I spoke of automaticity earlier and mentioned that all myocardial cells possess this wonderful property.
Sodium ions enter the cell and begin the depolarization.
Calcium ions follow and extend the depolarization even further. Once Calcium stops moving inward, Potassium ions move in and repolarization begins. In a nutshell, sodium starts the cells' stimulation. Calcium extends the stimulation thereby allowing the entire muscle to contract before Potassium finally comes along and tells the muscle to relax for a moment and prepare for the next cycle. The important part of this cycle is the period where the cells reset and prepare for the next wave.