• Muscle comprises the largest group of tissues in the body, accounting for approximately half of body weight. Skeletal muscle alone makes up about 40% of body weight in men and 32% in women, with smooth and cardiac muscle making up another 10% of total weight. • Although these three muscle types are structurally and functionally distinct, they can be classified in two ways according to common characteristics.

• First, muscles are categorized as striated (skeletal and cardiac muscle) or • unstriated (smooth muscle), depending on whether alternating dark and light bands, or striations (stripes), can be seen when the muscle is viewed under a light microscope. • Second, muscles are categorized as voluntary (skeletal muscle) or involuntary (cardiac and smooth muscle), depending on whether they are innervated by the somatic nervous system and subject to voluntary control or are innervated by the autonomic nervous system and not subject to voluntary control, respectively.

Skeletal muscle fibers are striated by a highly organized internal arrangement. • A single skeletal muscle cell, known as a muscle fiber, is relatively large, elongated, and cylinder shaped. • A skeletal muscle consists of a number of muscle fibers lying parallel to one another and bundled together by connective tissue. • The fibers usually extend the entire length of the muscle.

• During embryonic development, the huge skeletal muscle fibers are formed by the fusion of many smaller cells called myoblasts (myo means “muscle”; blast refers to a primitive cell that forms more specialized cells); thus, one striking feature is the presence of multiple nuclei in a single muscle cell. • Another feature is the abundance of mitochondria, the energy-generating organelles, as would be expected with the high energy demands of a tissue as active as skeletal muscle.

• A skeletal muscle fiber contains numerous myofibrils, which are cylindrical intracellular structures 1 mm in diameter that extend the entire length of the muscle fiber. • Myofibrils are specialized contractile elements that constitute 80% of the volume of the muscle fiber.

• Each myofibril consists of a regular arrangement of highly organized cytoskeletal microfilaments —the thick and the thin filaments. • The thick filaments, are special assemblies of the protein myosin, whereas the thin filaments, are made up primarily of the protein actin.