T-type channels contribute to which feature in SA and AV nodal cells?

T-type channels contribute to which feature in SA and AV nodal cells?

• A. L-type calcium current during plateau
• B. T-type calcium current contributing to early phase 4 pacemaker currents
• C. Sodium current during fast depolarization
• D. Chloride current during rest

Answer: (B)

T-type calcium channels provide a low-threshold inward Ca2+ current that helps drive the slow, spontaneous depolarization during diastole in SA and AV nodal cells. This early phase 4 pacemaker current gradually depolarizes the cell toward the threshold, setting the pace of automaticity. In nodal tissue, the upstroke and plateau involve mainly calcium entry through L-type channels, but the specific feature tested here is the contribution of T-type channels to the initial, negative-potential phase 4 depolarization that enables rhythmic firing.

T-type calcium channels provide a low-threshold inward Ca2+ current that helps drive the slow, spontaneous depolarization during diastole in SA and AV nodal cells. This early phase 4 pacemaker current gradually depolarizes the cell toward the threshold, setting the pace of automaticity. In nodal tissue, the upstroke and plateau involve mainly calcium entry through L-type channels, but the specific feature tested here is the contribution of T-type channels to the initial, negative-potential phase 4 depolarization that enables rhythmic firing.