pacemakerpotential

The pacemaker potential, also called diastolic depolarization, is the gradual, spontaneous depolarization of certain cardiac cells that brings the membrane potential to the threshold for initiating an action potential. It is characteristic of sinoatrial (SA) node cells and other pacemaker tissues (atrioventricular node, Purkinje fibers) and is responsible for the rhythmic, automatic firing that sets heart rate.

Physiologically, the pacemaker potential occurs during the late repolarization phase and is driven by a coordinated change in ionic currents. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels carry the "funny" current (If), permitting inward Na+ (and some K+) flow that begins depolarization. T-type and then L-type calcium channels open as the membrane approaches threshold, producing the action potential upstroke, while reduced outward potassium currents aid the depolarizing slope.

Autonomic modulation alters the slope of the pacemaker potential and thus heart rate. Sympathetic stimulation increases cAMP and enhances If and calcium currents, steepening depolarization and raising rate; parasympathetic stimulation activates muscarinic receptors and GIRK channels, increasing K+ efflux and slowing depolarization. Pharmacologic agents such as ivabradine selectively inhibit If to reduce heart rate.

Alterations in pacemaker potentials can cause bradycardia or tachyarrhythmias and are central to understanding sinoatrial dysfunction and certain therapeutic interventions. The concept also applies broadly to other biological pacemakers in development and specialized tissues.