A student investigates the effect of a toxin on nerve impulse transmission. The toxin blocks voltage-gated sodium ion channels in the axon membrane. Explain why the affected neurone is unable to generate an action potential after exposure to this toxin.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Voltage-gated sodium ion channels open in response to a change in membrane potential. In a healthy neurone at rest, the membrane potential is approximately −70 mV.
Model answer (4 marks)
Voltage‑gated Na⁺ channels are essential for the rapid depolarisation that initiates an action potential.
1. Depolarisation requires Na⁺ to rush into the axon.
2. The toxin blocks these channels, so Na⁺ cannot enter.
3. Without Na⁺ influx the membrane potential cannot become less negative or reach the threshold.
4. The membrane therefore stays at or near the resting potential and no action potential is fired.
1. Depolarisation requires Na⁺ to rush into the axon.
2. The toxin blocks these channels, so Na⁺ cannot enter.
3. Without Na⁺ influx the membrane potential cannot become less negative or reach the threshold.
4. The membrane therefore stays at or near the resting potential and no action potential is fired.
Examiner tips
- Use the term ‘depolarisation’ and ‘threshold potential’. Show the causal chain: block → no Na⁺ influx → no depolarisation → no AP. Keep each point short and directly linked to the marks.
Common mistakes
- Confusing resting potential with threshold potential. Failing to mention that Na⁺ influx is required for depolarisation. Using vague language such as ‘the neuron is damaged’ instead of the specific channel blockage mechanism.
Mark scheme (4 marks)
- Depolarisation requires sodium ions to rush into the axon
- The membrane potential cannot become less negative / cannot reach the threshold potential
- An action potential requires a sufficient change in membrane potential to trigger further channel opening
- The membrane remains at / close to resting potential so no action potential is fired
Key terms in this question
voltage-gated sodium ion channels · action potential
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Neuronal communication questions
- A student investigating action potentials places two recording electrodes on the…
- Explain why the refractory period is essential for the transmission of nerve imp…
- A student states that the refractory period of a neurone is simply a time delay …
- Explain why a neurone cannot be stimulated to produce a second action potential …