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3.4.2Additional lessons

Nerve Impulses — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 62 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Resting Potential

3.4.2 Nerve Impulses
Sodium-potassium pump
  • The pump uses ATP to move three sodium ions out of the axon for every two potassium ions moved in.
Potassium leakage
  • Potassium ions leak out through open channels, so the inside becomes negative relative to the outside.
Resting potential
  • The membrane potential of a neurone at rest is about −70 mV.
Polarised
  • A membrane with a difference in charge across it is said to be polarised.
Gradients
  • The pump creates concentration gradients for sodium and potassium across the membrane.

Part 2 of 6Action Potential

1
A stimulus opens some sodium channels; if the potential reaches the threshold of about −55 mV, voltage-gated sodium channels open.
2
Sodium ions rush in, so the membrane is depolarised and the inside reaches about +40 mV.
3
Sodium channels close and voltage-gated potassium channels open.
4
Potassium ions leave the axon, repolarising the membrane back towards the resting potential.
5
The membrane briefly overshoots, then the pump restores the resting potential.

Part 3 of 6Propagation

Propagation
Local currents
  • Sodium entering at one point spreads to the next region and brings it to threshold.
One direction
  • The region just passed is in a refractory period, so the impulse moves forward only.
All or nothing
  • An action potential occurs fully if the threshold is reached, and not at all if it is not.
Myelination
  • In myelinated axons the impulse jumps between nodes of Ranvier, which is much faster than in unmyelinated axons.

Part 4 of 6Speed of Conduction

Speed of Conduction
Myelinated
  • Impulses jump from one node of Ranvier to the next, which is called saltatory conduction.
Diameter
  • A wider axon has less resistance to the flow of ions, so it conducts impulses faster.
Temperature
  • Higher temperature increases the speed of conduction, up to the point where proteins are damaged.

Part 5 of 6Worked Example

Question

  • An impulse travels 0.6 m along a myelinated axon in 8 ms.
  • Calculate its speed in m s⁻¹.

  1. Convert time8 ms = 0.008 s.
  2. Speed = distance ÷ time = 0.6 ÷ 0.008 = 75 m s⁻¹.

Answer 75 m s⁻¹ (example data).

Part 6 of 6Exam Checklist & Tips

3.4.2 – Key Points for the Exam

  • Resting potential The potential difference across the membrane of an unstimulated neurone.
  • Depolarisation A change in the membrane potential to a less negative value.
  • Repolarisation Return of the membrane potential to its resting value.
  • Threshold The membrane potential that must be reached to trigger an action potential.

Common Mistakes

  • Saying that the impulse is a flow of electrons along the axon, when it involves ions crossing the membrane.
  • Mixing up depolarisation (sodium enters) with repolarisation (potassium leaves).
  • Stating that a stronger stimulus makes a bigger action potential, when it changes the frequency.
  • Forgetting that the pump needs ATP to restore the gradients.

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