Cardiac muscles action potential at plateau is due to
**Core Concept**
The cardiac muscle action potential at the plateau phase is a unique phenomenon compared to other excitable tissues. It is primarily caused by a slow inward movement of calcium ions (Ca²⁺) through the L-type calcium channels (LTCC), which is balanced by a slow outward movement of potassium ions (K⁺) through the potassium channels.
**Why the Correct Answer is Right**
The L-type calcium channels are activated during the depolarization phase of the action potential and remain open for a longer duration, allowing a slow influx of calcium ions into the cardiac muscle cells. This calcium influx is crucial for the plateau phase of the action potential, as it helps to maintain the depolarization and prevent the rapid repolarization seen in other excitable tissues. The slow outward movement of potassium ions through the potassium channels helps to balance the inward calcium current, thereby contributing to the plateau phase.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the plateau phase of the cardiac muscle action potential is not primarily due to the rapid depolarization of the sodium channels.
* **Option B:** This option is incorrect because the plateau phase is not caused by the inward movement of chloride ions (Cl⁻).
* **Option C:** This option is incorrect because the plateau phase is not primarily due to the rapid repolarization of the potassium channels.
**Clinical Pearl / High-Yield Fact**
The plateau phase of the cardiac muscle action potential is crucial for the contraction of the heart muscle, as it allows for the influx of calcium ions necessary for muscle contraction. This is why drugs that block L-type calcium channels, such as verapamil and diltiazem, can slow the heart rate and reduce contractility.
**Correct Answer: D**