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# Skeletal Muscle Approximately 40% of the body is skeletal muscles and almost another 10% is smooth and cardiac muscle. Many of the same principles of contraction apply to all these different types of muscles. ## Physiologic Anatomy * Each muscle is composed of a number of muscle fibers arrange...

# Skeletal Muscle Approximately 40% of the body is skeletal muscles and almost another 10% is smooth and cardiac muscle. Many of the same principles of contraction apply to all these different types of muscles. ## Physiologic Anatomy * Each muscle is composed of a number of muscle fibers arranged parallel to each other. * Each muscle fiber is cylindrical in shape about 10-100µ in diameter; its cell membrane is called sarcolemma and cytoplasm is called sarcoplasm. The sarcoplasm contains, in addition to the normal organoids (mitochondria, Golgi apparatus etc.), contractile elements called myofibrils. Each muscle fiber contains several hundred to several thousands myofibrils. * Two types of tubules: * Longitudinal tubules * Transverse tubules (T- tubules) * The skeletal muscle fiber show transverse striations hence the name striated muscles. * Each myofibril along its length shows alternating light (I) and dark (A) bands. The I band is bisected by a dark line called Z-line. The segment between two successive Z-lines is called sarcomere (the unit of the myofibril). The A band is bisected by a lighter band called H band. The transverse striations in a muscle fiber are due to the fact that the bands in adjacent myofibrils are in register with one another. Each myofibril is formed of finer myofilaments which are of two types: - **Thin filament (actin filament):** * It extends from Z-line towards, but not reaching, the middle of the H band (this is applied when the muscle fiber is at its normal fully stretched resting length, i.e. the length of the sarcomere is about 2µ). The thin filament is formed of:- * **Actin:** Each myofibril contains about 3000 thin filaments. * It is formed of globular actin molecules called G actin molecules. * It has on one side an active site with which the cross bridges of the myosin filament interacts to cause muscle contraction. The image shows a magnified diagram of a muscle fiber and myofibrils. It illustrates the position of the Z line and the arrangement of the thick and thin filaments.