Plateau phase of action potential of cardiac muscle starts with
**Core Concept**
The plateau phase of the action potential in cardiac muscle is a unique feature that distinguishes it from other excitable cells. It is characterized by a prolonged depolarization phase that lasts for approximately 200-300 milliseconds, allowing for a longer duration of contraction and a more efficient transmission of electrical impulses through the heart.
**Why the Correct Answer is Right**
The plateau phase is initiated by the activation of the L-type calcium channels (LTCCs) in the cardiac muscle cell membrane. These channels allow an influx of calcium ions (Ca²⁺) into the cell, which in turn activates the ryanodine receptors (RyRs) on the sarcoplasmic reticulum, releasing more calcium ions into the cytosol. This feedback loop maintains the depolarization of the cell membrane and prolongs the plateau phase. The slow inactivation of the LTCCs and the gradual closure of the sodium-potassium pumps (Na+/K+-ATPase) also contribute to the termination of the plateau phase.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the plateau phase of the action potential in cardiac muscle. The initial rapid depolarization phase is mediated by the opening of voltage-gated sodium channels, not the plateau phase.
**Option B:** This option is incorrect because the plateau phase is not initiated by the closure of potassium channels. In fact, the opening of certain potassium channels (such as the slow delayed rectifier potassium channels) helps to terminate the plateau phase.
**Option C:** This option is not accurate because the plateau phase is not caused by the influx of potassium ions. The movement of potassium ions out of the cell is actually a key factor in the repolarization phase that follows the plateau phase.
**Clinical Pearl / High-Yield Fact**
The plateau phase of the action potential in cardiac muscle is highly dependent on the activity of the L-type calcium channels, which are sensitive to calcium channel blockers (CCBs) such as verapamil and diltiazem. These medications can effectively reduce the heart rate and contractility by inhibiting the LTCCs and prolonging the repolarization phase.
**Correct Answer:** C.