T-type channel is described as

• A. Transient current; contributes to early phase 4 pacemaker currents in SA and AV nodal cells
• B. Continuous current for plateau
• C. Inactivated during depolarization
• D. Only in ventricular myocytes

Answer: (A)

T-type calcium channels are low-voltage-activated, transient calcium channels. They open with small depolarizations near the resting membrane potential and then inactivate quickly, producing a brief inward Ca2+ current. This transient current contributes to the early phase of diastolic depolarization (phase 4) in sinoatrial and atrioventricular nodal cells, helping to bring the membrane potential toward threshold and support automaticity. This distinguishes them from the L-type calcium channels, which sustain the plateau phase and require a higher depolarization to activate. They are not exclusive to ventricular myocytes, as they also appear in nodal tissue. Thus, describing T-type channels as a transient, low-voltage-activated current that contributes to early phase 4 pacemaker currents in SA and AV nodal cells is the best fit.