HyperpolarisationK+ channels slow to close → briefly more negative than -70 mV
Refractory periodTime after an action potential when sodium channels are inactivated: no new impulse can fire (absolute), or only a stronger stimulus can (relative).
All-or-nothingBelow threshold = nothing; at or above threshold = full AP every time
SynapseGap between two neurones — signal crosses chemically
NeurotransmitterChemical messenger released into synaptic cleft (e.g. acetylcholine)
AChR (cholinergic receptor)Postsynaptic ligand-gated channel — opens to Na+ when ACh binds
AcetylcholinesteraseEnzyme that hydrolyses ACh → acetate + choline (recycled)
Spatial summationSeveral presynaptic neurones add up to reach threshold
Temporal summationRapid repeated firing from one neurone adds up to reach threshold
NeuroneSpecialised cell that transmits electrical impulses
DendriteBranch receiving signals from other neurones
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Learn Coordination & Response step by step
Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board to open its course map, then tap L on this topic:
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What does the 'all-or-nothing' principle mean for action potentials?✗ Stronger stimuli produce larger action potentials.✓ Once threshold is reached, the action potential fires at full size — sub-threshold stimuli produce no action potential.
During an action potential, do Na+ and K+ move through the same channels?✗ Na+ and K+ swap places through the same channel at the same time.✓ No — voltage-gated Na+ channels open first (depolarisation, Na+ in), then voltage-gated K+ channels open (repolarisation, K+ out).
What is the purpose of the refractory period after an action potential?✗ The refractory period exists because the cell runs out of energy and needs a rest.✓ It ensures action potentials travel in one direction only and limits the maximum firing frequency.
What causes the depolarisation phase of an action potential?✗ K+ moves into the cell✓ Na+ moves into the cell
How are signals transmitted across a synapse between two neurones?✗ The electrical signal jumps directly from one neurone to the next across the synaptic gap.✓ Via neurotransmitter chemicals released from the presynaptic neurone, which bind receptors on the postsynaptic neurone.
Practise Coordination & Response — free games
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