Final answer:
The Atrioventricular (AV) Node is a clump of specialized myocardial conductive cells in the lower portion of the right atrium, crucial for delaying the cardiac impulse for proper heart function.
Step-by-step explanation:
The bundle of specialized cells located in the lower portion of the right atrium is known as the Atrioventricular (AV) Node. This node is critical in the cardiac conduction system as it receives the impulse from the Sinoatrial (SA) Node, which is the primary pacemaker of the heart. The AV node briefly delays the impulse, allowing time for the atria to contract and the ventricles to fill with blood before it sends the signal on to the atrioventricular bundle, or Bundle of His, and then to the ventricles.