During depolarisation the outer surface of the membrane becomes

Options

A. negatively charged
B. positively charged
C. neutrally charged
D. None of these

Explanation

### Step-by-Step Solution

1. **Understanding Resting Membrane Potential**:
- In the resting state of a neuron, the outer surface of the membrane is positively charged due to a high concentration of sodium ions (Na+) outside the cell, while the inner surface is negatively charged. The typical resting membrane potential is around -70 millivolts (mV).

2. **Threshold Stimulus**:
- When a threshold stimulus is applied, it causes a change in the membrane potential. The membrane potential changes from -70 mV to -55 mV, which is the threshold for depolarization.

3. **Sodium Influx**:
- If the stimulus continues, sodium channels open, allowing a rapid influx of sodium ions into the neuron. This influx of positively charged sodium ions causes the membrane potential to rise further.

4. **Reversal of Charge**:
- As sodium ions continue to enter the cell, the membrane potential can reach approximately +35 mV. This results in a reversal of charge across the membrane: the inner surface becomes positively charged, and the outer surface becomes negatively charged.

5. **Conclusion**:
- Therefore, during depolarization, the outer surface of the membrane becomes negatively charged due to the rapid influx of sodium ions.

### Final Answer:
During depolarization, the outer surface of the membrane becomes **negatively charged**.

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