Muscle contraction occurs through the sliding filament theory. When a nerve impulse reaches the neuromuscular junction, it triggers the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber. Calcium ions bind to troponin, causing a conformational change in tropomyosin, which exposes the binding sites on actin filaments. Myosin heads then attach to these binding sites, forming cross-bridges, and pull the actin filaments toward the center of the sarcomere, resulting in muscle contraction.
