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.
Cardiac-muscle contraction is actin-regulated, meaning that the calcium ions come both from the sarcoplasmic reticulum (as in skeletal muscle) and from outside the cell (as in smooth muscle). Otherwise, the chain of events that occurs in cardiac-muscle contraction is similar to that of skeletal muscle.