Which phase of the ventricular action potential is primarily responsible for the plateau phase (phase 2) in ventricular muscle cells, and what distinguishes it from the cardiac pacemaker (SA node) action potential?

• A Phase 2 plateau is caused by Na+ influx through fast channels; SA node has identical plateau due to slower Na+ entry
• B Phase 2 plateau is caused by L-type Ca2+ channel inward current balancing K+ outward current; SA node lacks a plateau phase, having a slow inward current responsible for slow depolarization (funny current If and T-type Ca2+) ✓
• C Phase 2 plateau is caused by K+ efflux overcoming Ca2+ influx; SA node has the same mechanism
• D Phase 2 plateau is caused by T-type Ca2+ channels; SA node uses L-type Ca2+ channels exclusively

Explanation

In ventricular myocytes, the plateau (phase 2) results from a balance between inward L-type Ca2+ current (ICaL) and outward K+ currents (primarily IKr, IKs). This prolonged plateau prevents tetanic contraction by keeping the cell refractory during systole. The SA node (pacemaker) lacks a true plateau — instead it shows a slow phase 4 (pacemaker potential/spontaneous depolarization) mediated by the 'funny current' If (HCN channels, inward Na+/K+) and T-type Ca2+ channels (ICaT); the upstroke itself is mediated by L-type Ca2+ (not fast Na+ channels).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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