7.1.3 (4)

The spontaneous diastolic depolarization that characterizes pacemaker cells requires net inward current flow during phase 4 of the action potential. This net inward current is thought to have three components:

1. The gradual decay of the potassium outward currents (Ik-in blue) responsible for repolarization (phases 2 and 3 of the action potential).
2. The “turn-on” or activation of an unusual inward current labeled the “funny current” or If (shown in yellow). It is unusual because it is activated by hyperpolarization rather than depolarization and is believed to be carried primarily by sodium ions.
3. The “turn-on” or activation of the L-Type calcium current (Ica-in red) towards the end of diastole. This current then continues to flow, generating the upstroke of the action potential.