The impulse for the activation of the heart is generated normally in the sinoatrial node (SA node). This structure covers an area of only a few square millimeters. The cells have an irregular contour and are small (diameter less than 10 μ). Furthermore, they contain sparse myofilaments which are often not well organized. The intercellular junctions are different from normal myocardium since there are no intercalated discs, no real nexus, and only rare membrane junctions. The nodal cells often form clusters divided by connective tissue. One of the characteristic features of the SA nodal structure is the abundant innervation by autonomic fibers [15]. Since West [28] made the first impalement with a microelectrode in a fiber of a mammalian SA node, many researchers have studied the electrophysiologic characteristics of the node. Most of these studies were done with the rabbit heart. Although only little information about the human SA node is available, it seems reasonable to assume that there is no principal difference between man and rabbit in respect to the electrophysiology of the SA node.