Plateau phase of ventricular muscle is due to opening of:
**Core Concept**
The plateau phase of ventricular muscle action potential is a unique feature of cardiac muscle cells, characterized by a prolonged depolarization phase. This phenomenon is attributed to the opening of specific ion channels that allow a slow influx of positive ions, prolonging the action potential duration.
**Why the Correct Answer is Right**
The plateau phase is primarily due to the opening of L-type calcium channels (LTCCs). These channels allow a slow influx of calcium ions (Ca²⁺) into the cardiac muscle cell, which in turn triggers the opening of potassium channels, maintaining a balance of positive and negative ions. The inward flow of calcium ions is balanced by the outward flow of potassium ions, resulting in a prolonged depolarization phase. This mechanism is crucial for cardiac muscle contraction and relaxation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because sodium channels are primarily responsible for the initial rapid depolarization phase (Phase 0) of the action potential, not the plateau phase.
**Option B:** This option is incorrect because potassium channels are responsible for repolarization (Phase 3) of the action potential, not the plateau phase.
**Option C:** This option is incorrect because chloride channels are not directly involved in the plateau phase of the action potential.
**Clinical Pearl / High-Yield Fact**
The plateau phase of the action potential is critical for cardiac muscle contraction and relaxation. Abnormalities in L-type calcium channels can lead to conditions such as long QT syndrome, which can increase the risk of arrhythmias.
**Correct Answer:** D. L-type calcium channels.