All the variety of movements that are constantly performed in the body, provide muscles. Thanks to them, the body is kept in balance and moves in space, respiratory movements of the chest and diaphragm, eye movements, voice formation, swallowing, the most important functions of internal organs, including the work of the heart, are carried out.
Distinguish between smooth and striated muscles.
Smooth muscles (Figure I) form the muscle membrane of the walls of the stomach and intestines. bladder, uterus and other hollow organs, as well as blood and lymph vessels, ducts of glands. They consist of small muscle cells, the length of which, as a rule, does not exceed 100 micrometers. The autonomic nervous system regulates the activity of smooth muscles. They do not obey volitional orders, therefore they cannot be reduced and relaxed arbitrarily.
The striated muscles make up approximately 40 percent of body weight. Among them, there are skeletal muscles, as well as a muscle of the heart.
The skeletal muscles (Figure II) cover the skeleton so that the bones only in some places lie directly under the skin. Muscle development to a large extent (especially in men) determines the shape, body shape.
Skeletal muscle is formed by muscle fibers (this is its working part) and interlayers of connective tissue. One or several nerves and blood vessels supplying it fit each muscle; both of them penetrate into the thickness of the muscle in the region of the so-called neurovascular field.
Muscle fibers are multinuclear muscle formations composed of thin fibrils – myofibrils. Under the microscope, the fibers of the skeletal muscles appear to be traversed by transverse light and dark bands (hence the name of the muscles – striated). The number of fibers in the muscles is different: in small ones, there can be several hundred, and in large ones, 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 fitness of the muscles. The more often and more intense the muscle works, the more it becomes thicker, and therefore stronger. Continuous training, physical labor can double the diameter of the fibers. The layers of connective tissue muscle fibers are bundled: connective tissue membrane surrounds the muscle and the outside. Intramuscular connective tissue passes into the tendon – a dense fibrous cord, with which the muscle is firmly attached to the bone. The muscle bundles are located differently with respect to the tendon (Figure 111). They can go almost parallel to it, forming a spindle-shaped muscle (1); in cirrus muscles, muscle bundles are attached to the tendon from different sides (2). at an angle from one side (3) or from two sides (4).
Muscle fibers have great elasticity, they are able to stretch and shorten. Contracting, the muscle with the help of a tendon pulls a bone that acts as a lever – in this way various movements are made.
Blood enters the skeletal muscle through several arteries. They branch between the muscle bundles in the layers of connective tissue, gradually tapering and passing into arterioles. Directly near muscle bundles, arterioles are divided into a large number of blood capillaries, which densely braid muscle fibers. In relaxed. muscle at rest, 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, and the amount of blood flowing to the muscle increases tens of times.
Together with arteries, one or more nerves enter the muscle. In different muscles, one nerve fiber can innervate a different number of muscle fibers. For example, in the muscle of the eye, 3-6 muscle fibers obey it, and in the triceps muscle of the lower 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 section of the muscle is the head, and the end opposite the head 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 muscles, as well as double-abdominal, multi-abdominal and multi-tendon muscles.
Almost all muscles are equipped with an auxiliary device (Figure IV). First of all, these are connective tissue membranes – fascia. Superficial fascia (5) covers the muscles on the outside, separating them from the skin and subcutaneous fat. From it, between the muscle groups or individual muscles, partitions continue – deep fascia (b), which, as a rule, are fused with the periosteum of bones. Thus, the muscles are enclosed, as it were, in a case formed by superficial and deep fascia, as well as bones. Fascias and bones support the muscles.