Spread of Excitation from the Sinus Node

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Abstract
Microelectrodes were employed to determine the mode of propagation of excitation from the sinus node in the rabbit heart. The excitation starts from the sinus node radially, at first, but then proceeds to the crista terminalis, often obliquely. This is because some special tissue having a slow conduction velocity lies behind the sinus node, and also because there is a zone of relatively faster conduction at the basal wall of the superior vena cava. Once the excitation wave enters the crista terminalis, it travels rapidly in two opposite directions through two branches of the ring-like structure which is formed by the crista terminalis and its extension. The excitation wave traveling through the branch encircling the inferior vena cava reached the atrioventricular node earlier in most instances. In the caval region inside the ring-like structure there is a zone of relatively faster conduction. The conduction to the atrioventricular node through this route is latent in the normal state, because conduction through the ring-like structure takes place beforehand. In the caval region near the sinus node, the conduction velocity of the excitation wave from the spontaneously beating sinus node was greater than that from the electrically driven atrium. The spontaneous action potential showed also a steeper rise than that produced by electrical stimulation. The mechanism responsible for this is discussed.