What is the key distinction of cardiac action potentials compared to nerve action potentials?

• A. Cardiac action potentials are much longer in duration
• B. Cardiac action potentials are initiated by nerves
• C. Cardiac action potentials have no pacemaker cells
• D. Cardiac action potentials are never spontaneous

Answer: (A)

The defining distinction is the plateau phase that prolongs the cardiac action potential. In cardiac myocytes, after the initial depolarization, calcium enters the cell through L-type calcium channels and sustains depolarization while potassium leaks outward, creating a prolonged plateau. This keeps the action potential much longer—on the order of hundreds of milliseconds—and produces a long refractory period, which prevents tetany and allows the heart to contract and then relax in a coordinated cycle. In contrast, nerve action potentials are rapid and brief, driven mainly by a quick Na+ influx followed by fast K+ repolarization, so they fire rapidly and repetitively. The heart does have pacemaker cells that generate impulses automatically and autonomic nerves modulate rate, but they are not required to initiate the impulse in the same way nerves drive nerve action potentials. Therefore, the key point is that cardiac action potentials are much longer in duration due to the plateau phase.