Do some cardiac cells have spontaneous pacemaker activity?

• A. No
• B. Yes
• C. Only during exercise
• D. Only in disease

Answer: (B)

Spontaneous pacemaker activity is a property of specific cardiac cells in the heart’s conduction system. The sinoatrial (SA) node cells automatically depolarize without any external trigger. Their resting potential isn’t stable; they slowly drift upward during phase 4 because of an inward current called the funny current (If) and transient calcium currents. When this depolarization reaches threshold, they fire an action potential, triggering the heartbeat. The SA node sets the natural pace, typically 60–100 beats per minute, and other nodal tissues like the AV node can take over if needed. Autonomic input (sympathetic speeds up, parasympathetic slows down) modulates the rate but doesn’t create the automaticity itself. In contrast, most ventricular and atrial myocytes have stable resting potentials and require an impulse from these pacemaker cells to depolarize. So, yes, some cardiac cells do have spontaneous pacemaker activity.