Ion channels are small pores in cell membranes. They open in response to changes in membrane potentials or stimulation by bio-active substances. They selectively pass specific ions to produce electrical signals. In the heart, there are many types of channels working in concert. Diversity of action potentials within the heart comes from different distribution of ion channels in different tissues. Na channels convey inward current to initiate action potentials. Inward current through Ca channels produces the plateau and contributes to automatic activities in some situations, including membrane depolarization. Many types of K channels are now identified in cardiac tissues, including those activated only under pathophysiological conditions. K channels contribute to membrane repolarization and the maintenance of resting potentials.