Final answer:
The D-papillary muscles are not a population of pacemaker cells in the heart; the SA node, AV node, and Purkinje fibers are involved in the heart's electrical conduction that regulates the heartbeat,the correct option is D).
Step-by-step explanation:
The pacemaker cells of the heart are specialized cardiac cells that regulate the heart's rhythm. The correct options for pacemaker cells mentioned in the question include the atrioventricular (AV) node, Purkinje fiber system, and the sinoatrial (SA) node. Among the choices provided, the D-papillary muscles are NOT a population of pacemaker cells but instead are muscles that attach to the valves in the heart and contract to prevent the inversion or prolapse of these valves during ventricular contraction.
It is essential to understand that the SA node initiates the action potential, which sweeps across the atria and then reaches the AV node where there is a delay that allows the atria to complete pumping blood before the impulse is transmitted to the ventricles via the Purkinje fiber system. This sequence ensures a coordinated cardiac cycle for effective blood pumping. If the AV node had the slowest rate of firing, the atria would contract, but the ventricles would not because the AV node passes the signal to the Purkinje fibers, which allow the ventricles to contract.