Muscle

Muscle tissue is responsible for producing force and facilitating movement. There are three main types: skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle attaches to bones via tendons and is under voluntary control; smooth muscle forms the walls of hollow organs and blood vessels and operates involuntarily; cardiac muscle makes up the heart and contracts rhythmically without conscious input.

Muscle structure consists of elongated muscle fibers that contain myofibrils, which are organized into repeating units called sarcomeres. Sarcomeres house the contractile proteins actin and myosin. A connective tissue framework, blood vessels, and nerves organize each muscle. Skeletal muscles are innervated by motor neurons at neuromuscular junctions and receive substantial blood supply to meet metabolic needs.

Contraction occurs through the sliding filament mechanism: calcium ions enable actin and myosin interactions, triggering cross-bridge cycling that shortens sarcomeres. This process requires adenosine triphosphate (ATP) and is regulated by excitation-contraction coupling from nerve impulses to the muscle fibers.

Muscles generate energy via aerobic and anaerobic pathways. Oxidative metabolism in mitochondria sustains sustained activity, while rapid, high-intensity efforts rely on glycolysis and phosphocreatine. Heat production also contributes to body temperature during activity.

Function and adaptation include movement, posture maintenance, and thermogenesis. Muscles can grow through hypertrophy in response to training, while repair involves satellite cells. Aging is associated with reduced muscle mass and strength, a condition known as sarcopenia.

Clinical considerations include muscle strains and tears, dystrophies and myopathies, cramps, and disuse atrophy.