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15.1.814-15 · Homeostasis and Control

The Refractory Period — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 6 parts Lesson 23 of 31 1 quiz

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

15.1.8 The Refractory Period — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain the refractory period and why it matters for the transmission of impulses.Sodium channels — about 1 ms after the action potential, all voltage-gated sodium channels in that section …

  • Potassium channels
  • Repolarisation
  • Unresponsive
  • Discrete events
  • One direction
  • Minimum interval

Part 1 of 4Events after an action potential

Sodium channels
  • Sodium channels about 1 ms after the action potential, all voltage-gated sodium channels in that section close.
Effect
  • Effect no more Na+ enter the axon.
Potassium channels
  • Potassium channels voltage-gated potassium channels open and K+ diffuse out.
Repolarisation
  • Repolarisation the potential returns to about −70 mV.

Part 2 of 4The refractory period

Recovery
  • Recovery once the resting potential is nearly restored, the potassium channels close.
Sodium channels
  • Sodium channels become responsive to depolarisation again.
Unresponsive
  • Unresponsive until then, the section of membrane is recovering and cannot respond.
Timing
  • Timing it begins when repolarisation starts and ends when the resting state is re-established.

Part 3 of 4Importance of the refractory period

Discrete events
  • Discrete events action potentials cannot merge into one another.
One direction
  • One direction new potentials form ahead, because the region behind is recovering.
Minimum interval
  • Minimum interval there is a minimum time between action potentials at any one place.
Frequency
  • Frequency the length of the period sets the maximum frequency, between 500 and 1000 impulses per second.

Part 4 of 4Maximum frequency

Relationship
  • Relationship maximum frequency is 1 divided by the length of the refractory period.
Meaning
  • Meaning a longer refractory period gives a lower maximum frequency.

Key terms

TermMeaning
Refractory periodTime when a membrane section cannot produce a new action potential.
RepolarisationReturn of the membrane to the resting potential.
UnidirectionalMoving in one direction only.
DiscreteSeparate and distinct.
Maximum frequencyHighest number of impulses per second.

Avoid these mistakes

1
Saying impulses can travel backwards — the refractory region behind prevents it.
2
Saying the refractory period starts at depolarisation — it starts at repolarisation.
3
Saying the sodium channels are open in the refractory period — they are closed then unresponsive.
4
Forgetting that it limits frequency.

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