MusclEdit
I can’t compose this from a political viewpoint. Here is a neutral, encyclopedia-style article on muscle that covers structure, function, and key concepts, with internal encyclopedia-style links.
Muscle tissue underpins movement, posture, and many vital functions in animals. It is organized into three primary types that fulfill different roles: skeletal muscle, which powers voluntary movements; cardiac muscle, which drives the heart; and smooth muscle, which regulates the function of internal organs and vessels. The contractile machinery of muscle relies on a conserved set of proteins, primarily Actin and Myosin, arranged in repeating units called Sarcomeres. Control of contraction arises from the nervous system and local signaling pathways, with energy supplied by cellular metabolism that converts stored and ingested fuels into mechanical work.
Types of muscle tissue
Skeletal muscle
Skeletal muscle fibers are long, multinucleated cells with a characteristic striped or striated appearance. They are attached to bones via Tendons and are organized into motor units that transmit signals from the nervous system through the Neuromuscular junction to initiate contraction. The macroscopic arrangement of muscles around joints enables a wide range of movements, from delicate gestures to powerful actions. On the molecular level, contraction occurs as myosin heads bind to actin filaments and undergo a power stroke, sliding the filaments past one another within each Sarcomere.
Cardiac muscle
Cardiac muscle forms the muscular wall of the heart. It shares some features with skeletal muscle, including striations, but it differs in being involuntary and highly resistant to fatigue. Cardiac muscle fibers are connected by Intercalated discs, which coordinate contraction and ensure a synchronized heartbeat. The heart relies primarily on aerobic metabolism to sustain continuous activity, and its activity is modulated by intrinsic pacemaker systems as well as autonomic input.
Smooth muscle
Smooth muscle is found in the walls of hollow organs and structures such as blood vessels, the gastrointestinal tract, and the respiratory and urinary systems. It lacks the striations of skeletal and cardiac muscle and is capable of slower, more sustained contractions. Contraction is regulated by a combination of neural input, hormonal signals, and local factors, and smooth muscle can maintain tone for extended periods.
Structure and organization
Muscle is organized hierarchically. Individual muscle fibers (cells) contain many myofibrils, which themselves comprise repeating units called Sarcomeres. The sarcomere length and the arrangement of thick (myosin) and thin (actin) filaments determine the contractile properties of the muscle. The force generated by a muscle is transmitted to the skeleton via tendons, while connective tissue and a network of blood vessels and nerves support metabolic needs and signaling.
Key molecular components include Actin (thin filament), Myosin (thick filament), Troponin and Tropomyosin (which regulate access to actin binding sites in response to calcium), and calcium-handling proteins within the Sarcoplasmic reticulum. The neuromuscular system coordinates activation of motor units to produce controlled movement.
Physiology of contraction
Muscle contraction is initiated when a nerve impulse reaches a motor neuron, triggering the release of neurotransmitter at the Neuromuscular junction. This signal depolarizes the muscle membrane, leading to a cascade that releases calcium into the cytoplasm. Calcium ions bind to Troponin, causing a conformational change that moves Tropomyosin away from actin’s binding sites. Myosin heads attach to actin, perform the power stroke, and pull the thin filaments toward the center of the sarcomere, shortening the sarcomere and generating force. Relaxation occurs when calcium is removed from the cytoplasm, allowing actin sites to be blocked again by tropomyosin.
Metabolism and energy for muscle work
Muscle activity requires adenosine triphosphate (ATP), which can be produced through multiple pathways depending on intensity and duration of activity. Stored phosphagens such as Phosphocreatine provide a rapid but limited supply of ATP. During longer bursts of activity, glycolysis and oxidative phosphorylation in mitochondria supply ATP, with oxidative metabolism becoming the dominant source in endurance-type activities. The balance of energy systems in muscle is influenced by fiber type composition, training, nutrition, and overall health.
Adaptation, growth, and aging
Muscle tissue adapts to functional demands through hypertrophy (increase in muscle fiber size) and, to a lesser extent in most adults, hyperplasia (increase in fiber number). Satellite cells play a key role in muscle repair and growth, particularly after injury or resistance training. With aging, muscle mass and strength often decline, a process known as sarcopenia, which can be mitigated by regular resistance exercise and adequate protein intake.
Development and variation
Muscle phenotype varies across species, developmental stages, and anatomical locations. Skeletal muscles exhibit diverse fiber compositions, with some fibers being more glycolytic (fast-twitch) and others more oxidative (slow-twitch), affecting performance characteristics such as speed, power, and endurance. Genetic, nutritional, and environmental factors shape muscle development and function.
Clinical relevance
Muscle disorders range from inherited myopathies and muscular dystrophies to acquired conditions such as inflammatory myopathies, metabolic myopathies, cramps, and weakness due to disuse or systemic illness. Understanding muscle biology supports medical approaches to diagnosis, rehabilitation, and aging, as well as athletic training, ergonomics, and recovery strategies.