Muscle tissue

Muscle tissue (textus muscularis) is a group of tissues (striated, smooth, cardiac) of different origin and structure, united by a functional feature - the ability to contract - shorten. Along with the mentioned varieties of muscle tissue formed from the mesoderm (mesenchyme), in the human body there is muscle tissue of ectodermal origin - myocytes of the iris of the eye.

Striated (cross-striated, skeletal) muscle tissue is formed by cylindrical muscle fibers from 1 mm to 4 cm or more in length and up to 0.1 mm in thickness. Each fiber is a complex consisting of a myosymplast and myosatellite cells covered with a plasma membrane called a sarcolemma (from the Greek sarkos - meat). The basal plate (membrane) formed by thin collagen and reticular fibers adjoins the sarcolemma on the outside. The myosymplast, located under the sarcolemma of the muscle fiber, is called the sarcoplasm. It consists of many ellipsoid nuclei (up to 100), myofibrils and cytoplasm. Elongated nuclei oriented along the muscle fiber lie under the sarcolemma. The sarcoplasm contains a large number of elements of the granular endoplasmic reticulum. Approximately 1/3 of the dry mass of muscle fiber is accounted for by cylindrical myofibrils, which extend longitudinally through almost the entire sarcoplasm. Between the myofibrils are numerous mitochondria with well-developed cristae and glycogen.

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

T-tubules play an important role in the rapid conduction of the action potential to each myofibril. The action potential generated in the sarcolemma of the muscle fiber under the influence of a nerve impulse spreads along the T-tubules, and from them to the non-granular endoplasmic reticulum, the canals of which are located near the T-tubules, as well as 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 is a light zone - stripe H, in the center of which is line M, or mesophragm. Line Z - the so-called telophragm - passes through the middle of disk I. The alternation of dark and light disks in neighboring myofibrils located at the same level on a histological preparation of skeletal muscle creates the impression of transverse striation. Each dark disk is formed by thick myosin threads with a diameter of 10-15 nm. The length of the thick threads is about 1.5' μm. The basis of these threads (filaments) is the 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 the low molecular weight protein actin, as well as the low molecular weight proteins tropomyosin and troponin.

The section of the myofibril between two telophragms (Z-lines) is called a sarcomere. It is the functional unit of the myofibril. The sarcomere is about 2.5 µm long and includes a dark disk A and halves of the light disks I adjacent to it on both sides. Thus, thin actin filaments go from the Z-line towards each other and enter disk A, into the spaces between the thick myosin filaments. When the muscle contracts, the actin and myosin filaments slide towards each other, and when relaxed, they move in opposite directions.

Sarcoplasm is rich in the protein myoglobin, which, like hemoglobin, can bind oxygen. Depending on the thickness of the fibers, the content of myoglobin and myofibrils in them, so-called red and white striated muscle fibers are distinguished. Red muscle fibers (dark) are rich in sarcoplasm, myoglobin and mitochondria, but they have few myofibrils. These fibers contract slowly and can be in a contracted (working) state for a long time. White muscle (light) fibers contain little sarcoplasm, myoglobin and mitochondria, but they have many myofibrils. These fibers contract faster than 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 a quick reaction (contraction) and long-term performance.

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

Non-striated (smooth) muscle tissue consists of myocytes, which are located in the walls of blood and lymphatic vessels, hollow internal organs, where they form their contractile apparatus. Smooth myocytes are elongated spindle-shaped cells 20 to 500 μm long and 5 to 15 μm thick, devoid of transverse striation. The cells are located in groups, the pointed end of each cell is embedded between two adjacent cells. Each myocyte is surrounded by a basal membrane, collagen and reticular fibers, among which elastic fibers pass. The cells are connected to each other by numerous nexuses. The elongated rod-shaped nucleus, reaching 10-25 μm in length, takes the shape of a corkscrew when the cell contracts. From the inside, spindle-shaped dense (attachment) bodies located in the cytoplasm are adjacent to the cytolemma.

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

In the cytoplasm of smooth myocytes there are myofilaments of two types - 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 are attached to dense bodies. Thick short myosin myofilaments with a diameter of about 15 nm are located in the cytoplasm longitudinally. Thin and thick threads do not form sarcomeres, therefore smooth myocytes do not have transverse striation. When myocytes contract, actin and myosin myofilaments shift towards each other, and the smooth muscle cell shortens.

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

The speed of contraction of smooth myocytes is significantly less than that of striated muscle fibers (100-1000 times), while smooth myocytes expend 100-500 times less energy.

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

Striated cardiac muscle tissue is striated, but its structure and function differ from skeletal muscles. It consists of cardiac myocytes (cardiomyocytes) that form interconnected complexes. Contractions of the cardiac muscle are not controlled by human consciousness. Cardiomyocytes are irregularly cylindrical cells 100-150 μm long and 10-20 μm in diameter. Each cardiomyocyte has one or two oval nuclei located in the center and surrounded by myofibrils located strictly longitudinally along the periphery. Near both poles of the nucleus, cytoplasmic zones devoid of myofibrils are visible. The structure of myofibrils in cardiomyocytes is similar to their structure in skeletal muscles. Cardiomyocytes contain a large number of large mitochondria with well-developed cristae, which are located in groups between the myofibrils. Under the cytolemma and between the myofibrils are glycogen and structures of the non-granular endoplasmic reticulum. This network forms the canals of the L-system, with which the T-tubules come into contact.

Cardiomyocytes are connected to each other by so-called intercalated discs, which look like dark stripes when examined with light. An intercalated disc is a contact zone between two cardiomyocytes, including the cytolemma of these cells, desmosomes, nexuses, and zones of attachment of the myofibrils of each cardiomyocyte to its cytolemma. Desmosomes and nexuses connect adjacent cardiomyocytes to each other. Nexuses are used to transmit nerve excitation and exchange ions between cells.

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