Each skeletal muscle is composed of thousands of long, cylindrical muscle fibers. When viewed under a microscope, these fibers show alternating light and dark bands called striations. For this reason, skeletal muscles are also called striated muscles. The thick filaments consist of a protein called myosin. The thin filaments are made up chiefly of the protein actin.
Muscle fibers have a variety of other specialized parts.
Each skeletal muscle fiber has many elements called nuclei. These nuclei contain growth-producing substances that repair or remake various parts of the muscle fiber as they wear out. Each muscle fiber also has thousands of tiny sausage-shaped mitochondria. These structures produce the energy that the fiber needs in order to live and do its work. Muscle fibers are held together by connective tissue.