Cardiac-muscle cells are striated, and are a lot like skeletal-muscle cells except that in cardiac muscle, the fibers are interconnected. The sarcoplasmic reticulum of cardiac-muscle cells is not as well-developed as that of skeletal-muscle cells. 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.