Pacemaker Cells: A Case Study

Action potential, impulse conduction (italics), and cardiac cycle (bold)

Pacemaker cells in the SA node, found in the wall of right atrium, begin as polarized, with Na+ on the outside of the cell (sarcolemma) and the cell at its resting potential (-80 mV in the atria, -90 mV in the ventricles). The atria are full or filling with blood. The SA (and technically the AV) nodal cells gradually drift towards their threshold after each repolarization and depolarize (the prepotential: spontaneous depolarization). An impulse is sent (a property called automaticity or autorhythmicity) as a stimulus, causing the SA nodal cells to reach their threshold of -65 mV. 80-100 such action potentials are generated in the SA per minute (40-60 in the AV, but the SA reaches potential first - is faster- and thus sets the pace). This begins the rapid depolarization. The fast, voltage-gated Na+ channels open quickly and there is a massive influx of Na+ ions into the
K+ ions rush out of the cell, and the net result is rapid repolarization that restores the resting potential. ATRIAL DIASTOLE/ VENTRICULAR SYSTOLE. During atrial repolarization, the ventricles are depolarizing (seen as the QRS on the EKG) because the impulse has passed the AV node, travelled along the septum within the bundle of His, down the bundle branches and Purkinje fibers. The moderator band is a ‘shortcut’ so that the impulse stimulates the papillary muscles of the right ventricle (bracing the chordate tendinaea) before the ventricles contract to prevent backflow into the atria. The contraction of the ventricles closes the AV valves (producing the S1, or “lubb”,sound) and forces blood out past the semilunar valves and into the pulmonary circuit (from the right ventricle) via the pulmonary trunk, and into the systemic circuit (from the left ventricle) via the aorta, both ending back into the atria via the pulmonary veins and vena