Chapter 9: Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle

The sliding filament theory forms the mechanistic foundation, explaining how myosin heads bind to actin and use energy from ATP hydrolysis to perform power strokes that pull thin filaments toward the sarcomere center, thereby shortening the muscle fiber and generating tension. A critical link between electrical and mechanical events occurs through excitation-contraction coupling, wherein action potentials propagate along the sarcolemma and penetrate into transverse tubules, triggering calcium release from the sarcoplasmic reticulum through the coordinated action of dihydropyridine receptors and ryanodine receptors. Calcium ions bind to troponin, causing tropomyosin to shift position on actin filaments and expose myosin-binding sites, permitting cross-bridge interactions. Muscle relaxation depends on the restoration of low resting calcium concentrations via sarcoplasmic reticulum calcium ATPase pumps. The chapter differentiates among muscle fiber types based on metabolic capacity, contraction speed, and fatigue characteristics, with slow-twitch oxidative fibers suited for sustained activity and fast-twitch glycolytic fibers optimized for rapid powerful contractions. The physiology of cardiac and smooth muscle differs substantially from skeletal muscle in regulatory mechanisms, ultrastructure, and functional properties. Clinical applications include understanding malignant hyperthermia as a calcium-handling disorder, Duchenne muscular dystrophy resulting from dystrophin deficiency, and various myopathic conditions that compromise force generation and functional capacity.