The nerve impulses from the central nervous system cause the muscles in the human body to contract. When an impulse reaches the muscle fibers of a motor unit, it stimulates a reaction in each sarcomere that are located in between the myosin and the actin filaments.This reaction initiates a contraction. The head of the myosin filament reach forward and attach to the actin filament, pulling the actin towards the center of the sarcomere; this results in the shortening of the sarcomere. When the muscle is stimulated to contract by the nerve impulse, the calcium channel in the sarcoplasmic reticulum will be opened and calcium ions will be released into the sarcoplasm. Some of the released calcium will attach to troponin, this will cause a change in the muscle cells which move tropomyosin out of the way so that the cross bridge can link up and produce muscle contraction using ATP as energy source.
