Calcium is stored in the terminal cisternae in the sarcoplasmic reticulum of the muscle cell. Calcium ions are stored there until muscle contraction occurs. During muscle contraction, calcium ions are released in order to initiate the contraction.
In skeletal muscle, there are two terminal cisternae located with each T tubule in muscle cells. The T tubule plus the two terminal cisternae make up a physiological structure known as the triad. Calcium ions are released from the triad in order to initiate contraction and keep muscle contraction smooth and regular. Once the calcium ions are released, an action potential will depolarize the cell membrane and propagate throughout the muscle fibers. The action potential leads to muscle contraction.