1 Electrical activity in single cells
An electrical perturbation generated by the sino-atrial node (green) generates an electrical response in the atrial (blue) and ventricular (red) muscles, known as the action potential. The perturbation alters the permeability of the cell membrane to different ions. Sodium ions are first allowed to flow into the cell, rapidly increasing the membrane potential by more than 100 mV. The membrane then allows potassium ions to flow out of the cell, reducing the membrane potential to its normal value and restoring the cell to its resting state.