Which of the following will cause the plasma membrane to hyperpolarize when at its resting membrane potential (RMP)?

• A. Opening of potassium (K^+) channels
• B. Opening of sodium (Na^+) channels
• C. Closing of chloride (Cl^-) channels
• D. Opening of calcium (Ca^{2+}) channels

Answer: (A)

Verified step by step guidance

1. Understand the concept of resting membrane potential (RMP): The RMP is the electrical potential difference across the plasma membrane of a cell at rest, typically around -70 mV in neurons. It is maintained by the distribution of ions such as potassium (K⁺), sodium (Na⁺), chloride (Cl⁻), and calcium (Ca²⁺) across the membrane.
2. Recall the role of potassium (K⁺) in RMP: Potassium ions are more concentrated inside the cell than outside. At rest, potassium channels allow K⁺ to leak out of the cell, making the inside of the cell more negative relative to the outside. This contributes significantly to the RMP.
3. Define hyperpolarization: Hyperpolarization occurs when the membrane potential becomes more negative than the RMP. This can happen if positive ions (like K⁺) leave the cell or if negative ions (like Cl⁻) enter the cell.
4. Analyze the effect of opening potassium (K⁺) channels: If K⁺ channels open, more K⁺ ions will flow out of the cell, increasing the negative charge inside the cell. This will cause hyperpolarization.
5. Compare the effects of other ion channels: Opening sodium (Na⁺) or calcium (Ca²⁺) channels would allow positive ions to enter the cell, making the inside less negative (depolarization). Closing chloride (Cl⁻) channels would reduce the entry of negative ions, also leading to depolarization. Therefore, only the opening of potassium (K⁺) channels will cause hyperpolarization.