Structure of a skeletal muscle Muscle Physiology Animations USMLE Step 1

Skeletal muscle forms striated, multinucleated fibers organized hierarchically from whole muscle to sarcomeres, enabling powerful, voluntary contractions via the sliding filament mechanism. This structure supports USMLE-level understanding of excitation-contraction coupling, crucial for topics like tetanus, rigor mortis, and myopathies.​ Hierarchical Organization Whole muscles wrap in epimysium (outer sheath), dividing into fascicles via perimysium, with individual fibers encased in endomysium containing capillaries and nerves. Each fiber's sarcolemma (plasma membrane) invaginates into T-tubules for rapid signal spread, surrounding myofibrils that give the striated look.​ Sarcomere: Contractile Unit Myofibrils consist of repeating sarcomeres bounded by Z-lines, featuring A-band (myosin length, constant), I-band (actin only, shrinks), H-zone (myosin only, shrinks), and M-line (myosin anchor). Thin filaments (actin, tropomyosin, troponin) interdigitate with thick myosin filaments.​ Contraction Animation Key Neural impulse triggers acetylcholine release, depolarizing sarcolemma; T-tubules activate voltage-gated channels, releasing Ca2+ from sarcoplasmic reticulum. Ca2+ binds troponin, exposing myosin-binding sites on actin; myosin heads (ATP-powered) form cross-bridges, power stroke slides filaments, shortening sarcomere.​ USMLE Structure Table Level Components Role ​ Muscle Epimysium, fascicles Organ-level bundling, tendon link Fiber Sarcolemma, T-tubules, myofibrils Signal conduction, striations Sarcomere Z/I/A/H/M bands, actin/myosin Sliding for contraction ​ Filaments Thick (myosin), thin (actin+regulatory) Cross-bridge cycling