Explain how an action potential is generated at the axon hillock of a neuron following stimulation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Neurons integrate incoming signals at the axon hillock, where a decision to 'fire' or not is made. This process depends on changes in membrane permeability to specific ions.
Model answer (4 marks)
1. Depolarising stimuli raise the membrane potential at the axon hillock towards the threshold of about −55 mV.
2. When the threshold is reached, voltage‑gated Na⁺ channels open.
3. Na⁺ rushes into the cell along its electrochemical gradient, making the inside of the membrane positive and depolarising it to around +30 mV.
4. This follows an all‑or‑nothing rule: if the stimulus reaches threshold a full action potential is generated; if it does not, none occurs.
2. When the threshold is reached, voltage‑gated Na⁺ channels open.
3. Na⁺ rushes into the cell along its electrochemical gradient, making the inside of the membrane positive and depolarising it to around +30 mV.
4. This follows an all‑or‑nothing rule: if the stimulus reaches threshold a full action potential is generated; if it does not, none occurs.
Examiner tips
- Use the exact threshold value (−55 mV) and peak potential (+30 mV).
- Show the sequence: stimulus → threshold → Na⁺ channel opening → depolarisation → all‑or‑nothing.
- Mention voltage‑gated Na⁺ channels specifically.
Common mistakes
- Saying the membrane becomes negative instead of positive during the action potential.
- Using the wrong threshold value or omitting the all‑or‑nothing principle.
Mark scheme (4 marks)
- Depolarising stimuli cause the membrane potential at the axon hillock to rise towards the threshold potential (approximately −55 mV).
- When threshold is reached, voltage-gated sodium channels open, allowing Na⁺ to rush into the cell down its electrochemical gradient.
- The rapid influx of Na⁺ causes the inside of the membrane to become positive (depolarisation), reversing the membrane potential to approximately +30 mV.
- This follows an all-or-nothing principle: if the stimulus reaches threshold, a full-sized action potential is always generated; if it does not reach threshold, no action potential occurs.
Key terms in this question
action potential · axon hillock
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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