The fundamental process of muscle contraction is driven by the interaction between actin and myosin, two types of protein filaments. The sliding filament theory describes how these filaments slide past each other to shorten the muscle fiber:
Excitation: A nerve impulse triggers the release of calcium ions from the sarcoplasmic reticulum. Coupling: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin. Contraction: Myosin heads attach to actin to form cross-bridges and pull the actin filaments inward, powered by the hydrolysis of ATP. Relaxation: Calcium ions are pumped back into the sarcoplasmic reticulum, and the muscle fiber returns to its resting state.
