BIOCHEMISTRY OF MUSCLE, MUSCLE REDUCTION AND RELAXATION
General characteristics of the muscles
In animals and humans, there are two main types of muscles:
• Striped (attached to the bones, that is, to the skeleton, and therefore also called skeleton, and also distinguish the heart muscle, which has its own characteristics);
• smooth (musculature of the walls of hollow organs and skin).
Structure of muscle cells
The transverse striated muscle consists of numerous elongated muscle cells. The motor nerves enter at different points into the muscle fiber and transmit to it the electrical impulse causing the contraction. Muscle fiber is usually treated as a multinucleated giant cell covered with an elastic membrane - a sarcolemma. The diameter of a functionally mature striated muscle fiber is usually 10 to 100 μm, and the length of the fiber often corresponds to the length of the muscle.
A number of structures are found in the sarcoplasm of muscle fibers: mitochondria, microsomes, ribosomes, tubules and cisterns of the sarcoplasmic network, various vacuoles, glycogen clumps and lipid inclusions that play the role of spare energy materials, etc.
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In each muscle fiber in the semiliquid sarcoplasm, along the length of the fiber, a number of filamentous formations-myofibrils (their thickness usually less than 1 μm), which, like all fibers in general, have a transverse striation, is located in the form of bundles. The transverse striation of the fiber, which depends on the optical heterogeneity of the protein substances localized in all myofibrils at the same level, is easily revealed when studying the fibers of skeletal muscles in a polarizing or phase-contrast microscope (Figure 2).
The repeating element of the cross-striped myofibril is the sarcomere-a portion of myofibril, whose boundaries are narrow 2-lines. Each myofibril consists of several hundred sarcomeres. The average length of the sarcomere is 2.5-3.0 μm. In the middle of the sarcomere is a zone of 1.5-1.6 μm in length, dark in a phase contrast microscope. In polarized light, it gives a strong birefringence. This zone is usually called disk A (anisotropic disk). In the center of disk A there is a line M, which can be observed only in an electron microscope. The middle part of the disk A is occupied by the zone H of the weaker birefringence. Finally, there are isotropic disks, or disks I, with very weak birefringence. In a phase contrast microscope, they appear lighter than disks A. The length of the disks I is about 1 μm. Each of them is divided into two equal halves by a Z-membrane, or Z-line. According to modern ideas, disks A contain thick filaments consisting mainly of the myosin protein, and thin filaments, consisting, as a rule, of the second component of the actomyosin system, the actin protein. Thin (actinic) filaments begin within each sarcomere in the Z-line, stretch through the disk I, penetrate into the disk A and are interrupted in the region of the zone H.
Fig. 2. Photograph of a microscopic preparation of striated muscle tissue
Fig. 3. Scheme of the structure of the sarcomere
In the study of thin sections of muscles under an electron microscope, it was found that the protein strands are located strictly ordered. Thick yarns with a diameter of 12-16 nm and a length of about 1.5 μm are laid in the form of a hexagon with a diameter of 40-50 nm and pass through the entire disk A. Between these thick filaments there are thin threads 8 nm in diameter, extending from the 2-line for a distance of about 1 μm (Figure 3). A study of the muscle in the contraction state showed that the disks I almost disappeared in it, and the area of overlap between thick and thin threads increases (in the skeletal muscle, in the contraction state, the sarcomer shortens to 1.7-1.8 μm).
According to the model proposed by E. Huxley and R. Niedergerke, as well as H. Huxley and J. Henson, with the contraction of myofibrils, one system of filaments penetrates into another, that is, the threads begin to slide along one another as if is the cause of muscle contraction.
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