Conduction pathway

The heart possesses automaticity or autorhythmicity, meaning that it can depolarize on its own without external stimulation. Thus, the myocardium can essentially contract spontaneously, through a very intricate pathway, and allow the heart to pump blood through each of its four chambers and into the rest of the body.

1. Conduction begins at the sinoatrial (SA) node, which is also known as the heart’s pacemaker: it generates the initial action potential (AP) which induces the rest of the conducting system to stimulate heart contractile cells, establishing the heartrate.
   • The SA node is located in the right atrial wall.
2. The initial AP is then propagated through the internodal pathway and Bachmann’s bundles which are located in the right atrium, and across the interatrial septum, respectively. The internodal pathway is responsible for distributing the stimuli to right atrial cells, and Bachmann’s bundles allows the SA-node-generated AP to depolarize the left atrium as well.
3. The APs then converge at the atrioventricular (AV) node.
   • The AV node is located at the junction between the atria and the ventricles.
   • Importantly, there is a delay in the impulse before it reaches the AV node (~0.1 seconds), which allows the atria to fully contract before the ventricles do. This is due to decreased numbers of gap junctions and smaller diameter fibers which lower conduction speed.
4. Then it travels down the Bundle of His, or the AV bundle.
5. The AP travels down the interventricular septum to the right and left bundle branches, allowing it depolarize both the right and left ventricles.
6. Finally, the AP reaches the myocardium of the ventricles via the Purkinje fibers.