musculus

Musculus is the Latin term for a muscle, used in anatomical nomenclature to name individual muscles. The singular is musculus, the plural musculi. The English word muscle derives from this Latin root. The term is often accompanied by the root of a specific muscle, such as Musculus biceps brachii. The name musculus is historically linked to the idea that contracting muscles resemble a small mouse moving under the skin, a description reported in early anatomical writings.

In anatomy and physiology, muscles are tissues that generate force and movement. There are three major muscle tissue types: skeletal (striated and usually under voluntary control, attaching to bones or skin), smooth (non-striated and involuntary, found in walls of hollow organs and vessels), and cardiac (striated and involuntary, forming the heart). Skeletal muscles are organized into muscle fibers, each surrounded by endomysium; bundles of fibers form fascicles wrapped by perimysium; the entire muscle is enclosed by epimysium. A muscle fiber contains myofibrils, which consist of repeating sarcomeres—the contractile units containing actin and myosin filaments. Contraction occurs through the sliding filament mechanism, regulated by calcium ions and the actin–myosin cross-bridge cycle, initiated at the neuromuscular junction by motor neurons releasing neurotransmitters.

Functionally, muscles produce movement, maintain posture, and generate heat. They require energy in the form of ATP and rely on metabolic pathways that are aerobic or anaerobic, with oxygen delivery and blood supply closely linked to function. Developmentally, muscles arise from mesoderm, and repair involves satellite cells. Clinically, disorders of musculature include atrophy, hypertrophy, and myopathies, reflecting the essential role of musculi in movement and homeostasis.