Phase 3 repolarization in pacemaker cells is due to

Phase 3 repolarization in pacemaker cells is due to

• A. Sodium influx
• B. L-type calcium current
• C. High K+ outward currents, low Ca++ currents
• D. Chloride influx

Answer: (C)

Phase 3 repolarization in pacemaker cells is driven by outward potassium currents while inward calcium currents diminish. As repolarization begins, voltage-gated K+ channels open and K+ leaves the cell, pulling the membrane potential back toward the resting level. At the same time, L-type calcium channels close/inactivate, reducing Ca2+ entry that would oppose repolarization. In pacemaker cells, the upstroke is actually produced by calcium influx, not sodium, so the key to phase 3 is the shift to strong K+ efflux with reduced Ca2+ influx. Chloride influx is not a major factor here.

Phase 3 repolarization in pacemaker cells is driven by outward potassium currents while inward calcium currents diminish. As repolarization begins, voltage-gated K+ channels open and K+ leaves the cell, pulling the membrane potential back toward the resting level. At the same time, L-type calcium channels close/inactivate, reducing Ca2+ entry that would oppose repolarization. In pacemaker cells, the upstroke is actually produced by calcium influx, not sodium, so the key to phase 3 is the shift to strong K+ efflux with reduced Ca2+ influx. Chloride influx is not a major factor here.