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Learn9700Paper 4 · A Level Structured QuestionsControl and coordinationNerve impulses and synapses
Control and coordination · Subtopic 1 of 2

Nerve Impulses and Synapses

At rest (about −70 mV), the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in; the membrane is more permeable to K⁺, which leaks out. A stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in: depolarisation to about +40 mV. Na⁺ channels close and K⁺ channels open: repolarisation, overshooting (hyperpolarisation). The refractory period ensures one-way, discrete impulses. Action potentials are all-or-nothing; stimulus strength is coded by frequency. Myelin (Schwann cells) insulates the axon so impulses jump between nodes of Ranvier (saltatory conduction), much faster. Speed also rises with axon diameter and temperature. At a cholinergic synapse, Ca²⁺ enters the presynaptic knob, vesicles release acetylcholine by exocytosis; ACh diffuses and binds receptors, opening Na⁺ channels; acetylcholinesterase breaks it down. Synapses allow summation, one-way transmission and inhibition.

Cambridge 9700 Paper 4Edexcel YBI11 WBI15AQA 7402 Paper 2≈ 55 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Explain resting and action potentials.
  • Explain saltatory conduction and factors affecting speed.
  • Describe transmission at a cholinergic synapse.
  • Explain effects of drugs on synapses.
Pause and think

If action potentials are always the same size, how does your brain tell a gentle touch from a hard press?

Section 2 of 14

Core idea

Start hereElectric messages

Touch something hot and you pull away before you feel pain. Action potentials carry that message in milliseconds.

The big idea

Ion channels open and close to make a travelling wave of depolarisation; synapses pass it on with chemical messengers.

Phases of an action potential

Ion movements at each stage.

Worked exampleConduction time

An impulse travels 0.60 m at 30 m s⁻¹.

Goal
Time in ms.
Strategy
t = d/v.
  1. t=0.6030=0.020 st=\frac{0.60}{30}=0.020\ \text{s}
    Distance ÷ speed.
  2. =20 ms=20\ \text{ms}
    ×1000.
Answer20 ms.
✓ CheckMyelinated fibres would be faster ✓.

Summary

  • Na⁺ in, K⁺ out.
  • Saltatory conduction.
  • ACh synapse.
Key words
Action potential
Rapid reversal of membrane potential.
Saltatory conduction
Impulse jumping between nodes of Ranvier.
HistorySquid giant axon. Hodgkin and Huxley recorded action potentials inside a squid's 1 mm-wide giant axon in 1939–1952, earning a Nobel Prize.
Fun factFast as a car. Myelinated human motor neurones conduct at up to about 120 m/s, over 400 km/h.
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