In SA nodal depolarization, which ion influx most directly triggers the action potential?

In SA nodal depolarization, which ion influx most directly triggers the action potential?

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

Answer: (C)

Calcium entry through voltage-gated calcium channels triggers the SA nodal action potential. In SA nodal cells, the slow, spontaneous depolarization (pacemaker potential) brings the membrane to threshold, and then the opening of L-type calcium channels allows Ca2+ to flow into the cell, producing the rapid depolarization of the action potential. This Ca2+ influx is the direct trigger for the AP upstroke in the SA node. Sodium plays a key role in the upstroke of other cardiac cells, but not in the SA nodal AP; potassium efflux drives repolarization, and chloride isn’t a major player here.

Calcium entry through voltage-gated calcium channels triggers the SA nodal action potential. In SA nodal cells, the slow, spontaneous depolarization (pacemaker potential) brings the membrane to threshold, and then the opening of L-type calcium channels allows Ca2+ to flow into the cell, producing the rapid depolarization of the action potential. This Ca2+ influx is the direct trigger for the AP upstroke in the SA node. Sodium plays a key role in the upstroke of other cardiac cells, but not in the SA nodal AP; potassium efflux drives repolarization, and chloride isn’t a major player here.