Final answer:
The PR interval on an ECG reflects the time span for the electrical impulse to travel from the onset of atrial depolarization to the onset of ventricular depolarization, which includes the spread through the AV node, bundle of His, and Purkinje fibers.
Step-by-step explanation:
On the ECG, the time necessary for the spread of an electrical impulse through the AV node, bundle of His, right and left bundle branches, and the Purkinje fibers is reflected by the PR interval.
The PR interval encompasses the onset of atrial depolarization to the onset of ventricular depolarization. When the sinoatrial (SA) node initiates the action potential, it first spreads across the atria and reaches the atrioventricular node, where there is a crucial delay of approximately 100 ms. This pause, which is an essential aspect of the heart's electrical rhythm, allows the atria to finish contracting and effectively pump blood into the ventricles before the ventricular contraction begins.
After the delay at the AV node, the electrical impulse travels rapidly through the atrioventricular bundle and its branches to the Purkinje fibers, which then disseminate the impulse to the ventricular myocardium. This part of the conduction system ensures synchronized ventricular depolarization and subsequent contraction, as portrayed in the large QRS complex on the ECG.