The heart has its own electrical system. Its function is to synchronize the muscle contraction in an effective way.

The superior cavities of the heart (the atria), must initially contract to initiate the filling of the lower cavities (the ventricles). The ventricles then contract to push the blood from the bottom of the ventricles to into the large vessels located at their tops.

It takes an orchestra conductor

This electrical system is located at the top of the heart, at the level of the right atrium. It is a small cluster of cells called the sinoatrial node which is in fact the natural “pacemaker” of the heart. It initiates all the heartbeats and determines their rate.

It is said that the normal rate ranges between 60 and 100 beats per minute. It is however normal for the heart rate to vary during the day. The heart beats more slowly when one is at rest and accelerates during physical activity or a stressful situation.

Like a wave propagating in water

The electricity generated by this small cluster of cells (the sinoatrial node), is transmitted to the atria then to the ventricles.

This electric signal in the atria acts like a wave seen after a stone has been thrown in the water.  The electrical “wave” travels from the sinoatrial node towards the right and left atria.

A Customs officer between the atria and the ventricles

The transmission between the atria and the ventricles is only possible at a specific place. This “customs officer” is another small cluster of cells called the atrioventricular node. It controls the electrical signal so the ventricles can contract at an optimal rate.

The atrioventricular node plays a central role in some cardiac rhythm disturbances

Electric signal organized in the ventricles

Once it “passed the customs”, the electric signal is transmitted to the right and left ventricles through two specialized electric circuits, the Purkinje network.

The Purkinje Network

The Purkinje network is as a motorway which makes it possible for the electrical signal to pass promptly from the “customs officer” to the bottom of the ventricles at a very high speed, like the transmission lines of our power grid.

This organizes the contraction of the ventricles from the bottom to the top. Blood is thus driven out the ventricles to the pulmonary and aortic valves respectively.

Electrocardiogram

The electricity of the heart can be recorded by placing small electric sensors on the skin. This recording is called an electrocardiogram (ECG).

The ECG makes it possible to record the sequence of the various electrical events just described above, which one calls the “cardiac cycle”, beat by beat.

Any change to the normal electrical pathway of the heart will cause modifications to the ECG. An abnormal pathway, a heart attack, a thickening or a dilation of the heart will cause specific electrical anomalies.

The ECG is thus an easy, quick and painless test that can provide a lot of information.