Final answer:
The pacemaker that connects the AV node with the bundle branches, located in the upper portion of the interventricular septum, is the atrioventricular bundle or bundle of His.
Step-by-step explanation:
The pacemaker in question, located in the upper portion of the interventricular septum and connecting the AV node with the bundle branches, is called the atrioventricular bundle or bundle of His. It arises from the AV node, transmits the impulse through the interventricular septum, and divides into the left and right atrioventricular bundle branches. The sinoatrial (SA) node, on the other hand, is the primary pacemaker of the heart, known for initiating the sinus rhythm, but it is not located in the interventricular septum.