A level Biology B · Edexcel 9BI0 · Topic 9D
Nerve impulses and synapses
Key points
- Resting potential is about −70 mV, kept by the sodium–potassium pump (3 Na out, 2 K in) and K leaking out.
- Action potential: a stimulus opens voltage-gated Na channels, Na floods in (depolarisation); then K channels open and K leaves (repolarisation), overshooting (hyperpolarisation).
- It is all-or-nothing; stronger stimuli give more frequent impulses. The refractory period makes impulses one-way and separate.
- Conduction is faster in myelinated axons (saltatory conduction between nodes of Ranvier), wider axons and at higher temperatures.
- At a synapse, Ca enters the presynaptic knob, vesicles release neurotransmitter (e.g. acetylcholine) which binds receptors and opens Na channels on the postsynaptic membrane.
- Synapses allow summation, inhibition and one-way transmission; many drugs act on synapses.
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Neurones carry information as action potentials: brief reversals of the voltage across the membrane, caused by ions moving through channel proteins. At synapses the electrical signal becomes chemical, which allows signals to be combined…
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