What best describes pacemaker cells' resting state?

• A. They have no true resting potential
• B. They maintain a stable resting potential
• C. They have a higher resting potential than non-pacemaker cells
• D. They have a definite resting potential similar to nerve fibers

Answer: (A)

Pacemaker cells are specialized for automaticity, so they don’t sit at a fixed resting membrane potential. After each cycle they begin to depolarize again during a slow, spontaneous rise called the pacemaker potential (diastolic depolarization). This gradual depolarization is driven by inward currents, notably the funny current (If) that flows through HCN channels when the membrane is hyperpolarized, and by calcium entry as they approach threshold. Because the membrane potential continuously drifts toward threshold rather than stabilizing at a single negative value, there is no true resting potential. This contrasts with non-pacemaker cells and neurons, which have a definite resting potential.