Basic structure of a skeletal muscle
A typical skeletal muscle consists of two types of tissues: Contractile tissue and Supporting tissue.
The contractile tissue of each muscle is composed of numerous muscle fibers. Each muscle fiber is a multinucleated, cross-striated cylindrical cell. The length of each muscle fiber is between 1 and 300 mm. It consists of a cell membrane (sarcolemma), which encloses the cytoplasm (sarcoplasm).
There are two types of substances embedded in the sarcoplasm.
- Several nuclei arranged at the periphery beneath the sarcolemma.
- A number of evenly distributed longitudinal threads called myofibrils.
Each myofibril shows alternate light and dark bands. Dark bands are Anisotropic and thus are known as A-bands. The light bands are Isotropic and thus are known as I-bands. The bands of adjacent fibrils are aligned transversely so that the muscle fiber appears cross striated. In the middles of the A band (dark band) there is a light H band. In the middle of the H band there is a dark M line. In the middle of the I band (light band) there is a dark Z disk also known as Krause’s membrane. The segment of myofibril between two Z discs is called sarcomere.
It helps in organization of the muscle. Endomysium surrounds each muscle fiber separately. Perimysium surrounds bundles (fasciculi or myonemes) of muscle fibers of various sizes. Epimysium surrounds the entire muscle. The connective tissue of the muscle becomes continuous with the tendon.