Chapter 9: Skeletal Muscle Tissue

Skeletal muscle tissue serves multiple essential functions including voluntary body movement, postural maintenance, thermoregulation, and nutrient storage. The tissue is organized hierarchically, with individual muscle fibers wrapped by the endomysium, bundled into fascicles by the perimysium, and enclosed by the outermost epimysium. Within each fiber, the sarcolemma and sarcoplasm contain numerous myofibrils composed of repeating contractile units called sarcomeres. The sliding filament theory explains how muscle contraction occurs through the interaction of thin filaments containing actin, tropomyosin, and troponin, and thick filaments composed of myosin proteins. Muscle contraction is initiated when motor neurons release acetylcholine at the neuromuscular junction, triggering an action potential that propagates along the sarcolemma and through transverse tubules. This depolarization signals the sarcoplasmic reticulum to release calcium ions, which bind to troponin and expose binding sites on actin for myosin heads. The subsequent power stroke pulls thin filaments toward the sarcomere center, powered by ATP hydrolysis in a continuous contraction cycle. Tension production depends on the number of activated motor units and the optimal resting length of muscle fibers to maximize cross-bridge formation. Contraction types include isometric contractions, which generate tension without changing fiber length, and isotonic contractions, which may be concentric or eccentric depending on whether muscle shortens or lengthens. Energy metabolism shifts based on activity intensity, with aerobic metabolism in mitochondria supporting sustained activity and anaerobic glycolysis providing rapid but inefficient ATP during peak exertion. Lactate accumulation from anaerobic metabolism lowers pH and contributes to muscle fatigue, while recovery involves the Cori cycle and oxygen debt restoration. Muscle fibers exist in three genetically determined types: slow oxidative fibers with high mitochondrial content and fatigue resistance, fast glycolytic fibers producing powerful contractions but fatiguing quickly, and intermediate fibers with mixed characteristics. Chronic use causes hypertrophy through increased myofibril and mitochondrial development, while disuse leads to atrophy. Clinical conditions affecting skeletal muscle include rigor mortis from ATP depletion, inherited muscular dystrophy, tetanus from bacterial toxins blocking neurotransmitter release, polio from motor neuron destruction, and botulism from acetylcholine release inhibition.