside. Intramuscular connective tissue enters the tendon – a dense fibrous cord, with which the muscle is firmly attached to the bone. Muscle bundles are arranged differently with respect to the tendon. They can go almost parallel to him, forming a spindle-shaped muscle; in the cirrus muscles, muscle bundles are attached to the tendon from different sides at an angle on one side and on both sides. Muscle fibers are very elastic, they are able to stretch and shorten. By contracting the muscle with the help of a tendon, it pulls the bone, which acts as a lever, so various movements are performed.
Blood enters the skeletal muscle through several arteries. They branch between the muscle bundles in the layers of connective tissue, gradually narrowing and turning into arterioles. Directly near the muscle bundles, arterioles are divided into a large number of blood capillaries, which densely braid muscle fibers. In a relaxed. in rest muscle, a significant part of the capillaries is closed, blood does not flow through them. But as soon as the muscle begins to contract, the capillaries in the reserve open up, and the amount of blood flowing to the muscle increases tenfold.
Together with the arteries, one or more nerves enter the muscle. In different muscles, a single nerve fiber may innervate a different amount of muscle fibers. For example, in the eye muscle, 3 – 6 muscle fibers obey it, and in the triceps muscle of the leg – 120 – 160. The central nervous system regulates the activity of skeletal muscles, and, unlike smooth muscles, a person can consciously strain or relax a particular group. skeletal muscle.
Each muscle has a thickened part – the body, or abdomen; the initial part of the muscle is the head, and the opposite end is the tail. If a muscle has only one head, one tail and one abdomen, it is called simple. If the number of individual parts of the muscle, for example, doubles, triples, the muscle is called complex. There are biceps, triceps, quadriceps, and double-abdominal, multi-abdominal, and multi-vein muscles.
Almost all the muscles are equipped with an auxiliary device (Figure IV). First of all, it is the connective tissue sheath – fascia. The superficial fascia (5) covers the muscles outside, separating them from the skin and subcutaneous fat. From it between the muscle groups or individual muscles continue the septum – the deep fascia (b), which, as a rule, spliced with the periosteum of the bones. Thus, the muscles are, as it were, in a case formed by superficial and deep fascia, as well as bones. Fascia and bones serve as a support for the muscles. In addition, the fascia have a lateral resistance to contracting muscles and thus help them to perform motor function.
The tendons of some muscles are also enclosed in shells – the vagina, which hold the tendon near the bones and joints, make it move in a strictly defined direction. Inside the tendon sheaths are lined with a thin synovial membrane, the cells of which release a viscous fluid – synovium. The fluid serves as a biological lubricant for facing the vaginal and tendon surfaces, which significantly reduces the friction between them.
Auxiliary formations (Figure V) also include bones. located inside the tendons (7). They increase the angle of attachment of the tendon to the bones of the skeleton (8), which contributes to a more economical work of the muscles.
Muscle acts on bones interconnected by joints, like a lever (Figure VI). In the limbs, the long bones form a system of levers. The point of support of the lever is the joint (9), in which the movement takes place, the point of application of force is the place of attachment of the muscle (10) that performs the movement, and the point of resistance is the place of action of gravity on the bone (11). As you know, the easier it is to move something with a lever, the closer the resistance is applied to the pivot point and the farther from it the point of application of force. In the arms of our body, the point of application of force (the place of attachment of the muscle) is often close to the fulcrum (joint). Therefore, the muscles are forced to develop greater strength. For example, in order to lift a weight of 10 kilograms in an outstretched arm, the muscles of the arm must develop an effort of 100 kilograms.
By the nature of the movements performed, the muscles are divided into flexors and extensors, which lead and retract, lift and lower, rotate, compress, expand. Separate muscles or a group of muscles can co-operate in one direction; these are synergistic muscles. When muscles perform opposite actions and one group of muscles opposes another, they are called antagonists. Even synergists and antagonists are usually involved even in the simplest motor acts.