Skeletal muscle tissue is distinctive due to its long, tube-shaped multinucleated fibers (cells) with visible cross-striations. The striations are the result of highly ordered actin and myosin filaments within sarcomeres, the basic contractile units of muscle fibers (see images below). Sliding of the myosin thick filaments along the actin thin filaments within the sarcomeres leads to contraction of striated muscle fibers. Review the organization of the bands in the sarcomeres.

Skeletal muscle contains three main fiber types that are distinguished based on several properties, such as size of the fibers, their contractile rates, and the main pathways used to generate ATP. Review the table below describing the three types of fibers: type I slow oxidative, type IIa fast oxidative glycolytic, and type IIb fast glycolytic fibers.

Examine skeletal muscle in the images below and in these slides of the larynx (sample 1  and sample 2) and tongue using low power. Study its organization into fascicles, and identify the connective tissues of muscle:

• Epimysium (Ep), a dense irregular CT that surrounds the entire muscle
• Perimysium (P), a thinner CT layer that bundles muscle fibers into a fascicle
• Endomysium (En), a fine CT layer that surrounds each muscle fiber

At higher power, examine muscle fibers, and identify endomysium and blood vessels of various sizes. Examine the transversely cut skeletal muscle fibers in the H&E stained specimen of larynx again. Try to distinguish individual myofibrils within the muscle fibers. Note the position of the nuclei at the periphery of the muscle fibers. Nuclei of satellite cells (a limited population of progenitor cells capable of giving rise to new muscle fibers after muscle damage due to disease or injury) are also located here. Examine the images below and developing skeletal muscle of the fetal tongue and face to see myoblasts (that give rise to satellite cells) and developing muscle fibers (myotubes).

Finally, examine the dense regular connective tissue of tendon slide and compare its features to those of skeletal muscle. Note the differences in the location and shape of the nuclei and absence of striations in the tendon (seen in the image at the right).

Clinical note: Exercise or increased use of specific muscles produce hypertrophy or increased fiber size, while disuse results in muscle atrophy. Muscular dystrophies, a group of hereditary diseases, are characterized by muscle weakness and atrophy. Recent research has shown that in one form of this disease, Duchenne muscular dystrophy, abnormal function of satellite cells leads to impaired regeneration.
 

Next is a little more about muscle contraction.