The electrical activity of cells in the heart can be understood if you can envisage the movements of charged ions across the cell membrane. The important ions are K+, Na+ and Ca2+. Ion pumps push K+ into the cell, and Na+ and Ca2+ out of the cell. This sets up concentration gradients so that K+ tries to flow out, and Na+ and Ca2+ try to flow in. However, none of these ions can cross the phospholipid membrane. Instead, they have to wait until a specific set of ion channels in the membrane open. We will use this background knowledge to explain the basis of the action potential in cardiac myocytes.