Chapter 5: Excitable Tissue: Muscle

Excitable Tissue: Muscle begins by detailing the complex ultrastructure of skeletal muscle, identifying the muscle fiber as the basic cellular unit, which contains myofibrils composed of contractile thick filaments (myosin-II) and thin filaments (actin, tropomyosin, and troponin) organized into sarcomeres. The text explains the molecular mechanics of contraction through the sliding filament theory, where calcium ions (Ca2+) bind to troponin C, causing a conformational change that exposes myosin-binding sites on actin, allowing myosin heads to execute a power stroke fueled by ATP hydrolysis. The critical process of excitation-contraction coupling is described, highlighting how action potentials travel down transverse (T) tubules to trigger calcium release from the sarcoplasmic reticulum via the interaction between dihydropyridine receptors (DHPR) and ryanodine receptors (RyR). The chapter further differentiates between isometric and isotonic contractions, explores the length-tension relationship, and defines the mechanics of summation and tetanus. Metabolic pathways are examined, including the roles of phosphorylcreatine, aerobic glycolysis, and anaerobic mechanisms that lead to oxygen debt. Muscle fiber diversity is categorized into slow (Type I) and fast (Type IIA and IIB) fibers, which are organized into motor units recruited according to the size principle. The discussion transitions to cardiac muscle, noting its striated nature, the presence of intercalated disks and gap junctions that allow for syncytial function, and its unique electrical properties such as the prolonged plateau phase and calcium-induced calcium release. Finally, the chapter covers smooth muscle, distinguishing between unitary (visceral) and multiunit types, and explains its distinct contraction mechanism which lacks troponin and instead relies on calmodulin and myosin light chain kinase, as well as its ability to maintain tension through the latch bridge mechanism and exhibit plasticity. Clinical correlations such as muscular dystrophy, malignant hyperthermia, and rigor mortis are also integrated to illustrate the pathophysiological consequences of protein or ion channel dysfunction.