The heart cells that create rhythmical impulses are called pacemaker cells, and they directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.
Although all of the heart's cells possess the ability to generate these electrical impulses (or action potentials), a specialised portion of the heart, called the sinoatrial node, is responsible for the whole heart's beat.
The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. They possess some contractile filaments, though they do not contract.
Cells in the SA node will naturally discharge (create action potentials) at about 70-80 times/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.
If the S.A node doesn't function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become the heart's pacemaker. These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.
The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.
Further down the electrical conducting system of the heart, the Bundle of His, the left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential if they aren't inhibited by other electrical activity. These tertiary pacemakers fire at a rate between 30-40 per minute.
Even individual cardiac muscle cells will contract rhythmically on their own.
The reason the SA node controls the whole heart is that its action potentials are released most often; this triggers other cells to generate their own action potentials. In the muscle cells, this will produce contraction. The action potential generated by the SA node, passes down the cardiac conduction system, and arrives before the other cells have had a chance to generate their own spontaneous action potential.
