Summary
Highlights
The heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. For blood to flow correctly, the heart must coordinate its contractions. This coordination is achieved through electrical signals, where cells depolarize (become more positively charged) to squeeze. The process begins in an area where cells can depolarize automatically.
The sinoatrial node (SA node) contains unique cells that depolarize by themselves, a property called automaticity. Once SA node cells depolarize, they send waves of depolarization (electrical signals) to neighboring muscle cells. These waves spread through the right atrium and are quickly transmitted to the left atrium via Bachmann's bundle, ensuring coordinated depolarization and contraction of both atria.
From the SA node, the electrical signal travels through internodal tracts to the atrioventricular node (AV node). The AV node is the primary electrical connection between the atria and the ventricles. A crucial function of the AV node is to create a small delay (approximately 0.1 seconds) in the signal transmission. This delay allows the atria to fully contract and move blood into the ventricles before the ventricles begin to contract, ensuring efficient blood flow.
After the delay at the AV node, the signal continues to the bundle of His (pronounced 'Hiss'). From there, it branches into the right bundle and the left bundle. The left bundle further splits into the left posterior fascicle and the left anterior fascicle. These specialized pathways, collectively called the electrical conduction system, transmit the signal rapidly to the Purkinje fibers, which distribute the signal throughout the ventricular muscle cells. This rapid transmission through the 'highways' of the conduction system and subsequent slower spread through muscle cells ('little roads') ensures that all ventricular muscle cells contract almost simultaneously for an effective pump.