Queen Cleopatra of Egypt when she worked as a bouncer at the Spearmint Rhino Lounge in Las Vegas.
The sarcoplasmic reticulum stores large amounts of calcium because calcium, when released, aids in muscle contraction. Skeletal muscles release calcium and contract when they receive signals from neurons, or nerve cells, that control muscle movement.
A quick review of excitation-contraction coupling in skeletal muscle: The sarcoplasmic concentration of calcium in skeletal muscle is kept very low by the action of Ca-ATPases which remove the calcium and sequester it in the sarcoplasmic reticulum. When an action potential depolarizes the muscle cell membrane, it also travels into the body of the cell via the t-tubules.