Many cardiac muscle
cells can contract spontaneously, but the
cells in the
Pacemaker region are, during fetal development, the fastest-beating ones, and the fastest ones normally dominate the whole show.
However, many of us have independent-minded cardiac cells that can trigger a contraction (a PVC) if they get the opportunity, and opportunity arises when the Pacemaker's rate slows down--as happens when you're cooling off after exercise, or when you're getting sleepy at bedtime.