The muscles of a person in relation to his total weight is about 40%. Their main function in the body is to provide movement due to the ability to contract and relax. For the first time the structure of muscles (8th grade) begins to be studied in school. There knowledge is given at a general level, without much deepening. The article will be interesting to those who want to go beyond this.
Muscle structure: general information
Muscular tissue is a group that combines striated, smooth and hearty varieties. Differing by origin and structure, they are united by the sign of the function performed, that is, the ability to contract and lengthen. In addition to these varieties, which are formed from the mesenchyme (mesoderm), in the human body there is also muscle tissue having ectodermal origin. These are myocytes of the iris of the eye.
The structural, general structure of the muscles is as follows: they consist of the active part, called the abdomen, and the tendon ends (tendons). The latter are formed from dense connective tissue and perform the function of attachment. They have a distinctive whitish-yellow color and brilliance. In addition, they have a significant fortress. Usually, with their tendons, muscles attach to the links of the skeleton, the connection with which is mobile. However, some can be attached to the fascia, to various organs (eyeball, laryngeal cartilage, etc.), to the skin (on the face). The blood supply to the muscles varies and depends on the loads they experience.
Control over their work is carried out, as in other organs, the nervous system. Receptors or effectors terminate its fibers in the muscles. The former are also located in the tendons, have the form of terminal ramifications of the sensory nerve or the neuromuscular spindle, which has a complex device. They react to the degree of contraction and stretching, so that a person has a certain feeling, which, in particular, helps to determine the position of the body in space. Effector nerve endings (the second name is motor plaques) belong to the motor nerve.
The structure of the muscles is also characterized by the presence in them of the endings of the fibers of the sympathetic nervous system (vegetative).
The structure of the striated muscle tissue
It is often called skeletal or striated. The structure of the skeletal muscle is quite difficult. It is formed by fibers having a cylindrical shape, with a length of from 1 mm to 4 cm and more, with a thickness of 0.1 mm. Moreover, each is a special complex consisting of myosatellitocytes and myosymplast, covered with a plasma membrane called the sarcolemma. Outside, the basement membrane (plate), which is formed from the thinnest collagen and reticular fibers, abuts on it. Myosymplast consists of a large number of ellipsoidal nuclei, myofibrils and cytoplasm.
The structure of the muscles of this type is distinguished by a well-developed sarcotubular network formed of two components: EPS tubules and T-tubules. The latter play an important role in accelerating the carrying out of the action potential to microfibrils. Miosatellitocytes are located directly above the sarcolemma. The cells have a flattened shape and a large nucleus rich in chromatin, as well as a centrosome and a small number of organelles; myofibrils are absent.
Sarcoplasm of skeletal muscle is rich in a special protein - myoglobin, which, like hemoglobin, has the ability to bind with oxygen. Depending on its content, the presence / absence of myofibrils and the thickness of the fibers, there are two types of striated muscles. The specific structure of the skeleton, the muscles - all these are elements of a person’s adaptation to the erect body, their main functions are support and movement.
Red muscle fibers
They have a dark color, rich in myoglobin, sarcoplasm and mitochondria. However, they contain little myofibrils. These fibers are reduced quite slowly and can remain in such a state for a long time (in other words, in the working state). The structure of the skeletal muscle and the functions it performs should be considered as parts of a whole, mutually causing each other.
White muscle fibers
They are distinguished by a light color, contain a much smaller amount of sarcoplasma, mitochondria and myoglobin, but they are characterized by a high content of myofibrils. This leads to the fact that they are reduced much more intensively than the red ones, but they also “get tired” too quickly.
The structure of human muscles is different in that the body has one and the other. Such a combination of fibers determines the speed of muscle response (contraction) and their long-term performance.
Smooth muscle tissue (uncharged): structure
It is built of myocytes stationed in the walls of the lymph, blood vessels, and constituting the contractile apparatus in the inner hollow organs. This elongated cells with a spindle shape, without cross-striations. Their location group. Every miocic surrounds the basal membrane, collagen and reticular fibers, among which are elastic. Among themselves cells are linked by the nexus. The structural features of the muscles of this group are that each myocite surrounded by connective tissue, suited one nerve fiber (for example, the sphincter of the pupil), and momentum are transported from one cell to another using a nexus. Its speed of 8-10 cm/s.
In smooth myocytes, the rate of contraction is much less than that of myocytes of striated muscle tissue. But energy is consumed sparingly. Such a structure allows them to make long-term contractions of a tonic nature (for example, sphincters of blood vessels, hollow, tubular organs) and rather slow movements, which are often rhythmic.
Cardiac muscle tissue: features
According to the classification, it belongs to the striated, but the structure and function of the muscles of the heart are markedly different from the skeletal. Cardiac muscle tissue consists of cardiomyocytes, which form complexes, connecting with each other. The contraction of the heart muscle is not under the control of the human mind. Cardiomyocytes are cells that have an irregular cylindrical shape, with 1-2 nuclei, a large number of large mitochondria. Between themselves, they are connected insert disks. This is a special zone, which includes the cytolemma, areas of attachment of myofibrils to it, desmosses, nexuses (through them the transmission of nervous excitation and ion exchange between cells takes place).
Classification of muscles depending on the shape and size
1. Long and short. The first are found where the greatest scope when moving. For example, upper and lower limbs. And short muscles, in particular, are located between separate vertebrae.
2. Broad muscles (in the photo - the stomach). They are mainly located on the body, in the cavity walls of the body. For example, the superficial muscles of the back, chest, abdomen. With a multi-layered arrangement of their fibers, as a rule, go in different directions. Therefore, they provide not only a large variety of movements, but also strengthen the walls of body cavities. In the wide muscles, the tendons have a flat shape and occupy a large surface, they are called sprains or aponeuroses.
3. Circular muscles. They are located around the orifices of the body and constrict them with their contractions, as a result of which they are called sphincters. For example, circular muscle of the mouth.
Complex muscles: structural features
Their names correspond to their structure: two-, three- (in the photo) and quadriceps. The structure of the muscles of this type differs in that their beginning is not one, but divided into 2, 3 or 4 parts (heads), respectively. Starting from different points of the bone, they then shift and merge into a common abdomen. It can also be divided across the intermediate tendon. Such a muscle is called digastric. The direction of the fibers may be parallel to the axis or be at an acute angle to it. In the first case, the most common, the muscle is sufficiently shortened during contraction, thereby providing a large sweep during movement. And in the second - the fibers are short, angled, but they are much larger in number. Therefore, the muscle is shortened slightly with contraction. Its main advantage is that it develops great strength. If the fibers are suitable for the tendon on one side only, the muscle has the name of single-stranded, if from two - two-circular.
The structure of human muscles is unique and has its own characteristics. So, for example, under the influence of their work, auxiliary devices are formed from the surrounding connective tissue. There are four of them.
1. Fascia, which is nothing but a shell of dense, fibrous fibrous tissue (connective). They cover both single muscles and entire groups, as well as some other organs. For example, kidneys, neurovascular bundles, etc. They influence the direction of thrust during contraction and prevent the muscles from moving sideways. The density and strength of fascias depends on their location (they differ in different parts of the body).
2. Bursal (pictured). The role and structure of many, perhaps, will remember from school lessons (Biology, 8th grade: “Building muscles”). They represent a kind of bags, the walls are formed by connective tissue and thin enough. Inside the fluid-filled type synovitis. As a rule, they are formed where tendons are in contact among themselves or experiencing a lot of friction on the bone when the muscle contraction, and the friction of her skin (e.g., elbows). Thanks to the synovial fluid is improved and easier to slide. They develop mainly after birth, and over the years the cavity increases.
3. Synovial vagina. Their development takes place inside the bone-fibrous or fibrous channels, with which the tendons of the long muscles are surrounded at the points of gliding along the bone. In the structure of the synovial vagina, two lobes are distinguished: the inner one, which covers the tendon from all sides, and the outer lining of the fibrous canal wall. They prevent the friction of the tendons against the bone.
4. Sesamoid bones. As a rule, they ossify inside the ligaments or tendons, strengthening them. This facilitates the work of the muscle by increasing the leverage of the force.