Cardiac Action Potential — FRCA Primary MCQ

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ModeratePhysiology - CardiovascularCardiac Action PotentialFRCA Primary

In a normal ventricular myocyte, which ionic flux provides the principal inward depolarising current that sustains phase 2 of the action potential by opposing outward potassium current?

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Correct answer: B — Calcium influx through L-type calcium channels

The correct answer is B. During phase 2, voltage-gated L-type calcium channels remain open and generate the principal inward depolarising current. This approximately counterbalances outward potassium currents, producing the ventricular action-potential plateau. The entering calcium also triggers calcium-induced calcium release from the sarcoplasmic reticulum and therefore links excitation to contraction. Potassium efflux is an essential opposing component, but increasing delayed-rectifier potassium current ultimately promotes phase 3 repolarisation. Fast sodium influx produces the phase 0 upstroke. The sodium-potassium ATPase maintains transmembrane ionic gradients rather than directly sustaining the plateau, and chloride influx is not its principal inward depolarising mechanism.

Reference: Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation, section/figure describing ventricular action-potential phases, 2021. https://pubmed.ncbi.nlm.nih.gov/34709746/