During muscle contraction the myofilaments myosin and slide toward each other and . This shortens the and the entire muscle. Muscle cells are "shocked" by nerve impulses from motor . The point of attachment of the nerve to the muscle is called a junction. A motor neuron and its muscle cells are referred to as a unit. The nerve impulse is carried from the neuron across the gap to the membrane () of the muscle cell by a chemical called . After the impulse is passed an enzyme called "de-activates" acetylcholine, readying the muscle for the next nerve impulse. Stimulation of the muscle cell causes to be released into the cell. This binds with the actin filaments causing them to expose active sites to the cross bridges. The cross bridges bind to the active sites, forming a new molecular structure which causes the cross bridge to toward the center, the actin filament with it. Energy from is used to break the , the cross bridge, and allow the cross bridge to form a new bond with another further down the actin filament. This cycle continues until the muscle is complete. Then ATP is used to cause active to move the calcium ions out of the muscle fiber causing of the muscle.