A 72-year-old woman from Mumbai is admitted with acute inferior wall myocardial infarction. Coronary angiography reveals complete occlusion of the right coronary artery. On the cardiac monitor, she develops bradycardia (HR 35 bpm) with AV dissociation and a junctional escape rhythm (narrow QRS complexes at 40 bpm). The bradycardia is refractory to atropine. Which anatomical feature of the blood supply to the cardiac conduction system explains the failure of atropine to restore AV conduction?