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C2.2.9 (HL)—Action potential propagation

Action potential propagation occurs when depolarized membrane regions trigger adjacent resting regions to open voltage-gated channels, linking cellular mechanisms to rapid communication in nervous systems.

Syllabus
First assessment 2025
Objective
C2.2.9
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2022
Most common paperPaper2
Typical marks4

Common command terms

  • Explain
  • Describe

Recent exam appearances

November 2022Paper2 ["HL"] · TZ06(b)[ 4 ]C2.2.9 (HL)—Action potential propagation
Practice this objective

Coverage 2022–2022 · Updated 15 Jul 2026

An Action Potential Propagates without Losing Amplitude

HL only

An action potential propagates along an axon when local currents from one depolarized region bring the adjacent membrane to threshold.

Na⁺ entering through open voltage-gated channels diffuses sideways inside the axon toward the next resting region. Compensating positive charge moves in the extracellular fluid, completing a local current circuit and depolarizing the next membrane segment to threshold.

The next segment opens its sodium channels and regenerates the action potential. The refractory region behind cannot immediately reopen sodium channels, so propagation normally proceeds forward.

Each newly depolarized segment supplies local current to the next segment, so a full-amplitude action potential is regenerated rather than gradually fading along the axon.

The action potential is not a single group of ions travelling the full axon length. Ions move short distances locally while sequential channel opening propagates the voltage change.

Action potential propagation

HL only

Assessment in practice

4–7 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Describe.

Command terms

Explain / Describe

What earns marks

Build the answer around this relationship: Local current from one depolarized region triggers adjacent membrane.

Representative question

Question 1

[Maximum number: 4]

Describe what occurs in a neuron when an action potential is propagated along the axon.

Neural Control And Evidence

HL only
  • At threshold, voltage-gated Na+ channels depolarize the axon; K+ channels then repolarize it. Local currents propagate the impulse and the refractory period enforces direction.
  • Action potentials are all-or-nothing; stimulus intensity is encoded by frequency. Myelin enables fast saltatory conduction between nodes of Ranvier.
  • Synapses integrate excitatory and inhibitory inputs through spatial and temporal summation. Chemicals may mimic, block or prolong neurotransmitter action.
  • Pain receptors use TRP channels that respond to heat, acid, capsaicin or tissue damage.
  • EEG, MRI and fMRI link conscious processing to coordinated activity across brain regions, providing neural correlates rather than a single consciousness centre.

Concept essentials

  • Local current from one depolarized region triggers adjacent membrane.
  • Voltage-gated sodium channels open in the next resting region.
  • Repolarization follows behind the advancing action potential.
  • Propagation is repeated regeneration along the axon membrane.