In the normal heart, the atrioventricular node (AVN) is part of the sole pathway between the atria and ventricles, and is responsible for the appropriate atrial-ventricular delay. Under normal physiological conditions, the AVN controls appropriate frequency-dependent delay of contractions. The AVN also plays an important role in pathology: it protects ventricles during atrial tachyarrhythmia, and during sinoatrial node failure the atrioventricular (AV) junction assumes the role of pacemaker. Finally, the AV junction provides an anatomic substrate for AV nodal reentrant tachycardia, which is the most prevalent supraventricular tachycardia in humans. Using fluorescent imaging with voltage-sensitive dye and immunohistochemistry, we have investigated the structure-function relationship of the atrioventricular (AV) junction during normal conduction, reentry, and junctional rhythm. We identified the site of origin of junctional rhythm at the posterior extension of the AV node (AVN) in 78% (n=23) of the studied hearts and we found that this pacemaker is sensitive to autonomic control. For instance, when the autonomic nervous system was activated using subthreshold stimulation, a transient accelerated junctional rhythm was observed when subthreshold stimulation was terminated. A very similar phenomenon is observed clinically during slow pathway ablations treating AV nodal reentrant tachycardia (AVNRT). The autonomic control of the AV junction was investigated using immunohistochemistry, showing that the AV junction of the rabbit is very densely innervated with both cholinergic and adrenergic neurons. The posterior AV nodal extension was similar to the compact AVN as determined by morphologic and molecular investigations. In particular, both the posterior extension and the compact node express the pacemaking channel HCN4 (responsible for the IF current) and neurofilament 160. In the rabbit heart, AV junction conduction, reentrant arrhythmia, and spontaneous rhythm are governed by heterogeneity of expression of several isoforms of gap junctions and ion channels, and these properties are regulated by the autonomic nervous system. Uniform neurofilament expression suggests that AV nodal posterior extensions are an integral part of the cardiac pacemaking and conduction system.