During the plateau phase of the ventricular action potential, which ion channels are primarily responsible for maintaining the membrane potential near 0 mV?

Explanation

## Plateau Phase of Ventricular Action Potential ### Ion Channel Activity During Plateau **Key Point:** The plateau phase (Phase 2) is characterized by a balance between inward (depolarizing) and outward (repolarizing) currents, maintaining the membrane potential near 0 mV. ### Ionic Mechanisms | Phase | Primary Inward Current | Primary Outward Current | Net Effect | |-------|------------------------|-------------------------|------------| | Plateau (Phase 2) | L-type Ca²⁺ channels | K⁺ channels (delayed rectifier) | Balanced → plateau | | Early repolarization | L-type Ca²⁺ (declining) | K⁺ channels (increasing) | Net outward | **High-Yield:** During the plateau: - **L-type calcium channels** remain open, providing sustained inward (depolarizing) current - **Delayed rectifier potassium channels** (particularly K~v~ channels) gradually open, providing increasing outward (repolarizing) current - The balance between these two currents keeps the membrane potential stable near 0 mV - Fast sodium channels are **inactivated** and do not contribute ### Clinical Pearl This prolonged plateau is unique to ventricular and atrial myocytes and is essential for: - Sustained contraction (allowing complete ventricular emptying) - Absolute refractory period (preventing tetanic contraction) - Coordinated electrical-mechanical coupling **Mnemonic:** **PLAT** = **P**rolonged **L**ow voltage with **A**ctive Ca²⁺ and **T**ransitional K⁺ channels [cite:Guyton & Hall Textbook of Medical Physiology Ch 10]