Structural unit of the tissue
Unlike many other tissues, skeletal muscle is formed not by single cells, but by complex structures—muscle fibers. The length of such a fiber varies from several centimeters to tens of centimeters and, as a rule, corresponds to the anatomical length of the muscle itself.
Each muscle fiber includes three mandatory components:
- Myosymplast. This is a long cylindrical structure that constitutes the main mass and volume of the fiber. It is precisely here that the contractile apparatus is located.
- Myosatellite cells (satellite cells). Undifferentiated cells that serve as a cambial reserve for growth and regeneration. They are located in special depressions of the myosymplast plasmalemma, but never fuse with it.
- Basal lamina. A universal external sheath that completely surrounds the complex of the myosymplast and myosatellites. In histology, the term sarcolemma is often used, which in the classical sense means the combination of the basal lamina and the plasmalemma.
Nuclear and contractile apparatus
The morphology of the myosymplast is best studied in longitudinal sections. One of the main features of the tissue is the colossal number of nuclei in each fiber. At the same time, 95% of them are narrow, rod-shaped nuclei of the myosymplast itself, while the remaining 5% belong to satellite cells. Cellular centers (centrioles) are completely absent in the myosymplast, which is why its nuclei have lost the ability to divide both under normal conditions and during injuries.
The myosymplast nuclei are always displaced to the very periphery of the fiber. The reason for this lies in the massive contractile apparatus—myofibrils, which occupy about 70% of the sarcoplasmic volume.
On a cross section at high magnification, myofibrils look like multiple dots (about 1,400 per fiber), with a diameter of approximately 1.5 µm each. At the light-optical level, they form the characteristic transverse striation of the entire fiber due to the regular alternation of light and dark regions (disks).
Tinctorial properties and microscopy
For a comprehensive study of tissue structure in histology, tongue preparations are often used. In this organ, bundles of muscle fibers run in three mutually perpendicular directions, which makes it possible to observe fibers in several projections at once on a single histological section: as longitudinal long strands and as rounded cross sections.
With standard hematoxylin and eosin staining, muscle fibers demonstrate pronounced oxyphilia—their cytoplasm stains bright pink with eosin. This is due to the extremely high concentration of proteins in the sarcoplasma. For the clearest visualization of transverse striation, iron hematoxylin staining is used.
Contraction mechanism and energy supply
The work of skeletal muscle is based on the sliding filament theory. Contraction occurs not due to the physical shortening of the protein molecules themselves, but as a result of a change in the degree of overlap of thin and thick myofilaments. The filaments slide toward each other due to the cyclical attachment and detachment of "cross-bridges". This leads to the approximation of Z-lines, shortening of the sarcomere, and, consequently, contraction of the entire myofibril.
The process is triggered by a nerve impulse, which causes a sharp increase in the concentration of calcium ions ($Ca^{2+}$) in the sarcoplasma. Such work requires colossal energy expenditures, which is why the fiber has a powerful life support system:
- Organelles: an extremely high content of mitochondria.
- Stores (inclusions): glycogen granules (carbohydrates) and lipid droplets (fats).
- Energy sources: ATP acts as the direct source (cleaved due to the ATPase activity of myosin). During peak loads and ATP deficiency, creatine phosphate comes into play, donating its phosphate group to instantly restore ATP levels.
Briefly: answers to topical questions
Which proteins form the thick and thin myofilaments of myofibrils?
Thick and thin myofilaments of myofibrils are formed by specific contractile and regulatory proteins.
- Thin myofilaments are based on a double helix of the globular protein actin, into which the fibrillar protein tropomyosin and the globular protein troponin are woven.
- Thick myofilaments are formed by the protein myosin, whose molecule consists of a long rod (tail) and a double head.
What structures limit the sarcomere within a myofibril?
The sarcomere in a myofibril is bounded by two sequentially located Z-lines.
- Z-line (telophragma) is a dark line passing through the center of each light I-band.
The sarcomere represents the functional unit of the contractile apparatus and includes one complete A-band in the center and two halves of I-bands at the edges adjacent to the Z-lines.
What is the composition of the muscle triad?
The muscle triad consists of one transverse T-tubule and two adjacent terminal cisternae.
- T-tubule is a deep channel-like invagination of the plasmalemma.
- Terminal cisternae are expanded terminal segments of the L-tubules of the sarcoplasmic reticulum (smooth endoplasmic reticulum), located closely on the sides of the T-tubule.
What types of muscle fibers exist in skeletal muscle tissue?
The following fiber types exist in skeletal muscle tissue. According to shortening velocity and metabolism:
- Red—slow, oxidative (type I).
- White—fast, glycolytic (type II).
- Intermediate.
According to the ability to generate an action potential:
- Phasic—generate a full-fledged AP.
- Tonic—incapable of generating a full-fledged AP.
By localization:
- Intrafusal—inside the muscle spindle.
- Extrafusal—other muscle fibers.
Which organelle performs the function of a calcium ion depot in a muscle fiber?
The function of a calcium ion depot in a muscle fiber is performed by the sarcoplasmic reticulum (a specialized form of smooth endoplasmic reticulum). At rest, calcium pumps in the membrane of the sarcoplasmic reticulum actively pump calcium ions from the sarcoplasm into the terminal cisternae. Upon excitation, calcium channels open, and calcium ions exit the depot into the sarcoplasm.
The symplast resembles a crowded subway car: contractile elements (myofibrils) occupy all the space in the center (70% of the volume), which is why the "passengers" (nuclei) are tightly pressed against the walls (periphery).
Class and exam questions
Why cannot nuclei in a muscle fiber divide?
Centrioles are completely absent in myosymplasts. As a result, nuclei lose the ability to divide both under normal conditions and during tissue regeneration.
What causes the shortening of a muscle fiber?
The process is based on the sliding filament theory. The protein molecules themselves do not change length, but thick and thin myofilaments slide toward each other. This brings the Z-lines closer together and reduces the length of the sarcomere.
What is the sarcolemma?
In literature, this term is interpreted in two ways. In the classical sense, this is a complex sheath consisting of the plasmalemma of the muscle fiber and the basal lamina covering it from the outside.
What else to study on this topic
- Molecular structure of the sarcomere: A-bands, I-bands, and Z-lines.
- The role of the sarcoplasmic reticulum in the storage and release of calcium ions.
- Stages of cross-bridge formation between filaments.
- Mechanism of activation and function of myosatellites during muscle injury.
- Comparison of the structure of skeletal, cardiac, and smooth muscle tissues.
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