From which two areas of the heart do electrical signals originate to create cardiac contractions?

The sinoatrial (SA) node and the atrioventricular (AV) node are crucial components in the heart's electrical conduction system that generate and regulate electrical signals for cardiac contractions. The SA node, located in the right atrium, serves as the primary pacemaker of the heart, initiating electrical impulses that spread through the atria, causing them to contract and push blood into the ventricles.

Following this, the AV node, situated between the atria and ventricles, receives the impulses from the SA node and briefly delays them before sending them through the bundle of His and into the ventricles. This ensures that the atria have enough time to fully contract before the ventricles contract, creating a coordinated heartbeat.

The other choices do not play a direct role in generating electrical signals for contractions. Ventricular nodes are not recognized entities in the conduction system, the coronary arteries and veins are involved in blood supply rather than electrical signaling, and while the right and left atria are important anatomical structures, they do not independently initiate electrical signals; this responsibility lies with the SA and AV nodes.