Skeletal muscle fibers (= myofibers) are long and narrow cells that often span the entire length of a muscle. The longest are found in the thigh muscles and measure up to 30 cm (12 in.) in length and 100 m in width.
The cytoplasm or sarcoplasm of skeletal muscle fibers is filled with bundles of contractile protein filaments called myofibrils. A typical, lipid bilayer membrane encloses the sarcoplasm. In skeletal muscle fibers, however, this membrane is referred to as the sarcolemma. Along the surface, skeletal muscle fibers appear striped or striated. The darker bands are referred to as A bands and the lighter ones are called I bands. This unusual surface appearance is due to the length, thickness, and alignment of the protein filaments in the myofibrils. Because of their exceptional length and high protein content, skeletal muscle fibers are multinuclear. The nuclei are dispersed all along the surface of the fibers, just underneath the sarcolemma. Each nucleus regulates the metabolic needs of the surrounding sarcoplasm.
Adjacent muscle fibers are surrounded and held together by thin layers of loose connective tissue called endomysium.
