The initiation of the heart beat

In the mammalian heart, electrical excitation arises from the specialized cells of the primary pacemaker region, the sino-atrial node, situated in the right atrium between the superior and inferior vena cavae. The rhythmic initiation of this electrical activity and its propagation throughout the heart are essential for coordinated cardiac contraction. During diastole, the membrane potential of sino-atrial nodal cells characteristically slowly depolarizes before reaching a threshold level and firing off the next action potential. There are at least nine different membrane currents underlying this phase of diastolic depolarization. The complex interplay between these inward and outward currents both provide exquisite control over heart rate and built-in redundancy, making the sinto-atrial (SA) node extremely hard to stop. This article briefly reviews the different currents underlying the function of the SA node and how these currents may be modulated by both endogenous and pharmacological agents.