Hyperpolarization is caused by which ions

**Core Concept**
Hyperpolarization is a change in the electrical potential of a cell membrane, resulting in a more negative charge inside the cell. This phenomenon is crucial in the regulation of neuronal excitability and is influenced by the movement of ions across the cell membrane.

**Why the Correct Answer is Right**
Hyperpolarization is primarily caused by an influx of potassium ions (K+) into the cell, which increases the negative charge inside the cell. This process is mediated by potassium channels, which allow K+ ions to flow out of the cell, increasing the concentration of K+ inside the cell. As a result, the electrical potential becomes more negative, leading to hyperpolarization. Additionally, certain neurotransmitters, such as glycine, can also cause hyperpolarization by opening chloride channels, allowing chloride ions (Cl-) to enter the cell.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because sodium ions (Na+) typically cause depolarization, not hyperpolarization. Sodium channels allow Na+ ions to flow into the cell, increasing the positive charge inside the cell.
* **Option B:** This option is incorrect because calcium ions (Ca2+) can cause depolarization or excitation in certain cells, such as muscle cells, but are not typically associated with hyperpolarization.
* **Option D:** This option is incorrect because anions, such as chloride ions (Cl-), can contribute to depolarization or hyperpolarization, but are not the primary cause of hyperpolarization.

**Clinical Pearl / High-Yield Fact**
The movement of potassium ions across the cell membrane is crucial in regulating neuronal excitability. In conditions such as hypokalemia, where potassium levels are low, neuronal excitability can be increased, leading to symptoms such as muscle weakness and tetany.

**Correct Answer: C. Potassium ions (K+)**