Electrophysiological evidence for specialized fiber types in rabbit atrium

Intracellular microelectrodes have been used to study the site of origin and direction of spread of activity in the rabbit atrium. In this study a number of fiber groups have been found which possess specialized electrophysiological characteristics and a consistent anatomical localization. A pacemaker potentiality has been found only in tissues derived from embryologically distinct structures such as the sinus venosus, the venosus valves and the lower segment of the auricular canal. Activity normally spreads slowly from the S-A node and excites the crista terminalis along a broad front. Spread is then rapid through the crista, the pectinate muscles and the fibers of the atrial roof. The septum is normally excited from the crista terminalis. Excitation reaches the A-V node almost simultaneously from the crista terminalis and the right segment of the S-A ring bundle. This latter shows many characteristics of specialized conducting tissue. Around the A-V ring conduction velocity slows markedly in fibers which have many of the electrophysiological characteristics of A-V nodal fibers.