Sinus Node Function

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Abstract
The mechanism responsible for spontaneous cardiac rhythmicity has been of interest to physicians since the time of Hippocrates.1From the 17th to the 18th century, a neurogenic theory was attributed to Willis.1However, a different explanation was proposed in 1754 by Haller; he thought that the heart beat originated from intrinsic irritability of myocardial cells.2This concept was further supported by the observation that embryonic myocardial cells beat rhythmically considerably before innervation.3 The presence of a specialized cardiac area responsible for the intrinsic rhythmicity of the mammalian heart was not identified until the late 19th and early 20th centuries. At that time, the pacemaker site was identified as an area between the superior and inferior vena caval insertions into the right atrium. Experiments were performed to confirm this area as the "pacemaker" of the heart by studying (1) the shape of the P wave, (2) the