Which of the following changes occur in axon membranes during repolarisation?

Options

A. Decrease in permeability of `K^+` leak channels
B. Opening of `Na^+` voltage gated channels
C. Inhibition of `Na^+/K^+` pump
D. Opening of voltage gated `K^+` channels

Answer

To answer the question regarding the changes that occur in axon membranes during repolarization, we can break down the process step by step:

### Step-by-Step Solution:

1. **Understanding Repolarization**:
- Repolarization is the process that occurs after depolarization during an action potential in a neuron. It involves restoring the membrane potential to its resting state after it has been depolarized.

2. **Ion Movement During Repolarization**:
- During repolarization, the permeability of the axon membrane changes, particularly with respect to potassium (K+) and sodium (Na+) ions.

3. **Role of Voltage-Gated Channels**:
- Voltage-gated sodium channels, which were open during depolarization, close during repolarization. This prevents Na+ from entering the cell.

4. **Opening of Voltage-Gated Potassium Channels**:
- As the membrane repolarizes, voltage-gated potassium channels open. This allows K+ ions to flow out of the neuron, which helps to restore the negative internal charge of the neuron.

5. **Evaluating the Options**:
- **Option 1**: Decrease in permeability of K+ leak channels - This is incorrect as the permeability to K+ actually increases due to the opening of voltage-gated K+ channels.
- **Option 2**: Opening of Na+ voltage-gated channels - This is incorrect as these channels close during repolarization.
- **Option 3**: Inhibition of Na+ by K+ pump - This is misleading; the Na+/K+ pump works continuously to maintain ion gradients but is not directly involved in the repolarization process.
- **Option 4**: Opening of voltage-gated K+ channels - This is correct as it allows K+ to exit the cell, contributing to repolarization.

6. **Conclusion**:
- The correct answer is **Option 4: Opening of voltage-gated K+ channels**.