The bundle branches are a part of the electrical system of the heart.
The electrical system controls the heartbeat and is made up of several parts that tell the ventricular muscle when to contract. The SA node starts the heartbeat, causing the atria, or upper chambers of the heart, to contract. The signal then travels through the AV node and the bundle of His and splits into the left and right bundle branches, directing the impulse to the respective left and right ventricles. The signals end at the Purkinje fibers causing the ventricles, the lower chambers of the heart, to contract. The usual flow of electrical signals produces a normal heartbeat.
If either the right or left bundle branch fail to conduct the impulse properly or cause a delay in the signal, it is called a “bundle branch block”.