Final answer:
The main pacemaker of the heart that fires at a rate of 60-100 bpm is the sinoatrial (SA) node, which is responsible for initiating the sinus rhythm. It is situated in the upper portion of the right atrium, near the superior vena cava. AV node, another crucial conductive cell cluster, creates a delay to allow the atria to contract before the ventricles.
Step-by-step explanation:
The node located in the upper portion of the right atrium that is the main pacemaker and fires at a rate of 60-100 bpm is the sinoatrial (SA) node. The SA node establishes the normal cardiac rhythm and has the highest inherent rate of depolarization. It is a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium, near the orifice of the superior vena cava. The SA node initiates the sinus rhythm, which is the normal electrical pattern followed by contraction of the heart.
In contrast, the atrioventricular (AV) node is a second clump of specialized myocardial conductive cells, located in the inferior portion of the right atrium. The AV node has a critical role in creating a pause before depolarization and transmission of the impulse to the ventricles, a process that allows the atria to complete their contraction before the ventricles contract. This pause ensures sequential contraction and efficient blood pumping throughout the heart.