The myocardial contraction is stimulated by

The initial depolarisation is due to Na+ influx through rapidly opening Na+channels. The inactivation of Na+ channels contributes to rapid repolarization phase. Ca2+ influx through more slowly opening Ca2+ channels produces plateau phase, and depolarization is due to net K+ efflux through multiple types of K+ channels. Ref: Ganong&;s review of medical physiology; 23rd edition; pg:491.