Which event occurs during repolarization in the SA node?

Which event occurs during repolarization in the SA node?

• A. Calcium channels close and voltage-gated potassium channels open, allowing potassium to flow out.
• B. Sodium channels open and calcium channels close, allowing sodium to flow in.
• C. Potassium channels close while calcium channels remain open.
• D. Sodium channels open and potassium channels close.

Answer: (B)

Repolarization in SA node cells is driven by closing the calcium channels and opening potassium channels, which lets potassium exit the cell and bring the membrane potential back toward the resting level. In pacemaker tissue, the upstroke is mainly due to calcium entry rather than sodium, so repolarization relies on stopping calcium influx and allowing an outward potassium current to dominate. If sodium were driving the process (sodium entering), that would promote depolarization, not repolarization, and would not reset the membrane potential.

Repolarization in SA node cells is driven by closing the calcium channels and opening potassium channels, which lets potassium exit the cell and bring the membrane potential back toward the resting level. In pacemaker tissue, the upstroke is mainly due to calcium entry rather than sodium, so repolarization relies on stopping calcium influx and allowing an outward potassium current to dominate. If sodium were driving the process (sodium entering), that would promote depolarization, not repolarization, and would not reset the membrane potential.