They can either be stimulated by a neuron, they can either be stimulated by a nearby muscle cell. So if a nearby muscle cell generates an action potential, that action potential can propagate two adjacent muscle cells as a result of the gap junctions found between cells. And the third reason is something called myogenic activity. So certain cardiac myosites can actually generate their own action potential. They can contract by themselves without actually any type of outside stimulus. So this is known as myogenic activity.