The initial impulse in a heart is usually formed in the Sinoatrial (SA) node and carried through the atria, down the internodal atrial pathways, and to the Atrioventricular (AV) node. In normal conduction, the impulse would travel across the “bundle of His” (AV bundle), down the bundle branches, and into the Purkinje fibers. This would depolarize the ventricles and cause them to contract.

In an SA block, the electrical impulse is delayed or blocked on the way to the atria, thus delaying the atrial beat.