Molecules of tropomyosin situated in the grooves of the actin helix prevent myosin heads from binding actin. Also present are molecules of troponin. Binding both actin and tropomyosin, troponin may also bind calcium ions. In a normal cell, cytoplasmic calcium is kept very low by pumping the calcium into the extracellular space or into mitochondria. In skeletal-muscle cells, the sacroplasmic reticulum is present as a calcium reservoir. The sarcoplasmic reticulum is interwoven with another specialized feature, T-tubules, which conduct an action potential into and throughout the muscle cell after sufficient acetylcholine has crossed the neuromuscular junction, causing calcium release through out the cell. The calcium binds troponin which shifts tropomyosin away from the myosin binding sites on actin. When the myosin head binds to actin the interaction with actin promotes a major change in conformation (the power stroke). The power stroke is not accompanied by the hydrolysis of ATP. Hydrolysis of ATP 'cocks's the myosin head, priming it for the power stroke, before attachment to actin.