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C2.2.3—Nerve impulses as action potentials

Nerve impulses are action potentials propagated along neuron membranes by local depolarization, ion channel opening and restoration of resting polarity.

Syllabus
First assessment 2025
Objective
C2.2.3
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper2
Typical marks3–8

Common command terms

  • Explain
  • Outline

Recent exam appearances

November 2024Paper2 ["SL"] · TZ16(c)[ 7 ]C2.2.3—Nerve impulses as action potentials
May 2022Paper2 ["SL"] · TZ15(b)[ 1 ]C2.2.3—Nerve impulses as action potentials
May 2018Paper2 ["SL"] · TZ25(c)[ 7 ]C2.2.3—Nerve impulses as action potentials
November 2015Paper2 ["SL"] · TZ07(b)[ 8 ]C2.2.3—Nerve impulses as action potentials
May 2015Paper2 ["SL"] · TZ13(d)[ 3 ]C2.2.3—Nerve impulses as action potentials
Practice this objective

Coverage 2010–2024 · Updated 15 Jul 2026

An Action Potential Is an All-or-none Nerve Impulse

An action potential is a brief, threshold-triggered reversal of membrane potential that propagates along an axon.

Voltage-gated sodium channels open rapidly during depolarization, then inactivate while potassium channels open to repolarize the membrane. Refractory periods prevent immediate re-firing in the same segment. The threshold is a condition, not a fixed promise: if it is not reached, the full action potential does not start.

Trace the phases:

  • threshold
  • Na⁺ influx and depolarization
  • K⁺ efflux and repolarization
  • hyperpolarization and recovery

A stimulus below threshold produces no full impulse, while a threshold stimulus produces an impulse with a similar amplitude.

Stronger stimuli are coded mainly by impulse frequency, not by a larger action-potential amplitude.

Nerve impulses as action potentials

Assessment in practice

1–8 marks
How it is assessed

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

Command terms

Explain / Outline

What earns marks

Build the answer around this relationship: Action potentials are regenerated along neuron membranes.

Representative question

Question 1

[Maximum number: 3]

Outline how nerve impulses are transmitted along a nerve fibre.

SL Transfer: Explain Core Neural Signalling

Neurons carry electrical impulses in the nervous system; motor, sensory, and relay neurons differ in axon, dendrite, and cell body arrangement. Sodium-potassium pumps use ATP to move 3 Na+ out and 2 K+ in; ion gradients make the resting axon polarized at about -70 mV. A nerve impulse is a propagated action potential along a nerve fibre; stimulus-triggered sodium influx reverses membrane polarity. Larger axon diameter lowers resistance and increases impulse speed; myelin sheaths and nodes of Ranvier enable faster saltatory conduction. Synapses connect neurons to neurons, muscles, or glands; chemical synapses transmit one way across a narrow synaptic cleft. Action potentials open voltage-gated Ca2+ channels in presynaptic terminals; Ca2+ causes vesicle fusion and neurotransmitter exocytosis into the cleft. Neurotransmitters diffuse and bind receptors on the postsynaptic membrane; EPSPs depolarize the membrane and make threshold more likely.

Concept essentials

  • Action potentials are regenerated along neuron membranes.
  • Sodium ion entry causes depolarization of the membrane.
  • Potassium ion exit helps repolarize the membrane.
  • Local current from one region triggers depolarization in the next region.