The cell is impermeable to sodium unless depolarization occurs. Early Repolarization Fast sodium channels close. This influx allows the cell to maintain a positive charge over this time period. Myocardial pacemaker cells depolarize differently. Even at RMP, pacemaker cells are permeable to sodium.
Pacemaker cells have no fast sodium changes, just a steady influx of sodium.
Usually this occurs with firing of the SA node at 60-100 bpm. This should occur 0.12-0.20 seconds after atrial depolarization.