The initiation of the heart beat

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