Specialized cells within the His-Purkinje system exhibit typical fast responses, but may also undergo what during phase 4?

Specialized cells within the His-Purkinje system exhibit typical fast responses, but may also undergo what during phase 4?

• A. Fast depolarization
• B. Slow spontaneous depolarization
• C. Early afterdepolarization
• D. Complete inactivity

Answer: (B)

Phase 4 automaticity. Although His-Purkinje cells are designed for fast conduction and have rapid depolarization for each impulse, they can under certain conditions develop a slow, spontaneous depolarization during phase 4. This gradual drift toward threshold means these fibers can generate impulses on their own, producing ectopic activity or arrhythmias when the normal pacemaker dictates are overridden. The other options don’t fit: fast depolarization is the normal phase 0 event in these cells, early afterdepolarizations occur during repolarization (phases 2–3), and complete inactivity isn’t characteristic of automatic pacing.

Phase 4 automaticity. Although His-Purkinje cells are designed for fast conduction and have rapid depolarization for each impulse, they can under certain conditions develop a slow, spontaneous depolarization during phase 4. This gradual drift toward threshold means these fibers can generate impulses on their own, producing ectopic activity or arrhythmias when the normal pacemaker dictates are overridden. The other options don’t fit: fast depolarization is the normal phase 0 event in these cells, early afterdepolarizations occur during repolarization (phases 2–3), and complete inactivity isn’t characteristic of automatic pacing.