Which tachycardia mechanism involves spontaneous depolarization during the resting potential?

• A. Automaticity
• B. Triggered activity
• C. Reentry
• D. Afterdepolarization

Answer: (A)

Automaticity is the ability of cardiac tissue to generate impulses on its own by diastolic depolarization. In nodal and some ectopic pacemaker cells, the resting membrane potential isn’t a fixed line; during phase 4, inward currents (notably the funny current, If) slowly depolarize the membrane toward threshold. When this threshold is reached, voltage-gated channels fire an action potential, creating rhythmic pacemaker activity. If a focus fires faster than the normal SA node pace, it can produce tachycardia. This mechanism is distinct from triggered activity, which comes from afterdepolarizations following an action potential, and from reentry, which requires a circuit.