Major current for depolarization in Purkinje fiber is of:
**Core Concept**
The **action potential** in cardiac cells, including **Purkinje fibers**, is generated by the movement of ions across the cell membrane. This process involves various ion channels and pumps, leading to rapid depolarization and repolarization phases. The primary ions involved are **sodium (Na+)** and **calcium (Ca2+)**.
**Why the Correct Answer is Right**
The major current for depolarization in **Purkinje fibers**, as in other cardiac cells, is carried by **sodium ions (Na+)**. This is because the rapid upstroke of the action potential is due to the opening of **voltage-gated sodium channels**, allowing an influx of Na+ into the cell. This influx of positive ions causes the membrane potential to become more positive, leading to depolarization.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while **potassium (K+)** channels are crucial for repolarization, they are not primarily responsible for the depolarization phase.
**Option C:** Incorrect as **calcium (Ca2+)** channels do contribute to the action potential, especially in the plateau phase, but are not the primary cause of initial depolarization.
**Option D:** This option is not provided, but typically, any other ion or mechanism not involving sodium would be incorrect for the initial depolarization phase.
**Clinical Pearl / High-Yield Fact**
Remember, the **sodium channel** is key to the initiation of the action potential in cardiac cells, including **Purkinje fibers**. Blocking these channels can lead to significant cardiac conduction abnormalities.
**Correct Answer:** D. Sodium