Inward flow of Na+  in heart leads to:
Phase 0 (depolarization), fast sodium channels open. When the cardiac cell is stimulated and depolarizes, the membrane potential becomes more positive. Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it. The membrane potential reaches about +20 millivolts before the sodium channels close.
Key Concept:
Inﬂux of Na+ via fast Na channel leads of phase 0 or beginning of AP in myocardium.
Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no111
Plateau phase
Action potential
Repolarization
No change