The Electrical Activity of the Heart

T heart beats in the absence of any nervous connections because the electrical (pacemaker) activity that generates the heartbeat resides within the cardiac muscle. After initiation, the electrical activity spreads throughout the heart, reaching every cardiac cell rapidly with the correct timing. This enables coordinated contraction of individual cells. The electrical activity of cardiac cells depends on the ionic gradients across their plasma membranes and changes in permeability to selected ions brought about by the opening and closing of cation channels. This chapter describes how these ionic gradients and changes in membrane permeability result in the electrical activity of individual cells and how this electrical activity is propagated throughout the heart.