Just like skeletal muscles, the heart is electrically stimulated to contract. This stimulation is also called ''activation'' or ''excitation''. Cardiac muscles are electrically charged at rest. The inside of the cell is negatively charged relative to the outside (resting potential). If the cardiac muscle cells are electrically stimulated they depolarize (the resting potential changes from negative to positive) and contract. The electrical activity of a single cell can be registered as the [[action potential]].
As the impulse spreads through the heart, the electric field changes continually in size and direction. The ECG is a graphical visualisation of these electric signals in the heart.
==The ECG represents the sum of the action potentials of millions of cardiomyocytes==