Final answer:
The Purkinje fibers do not have any pacemaker cells and are involved in spreading the electrical signal throughout the ventricles, causing them to contract. The SA node is the natural pacemaker of the heart, and the AV node provides a delay in the electrical signal between atrial and ventricular contraction.
Step-by-step explanation:
The Purkinje fibers do not have any pacemaker cells. The sinoatrial (SA) node is known as the heart's natural pacemaker, which initiates the electrical impulse causing the heart to beat. The atrioventricular (AV) node serves as a gate that slows the electrical signal before it enters the ventricles. This provides a necessary delay to ensure that the atria contract fully before the ventricles contract. The Bundle of His and its branches conduct the impulses to the Purkinje fibers, which then spread throughout the ventricles causing them to contract. Thus, the correct answer to the question is d. Purkinje.