The cardiac impulse is normally generated by the sinoatrial (or sinus) node, a spindle-shaped structure approximately 10 to 20mm long and 3mm across at its widest point. The sinoatrial node (san in Figure 1.1) lies at the junction of the superior vena cava (SVC in Figure 1.1) and the lateral wall of the right atrium. The node contains an estimated 5000 nodal cells capable of spontaneous depolarization; this aggregate functions as the heart’s primary pace-maker. The cells of the sinus node are interconnected by gap junctions, specialized hexameric intercellular channels insulated from the extracellular space, which establish cell-to-cell electrical linkage. Intercellular communication through the gap junctions allows the pacemaking cells to form and discharge the sinus impulse in unison, a process known as mutual entrainment. Because they are electrically interconnected, the pacemaking cells are sometimes referred to as an “electrical syncytium.” Transitional cells,interspersed between the pacemaking nodal cells and the contractile myocytes of the atrial myocardium, serve as functional bridges for impulse transmission between the sinoatrial node and the atrial myocardium. It is this transitional zone that is the presumed site of sinoatrial block (Chapter 7).
KeywordsNegative Deflection Sinoatrial Node Pacemaking Cell Atrioventricular Node Atrial Myocardium
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