The PR Segment resides between the end of the P Wave and the start of the Q or R wave.
The PR Segment represents depolarization of:
1. The AV Node
2. The top portion of the His Purkinje system.
The AV node contains specialized cells responsible for delaying the impulse traveling to the ventricles allowing the atriums time to contract and eject their contents into the ventricles before the ventricles contract.