In the normal heart, electrical activity is initiated by the Sinoatrial node (SAN) also referred to as the natural “pacemaker” of the heart. The SAN generates Action Potentials (AP’s) about 60-100 times a minute. The AP’s then spread throughout the myocardium via gap junctions between adjacent cells spreading first across both atria.

They then reach the Atrioventricular node (AVN) located between the atria and the ventricles. However, conduction at the ventricles is delayed for about 0.1s to allow time for the atria to contract fully before the ventricular muscle is excited.

The electrical impulses then spread down the conducting tissue (Bundle of HIS) branches that supply the right and left ventricles.

Finally, the wave of excitation then spreads to an extensive network of large fibres known as the Purkinje fibres, located just beneath the endocardium. The Purkinje fibres then spread this excitation rapidly to the ventricular myocytes leading to ejection of blood from the heart. The blood from the right ventricle goes to the lungs, and the blood from the left ventricle goes to the body. 

Diagram showing the wave of excitation as it passes from the SAN to the Right ventricle. The Image was hand drawn.


 The electrical activity can be detected by doing an electrocardiogram (ECG)

Diagram showing an Electrocardiogram normal Tracing with time on the x-axis and membrane potential on the y-axis. The image was hand drawn.

If it all goes wrong...

In certain heart diseases, myocytes in other parts of the heart can show independent pacemaker activity leading to irregular heart beats known as arrhythmia or tachycardia. These can cause a disruption in the ability of cardiac myocytes to pump blood effectively to the body causing alterations in the normal blood pressure.

In the worst case scenario, this can lead to cardiac arrest and if not restored in about 3 minutes, the brain and heart become significantly damaged resulting in death.