Skeletal muscles are composed of a bundle of muscle fibers and are attached to bones through tendons. Each skeletal muscle fiber is a single muscle cell. The sarcolemma, the plasma membrane of a skeletal muscle cell, consists of a lipid bilayer and glycocalyx that supports muscle fibers. The sarcolemma extends into the muscle cells to form tubular structures called transverse or T-tubules. Each side of the T-tubules consists of a membrane-bound structure called the sarcoplasmic reticulum, similar to the smooth endoplasmic reticulum. The storage and release of calcium ions in response to a neuronal signal.
Contractile elements called sarcomeres aid the contraction and relaxation of skeletal muscle fibers to control various body movements, protect internal organs, and maintain body posture. A sarcomere mainly consists of actin and myosin protein filaments. In a relaxed muscle cell, the active binding site of actin is masked by a protein called tropomyosin. This masking prevents actin from binding to myosin, preventing muscle contraction. When a nerve impulse stimulates a muscle fiber, the sarcoplasmic reticulum releases calcium into the sarcoplasm. The released calcium binds to a protein called troponin on the thin filaments of muscle fiber. This induces structural changes in troponin, allowing it to attach to tropomyosin. The binding of troponin to tropomyosin relieves the inhibition of actin and enables myosin to bind to actin, further causing muscle contraction.
