In this and the previous sections the sinus node was roughly described as a cluster of cells with the special property to depolarize spontaneously in diastole, eventually to the threshold of the next action potential. The cells in the cluster are electrically coupled. Not much has been said about the passive electrical properties of the cells and of the electrical coupling. The length constant seems to be less than 1 mm. The coupling enforces a certain synchrony in the activity of the cells. In microelectrode studies, therefore, one has to keep in mind that an electrode in one cell “sees” events occurring up to one mm away. Yet it is known that the cells in the sinus node do not have all the same ability to “spontaneously” depolarize after an action potential. There is a center normally in the head of the sinus node, the pacemaker, and there are other areas in the sinus node which show diastolic depolarization at a lesser rate and which are excited by propagation. Under certain conditions the pacemaker can shift from one area to another, as shown-in this section-by mapping the spread of excitation.
KeywordsPermeability Sucrose Norepinephrine Epinephrine Catecholamine
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