Karel Frederik Wenckebach "second degree AV block" and later named the "Wenckebach phenomenon" and reclassified as Mobitz type I block in Mobitz's 1924 paper. A similar phenomenon can also occur in the Sinoatrial node where it gives rise to Type I second degree SA block, and this is also known as a Wenckebach block; the two have distinct features on an ECG however. Wenckebach is credited for describing the median bundle of the heart's conductive system that connects the sinoatrial node to the atrioventricular node. This bundle was named Wenckebach's bundle, and is also known as the middle internodal tract. Wenckebach's bundle is
Sinoatrial node proteins called connexins. There are fewer gap junctions within the SA node and they are smaller in size. This is again important in insulating the SA node from the surrounding atrial cells. The sinoatrial node receives its blood supply from the sinoatrial nodal artery. This blood supply, however, can differ hugely between individuals. For example, in most humans this is a single artery, although in some cases there have been either 2 or 3 sinoatrial node arteries supplying the SA node. Also, the SA node artery mainly originates as a branch of the right coronary artery; however in some individuals