Muscle

Muscle tissue (textus muscularis) is a group of tissues (striated, smooth, cardiac), having a different origin and structure, combined according to a functional feature - the ability to contract - to shorten. Along with the aforementioned types of muscular tissue, formed from the mesoderm (mesenchyme), the human body secrete muscular tissue of ectodermal origin - the myocytes of the iris of the eye.

The striated (striated, skeletal) muscle tissue is formed by cylindrical muscle fibers from 1 mm to 4 cm in length and up to 0.1 mm in thickness. Each fiber is a complex consisting of myosymplast and myosatellite cells covered with a plasma membrane, which is called a sarcolemma (from the Greek sarkos - meat). Outside the sarcolemma, a basal plate (membrane) is formed, formed by thin collagen and reticular fibers. Myosymplast, under the sarcolemma of the muscle fiber, was called sarcoplasm. It consists of a set of ellipsoidal nuclei (up to 100), myofibrils and cytoplasm. Elongated nuclei, oriented along the muscle fiber, lie under the sarcolemma. In sarcoplasm there are a large number of elements of a granular endoplasmic reticulum. Approximately 1/3 of the dry mass of the muscle fiber occurs in cylindrical myofibrils, extending longitudinally almost through the entire sarcoplasm. Between myofibrils are numerous mitochondria with well-developed cristae and glycogen.

In the striated muscle fiber, the sarcotubular network is well developed, which is formed by two components: endoplasmic reticulum tubules located along the myofibrils (L-system) and T-tubules (T-tubules) beginning in the area of the sarcolemma invagination. T-tubes penetrate into the depth of the muscle fiber and form transverse tubules around each myofibril.

T-tubes play an important role in rapidly carrying out the action potential to each myofibril. Arising in the sarcolemma of the muscle fiber under the influence of a nerve impulse, the action potential spreads along the T-tubes, and from them onto the ungrain endoplasmic reticulum, the tubules of which are located near the T-tubes, and between the myofibrils.

The main part of the sarcoplasm of the muscle fiber is made up of special organelles - myofibrils. Each myofibril consists of regularly alternating sections - dark anisotropic disks A and light isotropic disks I. In the middle of each anisotropic disk A there passes a light zone - a strip H, in the center of which there is a line M, or a mesophragm. A line Z - the so-called telaphragm - passes through the middle of the disk I. The alternation of dark and light discs in neighboring myofibrils located on the same level on the histological preparation of the skeletal muscle creates the impression of transverse striation. Each dark disk is formed by thick myosin filaments 10-15 nm in diameter. The length of thick filaments is about 1.5'mkm. The basis of these filaments (filaments) is a high-molecular protein, myosin. Each light disk is formed from thin actin filaments 5-8 nm in diameter and about 1 μm in length, consisting of a low-molecular actin protein, as well as low-molecular proteins of tropomyosin and troponin.

The site of the myofibril between two telaphragms (Z-lines) is called a sarcomer. It is the functional unit of the myofibril. The length of the sarcomere is about 2.5 microns, it includes a dark disk A and halves of light discs I adjacent to it on both sides. Thus, the thin actin filaments run from the Z-line towards each other and enter the disk A, at intervals between thick myosin filaments. When the muscles contract, the actin and myosin filaments slip towards each other, while relaxing, they move in opposite directions.

Sarcoplasma is rich in protein myoglobin, which, like hemoglobin, can bind oxygen. Depending on the thickness of the fibers, the content of myoglobin and myofibrils in them are distinguished by the so-called red and white striated muscle fibers. Red muscle fibers (dark) are rich in sarcoplasm, myoglobin and mitochondria, but they have few myofibrils. These fibers are slowly shrinking and can be long in a reduced (working) state. White muscle fibers (light) contain little sarcoplasm, myoglobin and mitochondria, but they have many myofibrils. These fibers shrink faster than the red ones, but quickly "get tired". In humans, muscles contain both types of fibers. The combination of slow (red) and fast (white) muscle fibers provides the muscles with quick reaction (shortening) and long working capacity.

Myosatellitocytes are located directly above the sarcolemma, but under the basal plate (membrane). These are flattened cells with a large chromatin-rich nucleus. Each myosatellitocyte has a centrosome and a small number of organelles; spiral organelles (myofibrils) they do not. Myosatelliteocytes are stem cells of transversely striated (skeletal) muscle tissue, they are capable of DNA synthesis and mitotic division.

Non-uncircumcised (smooth) muscle tissue consists of myocytes, which are located in the walls of the blood, lymphatic vessels, hollow internal organs, where they form their contractile apparatus. Smooth myocytes are elongated spindle-shaped cells with a length of 20 to 500 μm and a thickness of 5 to 15 μm, devoid of transverse striation. The cells are arranged in groups, the pointed end of each cell is inserted between two adjacent cells. Each myocyte is surrounded by a basal membrane, collagen and reticular fibers, among which are elastic fibers. The cells are connected among themselves by numerous nexus. The elongated rod-shaped nucleus, reaching 10-25 microns in length, becomes a corkscrew when the cell shrinks. From the inside, the cytolemma is surrounded by spindle-shaped, dense (attached) bodies located in the cytoplasm.

Dense corpuscles are analogous to Z-strips of striated muscle fibers. They contain the protein a-actinin.

In the cytoplasm of smooth myocytes there are two types of myofilaments - thin and thick. Thin actin myofilaments with a diameter of 3-8 nm lie along the myocyte or obliquely in relation to its long axis. They attach to dense bodies. Thick short myosin myofilaments with a diameter of about 15 nm are located longitudinally in the cytoplasm. Thin and thick threads do not form sarcomeres, so smooth myocytes do not have transverse striation. With the contraction of myocytes, the actin and myosin myofilaments shift towards each other, while the smooth muscle cell is shortened.

A group of myocytes surrounded by a connective tissue is usually innervated by one nerve fiber. The nervous impulse is transmitted from one muscle cell to another due to nexus at a speed of 8-10 cm / s. In some smooth muscles (for example, the sphincter of the pupil), each myocyte is innervated.

The rate of contraction of smooth myocytes is much less than that of striated muscle fibers (100-1000 times), while the smooth myocytes consumes 100-500 times less energy.

Smooth muscles make prolonged tonic contractions (for example, sphincters of hollow - tubular - organs, smooth muscles of blood vessels) and relatively slow movements, which are often rhythmic.

The outlined cardiac muscle tissue is transversely striated, but its structure and function is different from skeletal muscles. It consists of cardiac myocytes (cardiomyocytes), which form complexes connecting with each other. The contractions of the heart muscle are not controlled by human consciousness. Cardiomyocytes are cells of irregular cylindrical shape with a length of 100-150 μm and a diameter of 10-20 μm. Each cardiomyocyte has one or two oval nuclei lying in the center and surrounded by myofibrils located along the periphery strictly longitudinally. Near the two poles of the nucleus are visible cytoplasmic zones devoid of myofibrils. The structure of myofibrils in cardiomyocytes is similar to their structure in skeletal muscles. In cardiomyocytes, a large number of large mitochondria with well-developed cristae, which are located in groups between myofibrils. Under the cytolemma and between myofibrils are glycogen and the structure of an ungrain endoplasmic reticulum. This network forms the tubules of the L-system, with which the T-tubes come into contact.

Cardiomyocytes are interconnected by so-called insertion discs, which, when light-optical, have the form of dark strips. The insertion disc is the contact zone of two cardiomyocytes, which includes the cytolemma of these cells, desmosomes, nexus and the attachment zones of myofibrils of each cardiomyocyte to its cytolemma. Desmosomes and neksusy connect adjacent cardiomyocytes with each other. Through neksusy there is a transfer of nervous excitation and exchange of ions between cells.

[1], [2], [3]