Final answer:
The main pacemaker of the heart is the sinoatrial (SA) node, which establishes the normal cardiac rhythm by having the highest rate of depolarization. Compared to other components of the conduction system, the Purkinje fibers have the slowest rate of firing.
Step-by-step explanation:
The main pacemaker of the heart is the sinoatrial (SA) node. The SA node is a specialized group of cells located in the right atrium and is responsible for initiating the cardiac cycle by generating an electrical impulse. This impulse then spreads through the heart, causing the muscles to contract and thus pump blood. The SA node has the fastest inherent rate of depolarization among the components of the heart's conduction system and sets the pace of the heartbeat.
When considering the firing rates of different components of the heart's conduction system, the Purkinje fibers have the slowest rate of spontaneous depolarization. Therefore, if the higher centers such as the SA or AV nodes were not functioning, the Purkinje fibers would be the last resort for initiating heartbeats, albeit at a slower rate which could lead to clinical conditions needing medical attention.