Your heart's conduction system carries electrical signals from the atrium through a gateway called the AV node (atrioventricular node). The AV node controls how signals pass from the upper chambers of your heart (atria) to the lower chambers (ventricles). From the AV node, the signals pass through several fiber bundles (bundle branches) before they spread out through your ventricles. A heart block can occur in any part of the conduction system.