Final answer:
Both the AV node and Purkinje fibers can act as secondary pacemakers for the heart, with the Purkinje fibers having the slowest intrinsic firing rate. If the primary SA node fails, these structures can take over to maintain a heart rhythm, albeit at a slower rate.
Step-by-step explanation:
The secondary pacemakers of the heart are components of the heart's conduction system that can take over the role of rhythm generation if the primary pacemaker, the sinoatrial (SA) node, fails. Both the AV node and Purkinje fibers can serve as secondary pacemakers due to their inherent ability to generate electrical impulses, though at a slower rate than the SA node. The AV node has an intrinsic firing rate that is lower than the SA node but higher than the Purkinje fibers, which have the slowest rate of firing of the components listed. If the signal is blocked at the AV node, the ventricles would still be able to contract due to the excitation provided by the backup pacemaker function of the Purkinje fibers.