heart beats over 10,000 times a day. To maintain such a systematic, routine,
day to day, rhythm the heart needs electrical impulses to control it's pace.
These impulses are provided by two electrical nodes: the sinoatrial
(SA) node and the atrioventricular(AV)
node. These nodes are responsible for the contraction of the atria (S-A node)
and ventricles (A-V node) and are the natural pacemakers
of the heart.
For more on this check out Understanding Action Potential
To see an animation of the cardiac cycle with explanations of the electrical activity of each phase click here.
There is no electrical activity during this phase; as a result all heart muscle relaxes, allowing blood to flow freely into the heart.
During this phase the sinoatrial node fires an electrical impulse which spreads throughout the right and left atria, causing the muscle to contract and push blood into the ventricles. Towards the end of the atrial systole the electrical impulse reaches and triggers the atrioventricular node.
The atrioventricular node (after a short delay) fires an impulse which spreads throughout the thick ventricular muscle causing it to forcibly contract and push blood through the lungs and body.