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8.1.4Unit 6: Practical Skills in Biology II

The Nerve Impulse — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 38 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5Resting Potential

8.1.4 The Nerve Impulse
Value
  • The inside of a resting axon is about −70 mV relative to the outside, so the membrane is polarised.
Sodium–potassium pump
  • Uses ATP to move 3 Na⁺ out and 2 K⁺ in, which sets up concentration gradients for both ions.
Potassium leakage
  • Open potassium channels let K⁺ diffuse out faster than Na⁺ can enter, leaving the inside more negative.
Permeability
  • The membrane is far more permeable to K⁺ than to Na⁺ at rest, and voltage-gated channels are closed.

Part 2 of 5Generating an Action Potential

1
A stimulus opens some sodium ion channels, so Na⁺ diffuses in and makes the inside less negative.
2
If the membrane reaches the threshold of about −55 mV, voltage-gated sodium channels open and many more Na⁺ ions rush in.
3
The potential reverses to about +30 mV, which is depolarisation.
4
The sodium channels close and voltage-gated potassium channels open, so K⁺ diffuses out and the inside becomes negative again, which is repolarisation.
5
The membrane briefly overshoots to a more negative value, called hyperpolarisation, before the pump restores the resting potential.

Part 3 of 5Properties of the Impulse

Properties of the Impulse
All or nothing
  • If the threshold is not reached no action potential occurs, and all action potentials have the same size.
Refractory period
  • The membrane cannot be stimulated again straight away, so impulses are discrete and travel in one direction.
Propagation
  • Na⁺ diffuses along the axon and depolarises the next section, creating a moving wave of depolarisation.
Intensity
  • A stronger stimulus produces a higher frequency of action potentials, not larger ones.

Part 4 of 5Worked Example

Question

  • A resting axon at −70 mV is stimulated.
  • Sodium ions enter and the potential rises to −58 mV.
  • State whether an action potential occurs, then calculate the total potential change from the resting value to the peak of +30 mV.

  1. Threshold is about −55 mV; −58 mV is more negative than this, so the threshold has not been reached.
  2. No voltage-gated channels open, so there is no action potential.
  3. Change to peak = 30 − (−70) = 100 mV.

Answer

  • No action potential occurs;
    • a full action potential changes the potential by 100 mV.

Part 5 of 5Exam Checklist & Tips

8.1.4 – Key Points for the Exam

  • Resting potential The potential difference across the membrane of an unstimulated neurone, about −70 mV.
  • Threshold potential The membrane potential at which voltage-gated sodium channels open and an action potential starts.
  • Depolarisation Reversal of the membrane potential as sodium ions enter.
  • Repolarisation Return of a negative internal potential as potassium ions leave.
  • Refractory period The time after an action potential when the membrane cannot respond to a new stimulus.

Common Mistakes

  • Saying an impulse is a flow of electrons along the axon like a current in a wire.
  • Stating that a stronger stimulus produces a bigger action potential.
  • Forgetting that potassium ions move out during repolarisation.
  • Confusing resting potential, which is negative inside, with the peak of the action potential, which is positive.

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