The independent units of skeletal muscle are called as muscle fibres. The fibres have many nuclei and are larger than most cells. Fibres are grouped together into bundles of fasciculi. Larger muscles are composed of many fasciculi.
The skeletal muscle fibres are elongated and cylindrical and 1.5 cm in length and 10-15 microns in diameter. The thickness varies with species, individual stage of nutrition, age, size and especially functions of muscle. With growth or exercise the size increases.
Surrounding the fibre is a covering or sheath called Sarcolemma. This sheath is elastic, transparent, homogenous and difficult to see except when broken in the fresh preparations. It can be seen in stained longitudinal sections. Inside the sarcolemma, are the nuclei and a cross striated substance composed principally of myofibrils. Surrounding fibrillae is sarcoplasm which corresponds to cytoplasm of other cells. The skeletal muscle fibre has many nuclei, several hundred appearing within a fibre of average size. The nuclei are directly under the sarcolemma. The nuclei are flattened and oval. They show a loose network of chromatin.
The skeletal muscle fibre shows a characteristic striated appearance due to regular alternation of dark and light stripes or striations across. They are produced by alternating light and dark segments of longitudinally arranged elements, by myofibrillae. The latter are 1.2 microns in diameter. They are visible within the fibres of teased fresh muscle as well as in fixed preparations. Similar segment of adjacent fibrillae are placed side by side in an intact fibre to form characteristic cross striations seen in the fibre as a whole.
In a cross section of a fibre the cut ends of myofibrils are visible as definite areas with individual size and shape. They are separated from each other by narrow spaces which represent the sarcoplasm. In routine preparations the fibrils appear to be arranged in irregular groups known as Cohanheimsfields.
In the cross striations of the fibrillae the dark band is called the A or Q band (anisotropic). The light band is designated I or J band (isotropic). Each of these bands is bisected by a narrow line, that in the 1 band stains deeply and is designated ‘Z” line and the line bisecting the A (q) band is pale deeply and is called H zone. The Z line is also called Krause’s membrane. The portion of a fibril between the two successive ‘Z’ line is called a sarcomere. In the relaxed condition, the sarcomere is to 3 microns.
Each myofibril is made up of thin, thread like elements myofilaments. Two types of myofilaments have been observed. One is thick and extends from one end of the A band to the other. These are myosin filaments. The thin filaments from either side of the ‘Z’ line extend across the adjacent I band into the ‘A’ band as far as the ‘H’ zone. These are made up of the actin and tropomyosin. Thus the cross striations seen with the light microscope are related to the distribution of myofilaments which can be seen only under electron microscope.
Sarcoplasm fills in the spaces between myofibrils is clear cytoplasmic ground substance. It contains Golgi apparatus, mitochondria, endoplasmic reticulum and has few ribosomes. It also contains glycogen and fat droplets. Fibres rich in sarcoplasma, appear darker with the naked eye or low magnifications. These are called dark fibres or red fibres. Those poor in sacroplasm are light fibres. Both kinds are intermingled in the muscles of mammals.
Changes during contraction
- During contraction the fibre as a whole becomes shorter and broader, when it contracts. Each sarcomere also becomes are stimulated to contract by about 50 per cent. The H zone of A band usually disappears at the same time.
- The total length of A band remains constant usually during contraction and relaxation.
- It is believed that during contraction the thin actin filaments slide over the thick myosin filaments, the actin filaments being more or less pulled into the “A” band or that the actin filaments fold or coil up during contraction.
- During contraction an acto-myosin complex is formed.
Muscle tendon attachment
- At the ends of a skeletal muscle the fibres are attached securely either to tendon, periosteum pr some fibrous connective tissue structure.
- Muscle fibrillae end abruptly and are not continuous with those of tendon or other connective tissue structure. But the connective tissue of muscle (see below) is continuous with the connective tissue of the tendon or other connective tissue structures to which muscles are attached.
Connective tissue of muscle
- Surrounding each muscle fibre is a network of fine reticular fibres the endomysium. Groups of muscles fibres are surrounded by collagenous connective tissue called perimysium. Thus a dozen or more muscle fibres are completely surrounded by perimysium to form fasiculi or muscle.
- Fasciculi are in turn bound into larger and larger orders of bundles by epimysium and the entire muscle has its outer investment the deep fascia of gross anatomy.
- Endomysium, perimysium and epimysium are continuous with one another in the order mentioned blood vessels and nerves ramify through the connective tissue.
Blood and nerve supply
- The larger branches of arteries penetrate the muscle by following laminae of perimysium.
- The arterioles penetrate the fasciculi capillary supply is very rich.
- Every skeletal muscle fibers receives at least one motor nerve ending from different nerves.
- The sensory or afferent fibres are associated with specialized endings called neuromuscular spindles (Refer Nervous tissue).
