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3.5.5Inquiry Process

Action Potentials — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 136 of 160

📚 IB Biology HL Hub / Topical Revision
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Part 1 of 5Phases of the Action Potential

3.5.5 Action Potentials
1
At rest the membrane is near −70 mV and most voltage-gated channels are closed.
2
A stimulus depolarises the membrane, and when threshold, about −55 mV, is reached, voltage-gated sodium channels open.
3
Na⁺ rushes in down its electrochemical gradient, and this positive feedback pushes the potential to about +40 mV.
4
Sodium channels close and voltage-gated potassium channels open, so K⁺ leaves the cell and the potential falls: this is repolarisation.
5
The potassium channels close slowly, so the potential briefly undershoots below −70 mV (hyperpolarisation) before the pump and leak channels restore rest.

Part 2 of 5Key Features

Threshold
  • Only a stimulus that depolarises the membrane to threshold triggers the opening of enough sodium channels to start an impulse.
All or nothing
  • Once triggered, every action potential in a given neurone has the same size, whatever the strength of the stimulus.
Refractory period
  • Shortly after firing, sodium channels are inactivated and cannot reopen, so a second impulse cannot be started immediately.
Direction
  • The refractory region behind the impulse prevents backward travel and also limits how often a neurone can fire.
Voltage-gated channels
  • These channels respond to the membrane potential itself,
    • so a small depolarisation can open more channels and make the change self-reinforcing.
PhaseChannelsIon movement
DepolarisationVoltage-gated Na⁺ openNa⁺ enters
RepolarisationNa⁺ closed, K⁺ openK⁺ leaves
HyperpolarisationK⁺ still openExtra K⁺ lost

Part 3 of 5Consequences

Consequences
Frequency
  • Because size is fixed, stimulus intensity is coded by the number of impulses per second.
Speed limit
  • The refractory period sets a maximum firing rate of a few hundred impulses per second.
Pump role
  • The pump does not cause the spike; it restores the ion distribution afterwards over a longer time.
Energy
  • Each impulse moves only a tiny fraction of ions, so many impulses can occur before the pump is critical.

Part 4 of 5Worked Example

Question

  • The refractory period of a neurone is 2 ms.
  • What is the maximum number of action potentials it can fire per second?

  1. Each action potential requires at least 2 ms before the next can begin.
  2. Convert2 ms = 0.002 s.
  3. Maximum frequency = 1 ÷ 0.002 = 500 per second.

Answer A maximum of about 500 action potentials per second.

Part 5 of 5Exam Checklist & Tips

3.5.5 – Key Points for the Exam

  • Action potential A rapid, temporary reversal of membrane potential that carries a nerve impulse.
  • Repolarisation Return of membrane potential to a negative value as K⁺ leaves.
  • Hyperpolarisation Membrane potential temporarily more negative than the resting value.
  • Refractory period Short time after an impulse when a new one cannot be triggered.

Common Mistakes

  • Saying Na⁺ leaves during depolarisation; it enters.
  • Believing a larger stimulus gives a larger action potential.
  • Stating that the pump produces repolarisation; K⁺ channels do.

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