Which sequence correctly lists the pacemaker action potential phases from left to right?

• A. 4, 0, 3
• B. 0, 4, 3
• C. 4, 1, 2, 3
• D. 1, 2, 3, 4

Answer: (A)

In pacemaker cells, the membrane potential starts with a gradual, spontaneous depolarization during phase 4 due to the funny current If and related currents, so the cell reaches threshold for action potential. When that threshold is reached, the upstroke (phase 0) occurs mainly because calcium ions enter through L-type Ca2+ channels, not sodium. After the upstroke, the cell repolarizes (phase 3) as outward potassium currents bring the membrane potential back down. So the correct left-to-right sequence is 4, then 0, then 3.

Other sequences imply a rapid upstroke before the diastolic depolarization or include phases (like 1 or 2) that aren’t characteristic of pacemaker action potentials, or they imply Na+-driven upstroke, which isn’t how nodal tissue depolarizes.