Final answer:
A junctional escape beat originates from the atrioventricular (AV) node, which serves as a secondary pacemaker when the sinoatrial (SA) node fails to generate an impulse.
Step-by-step explanation:
The origin of a junctional escape beat is the atrioventricular (AV) node. A junctional escape beat occurs when the heart's primary pacemaker, the sinoatrial (SA) node, fails to initiate a beat, or the impulse fails to reach the AV node. In response, the AV node can act as a secondary pacemaker and take over to maintain a rhythm necessary to sustain circulation. The AV node transmits the impulse to the atrioventricular bundle, initiating a contraction that is typically slower than a normal sinus rhythm initiated by the SA node.
Normal cardiac rhythm is established by the SA node, and when the SA node does not function properly, the AV node is ready to generate an impulse. The AV node is a critical part of the cardiac conduction system because it ensures that there is a pause (approximately 100 ms) between atrial and ventricular contractions, allowing the ventricles to fill completely with blood from the atria before they contract.