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15.1.614-15 · Homeostasis and Control

Transmission of Nerve Impulses — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 21 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

15.1.6 Transmission of Nerve Impulses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain the resting potential, the action potential and how the impulse travels along an axon.Impulse — travels along the neurone surface membrane extremely quickly.Not electrons —…

  • Action potentials
  • Sodium-potassium pumps
  • Impermeability
  • Closed channels
  • Local current
  • Sodium diffusion

Section 1 of 7Nature of an impulse

Impulse
  • Impulse travels along the neurone surface membrane extremely quickly.
Not electrons
  • Not electrons it is not a flow of electrons like an electric current.
Action potentials
  • Action potentials very brief changes in the distribution of charge across the membrane.
Ions
  • Ions caused by rapid movement of sodium and potassium ions across the axon membrane.

Section 2 of 7Resting potential

Value
  • Value the inside of the axon is about −70 mV compared with the outside.
Sodium-potassium pumps
  • Sodium-potassium pumps move Na+ out and K+ in using ATP, against their concentration gradients.
Anions
  • Anions large negatively charged molecules inside attract K+, reducing diffusion out.
Impermeability
  • Na+ cannot diffuse through the resting membrane.
Closed channels
  • Closed channels voltage-gated channels are closed at rest.

Section 3 of 7Generating an action potential

1
A stimulus opens sodium channels, and Na+ diffuse in.
2
The potential difference is reduced, which is depolarisation.
3
Voltage-gated sodium channels open, letting more Na+ in (positive feedback).
4
At about −50 mV (threshold), many more channels open.
5
The inside of the axon reaches about +30 mV.

Section 4 of 7Repolarisation

1
About 1 ms later, the sodium channels close.
2
Voltage-gated potassium channels open and K+ diffuse out.
3
The potential returns to about −70 mV (repolarisation).
4
There is a brief hyperpolarisation, more negative than the resting potential.
5
The potassium channels close and the resting state is re-established.

Section 5 of 7Travelling along the axon

Local current
  • Local current depolarisation at one site makes current flow to the next section of membrane.
Sodium diffusion
  • Sodium diffusion current is the diffusion of Na+ along the axon from high to low concentration.
Next section
  • Next section depolarises and opens sodium channels, so another action potential starts.
Result
  • Result action potentials pass along the entire length of the axon.

Section 6 of 7Key terms

TermMeaning
Resting potentialPotential difference across an unstimulated axon, about −70 mV.
Action potentialBrief reversal of the membrane potential.
DepolarisationPotential becoming less negative.
RepolarisationPotential returning to the resting value.
HyperpolarisationPotential briefly more negative than resting.
ThresholdPotential at which many sodium channels open, about −50 mV.
Voltage-gated channelChannel opened by a change in potential difference.

Section 7 of 7Exam technique

1
Saying an impulse is an electron flow — it is movement of ions across the membrane.
2
Mixing up the ions — Na+ enters for depolarisation and K+ leaves for repolarisation.
3
Saying the pump makes the action potential — it maintains the resting potential.
4
Saying the resting potential is positive — the inside is about −70 mV.

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