During the depolarization phase of SA nodal cells, which channels open to allow calcium entry?

During the depolarization phase of SA nodal cells, which channels open to allow calcium entry?

• A. Fast voltage-gated sodium channels
• B. Fast voltage-gated calcium channels
• C. Potassium channels
• D. Chloride channels

Answer: (B)

In SA nodal cells, the depolarization upstroke is driven by calcium entering the cell, not sodium. When these pacemaker cells depolarize, voltage-gated calcium channels open and let Ca2+ flow in, producing the rapid rise of the action potential. These fast voltage-gated calcium channels (the calcium channels responsible for the upstroke) are key because SA nodal cells lack the large fast sodium current that other cardiac cells use for rapid depolarization. After this rise, potassium channels open to repolarize. Chloride channels aren’t the main players in this depolarization phase.

In SA nodal cells, the depolarization upstroke is driven by calcium entering the cell, not sodium. When these pacemaker cells depolarize, voltage-gated calcium channels open and let Ca2+ flow in, producing the rapid rise of the action potential. These fast voltage-gated calcium channels (the calcium channels responsible for the upstroke) are key because SA nodal cells lack the large fast sodium current that other cardiac cells use for rapid depolarization. After this rise, potassium channels open to repolarize. Chloride channels aren’t the main players in this depolarization phase.