Muscle Fiber, Long, cylindrical, multinucleated skeletal muscle cell that can reach up to 100 μm in diameter and 30 cm in length. Myoblasts, Embryonic cells with their own nucleus that fuse together during early development to form multinucleated skeletal muscle fibers. Myosatellite Cells, Unfused embryonic myoblasts located in the endomysium that function as stem cells to repair or replace damaged muscle fibers. Sarcolemma, The plasma membrane of a muscle fiber cell. Sarcoplasm, The cytoplasm of a muscle fiber containing glycogen granules, mitochondria, and myofibrils. Transverse Tubules (T-tubules), Invaginations and branching tubes of the sarcolemma that carry the action potential deep into the center of the muscle cell. Myofibrils, Long cylindrical divisions within a muscle fiber responsible for muscle contraction. Myofilaments, Bundles of thick and thin protein filaments within a myofibril. Sarcoplasmic Reticulum (SR), Specialized smooth endoplasmic reticulum wrapping each myofibril that stores and releases calcium ions. Terminal Cisternae, Enlarged, expanded chambers of the sarcoplasmic reticulum attached to T-tubules where calcium ions are actively pumped and stored. Triad, The structural combination of one T-tubule flanked by two terminal cisternae surrounding a myofibril. Glycogen Granules, Stored form of glucose in the sarcoplasm broken down via glycolysis to supply ATP needed for muscle contraction. Sarcomere, The functional, contractile repeating unit of a skeletal muscle fiber spanning from Z-line to Z-line. Z-line (Z-disc), Border structures defining each sarcomere and serving as attachment sites for thin actin filaments. M-line, The exact center line of a sarcomere containing support proteins that stabilize thick myosin filaments. A-band, The dark central region of a sarcomere containing both thick myosin filaments and thin actin filaments. I-band, The light region of a sarcomere containing thin actin filaments only. H-zone (H-band), The central region of the A-band containing thick myosin filaments only on either side of the M-line. Zone of Overlap, The dark region of the A-band where thick and thin filaments overlap. Titin, A specialized elastic protein filament extending from the tip of thick filaments to the Z-line that stabilizes myosin and restores rest length. F-actin (Filamentous actin), A long, twisted strand made of two rows of spherical G-actin molecules forming the bulk of thin filaments. G-actin (Globular actin), Spherical actin molecules containing active sites that bind to myosin heads during contraction. Tropomyosin, A double-twisted, rope-like protein running along F-actin that covers G-actin active sites at rest to prevent myosin binding. Troponin, A globular 3-subunit protein attached to tropomyosin, G-actin, and calcium ions. Troponin-Tropomyosin Complex, The protein complex that blocks cross-bridge formation until calcium binds to troponin. Myosin, The thick filament protein containing long tails and double globular heads that act as motor proteins during muscle contraction. Cross-bridge, The connection formed when a myosin head attaches to an active site on G-actin during muscle contraction.

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