Chapter 9: Muscle

The structural foundation begins with the sarcomere, the basic contractile unit bounded by Z lines and containing thick myosin filaments and thin actin filaments arranged in a precise lattice that enables the sliding-filament mechanism of contraction. Supporting structures including the sarcolemma, T-tubules, sarcoplasmic reticulum, and terminal cisternae create an integrated system for excitation and calcium regulation, while titin provides elastic stability to the sarcomere. Neuromuscular transmission initiates contraction through acetylcholine release at the motor end plate, binding to nicotinic receptors and generating end-plate potentials that trigger action potentials along the sarcolemma. Excitation-contraction coupling mechanically links dihydropyridine receptors to ryanodine receptors, releasing calcium from internal stores to initiate cross-bridge cycling, during which myosin heads bind actin, undergo power strokes, and detach in a coordinated cycle powered by ATP hydrolysis. Single-muscle-fiber mechanics reveal how twitch responses, summation, and tetanic contractions relate to stimulation frequency, while length-tension relationships identify optimal sarcomere length for maximal force generation and load-velocity relationships distinguish between isometric, isotonic, concentric, and eccentric contraction modes. Energy metabolism relies on multiple ATP sources—creatine phosphate for immediate use, glycolytic pathways for rapid ATP production, and oxidative phosphorylation for sustained activity—with oxygen debt reflecting the metabolic cost of fatigue recovery. Muscle fiber types span slow-oxidative, fast-oxidative-glycolytic, and fast-glycolytic categories, differing in myoglobin content, metabolic capacity, and contraction speed. Whole-muscle function depends on motor unit recruitment, antagonistic muscle interactions, and adaptations including hypertrophy and atrophy in response to activity or denervation. Clinical disorders illustrate functional principles: myasthenia gravis disrupts neuromuscular transmission, muscular dystrophies compromise structural costameres and dystrophin function, and various conditions produce cramps or abnormal calcium handling. Smooth muscle lacks sarcomeric organization but relies on dense bodies and latch-state mechanisms regulated by myosin light chain kinase and phosphatase to sustain contraction with minimal energy expense, varying between single-unit and multiunit phenotypes with pacemaker activity and slow-wave propagation in many tissues. Cardiac muscle integrates these principles through intercalated disks, specialized calcium handling via L-type channels, and autonomous and autonomic control mechanisms that coordinate efficient, rhythmic pumping essential to circulation.