During repolarization in SA node, which ion movement predominates?

During repolarization in SA node, which ion movement predominates?

• A. Sodium influx
• B. Calcium influx
• C. Potassium efflux
• D. Chloride influx

Answer: (C)

Repolarization in SA nodal cells is driven by potassium moving out of the cell. When the repolarization phase begins, voltage-gated potassium channels open and potassium ions exit, making the inside of the cell more negative and restoring the membrane potential toward its resting level for the next beat. In SA node tissue, the upstroke (depolarization) is mainly caused by calcium influx, not sodium, so the key event that ends the action potential and resets the pacemaker potential is the loss of positive charge via potassium efflux. Sodium influx would push toward depolarization, and chloride influx isn’t the primary mechanism here, while calcium influx drives the earlier depolarization.

Repolarization in SA nodal cells is driven by potassium moving out of the cell. When the repolarization phase begins, voltage-gated potassium channels open and potassium ions exit, making the inside of the cell more negative and restoring the membrane potential toward its resting level for the next beat. In SA node tissue, the upstroke (depolarization) is mainly caused by calcium influx, not sodium, so the key event that ends the action potential and resets the pacemaker potential is the loss of positive charge via potassium efflux. Sodium influx would push toward depolarization, and chloride influx isn’t the primary mechanism here, while calcium influx drives the earlier depolarization.