when a neuron stimulates a muscle cell an action potential sweeps over the plasma membrane of the muscle cell the action potential releases internal stores of calcium that flow through the muscle cell and trigger a contraction muscle cells have an elaborate architecture that allows them to distribute calcium ions quickly throughout the cytool Deep tubular invaginations of the plasma membrane called T tubules crisscross the cell when the cell is stimulated a wave of depolarization that is an action potential spreads from the synapse over the plasma membrane and via the T tubules deep into the cell a voltage sensitive protein in these membranes opens a calcium release channel in the adjacent cytoplasmic reticulum which is the major calcium store in muscle cells thereby releasing a burst of calcium ions all throughout the cytool of the cell within a contracto bundle of a muscle cell called aof fibral the calcium interacts with protein filaments to trigger contraction in each Contracting unit or sarir thin actin and thick myosin filaments are just opposed but cannot interact in the absence of calcium this is because myosin binding sites on the actin filaments are all covered by a rod-shaped protein called tropomyosin a calcium sensitive complex called troponin is attached to the end of each tropomyosin molecule when calcium floods the cell troponin binds to it moving tropomyosin off the myosin binding sites opening the myosin binding site on the actin filaments allows the myosin Motors to crawl along the actin resulting in a contraction of the muscle fiber calcium is then quickly returned to the sarcoplasmic reticulum by the action of a calcium pump without calcium mein releases actin and the filaments slide back to their original positions e