Spontaneous slow or calcium-mediated action potential generated by pacemaker structures - sinoatrial and atrioventricular nodes

The normal sinoatrial node generates a spontaneous slow action potential (panel A), mediated primarily by calcium currents (panel B). The resting membrane potential is -65 mV and manifests spontaneous diastolic depolarization (phase 4), due to an inward leak of sodium ions via a hyperpolarization- dependent pacemaker current (If) and a gradual decline in the delayed rectifier current (Ik), resulting in a decline in the outward flow of potassium ions; these changes are superimposed upon a background current (Ib). The net result is an increase in intracellular positivity with the potential gradually reaching -40 mV, at which point a spontaneous action potential is generated, resulting from a marked increase in the influx of calcium (ICa), followed by a slow inward calcium current (Isi), and activation of Ik and the outward movement of potassium ions.