Impulses, synapses and receptors — Edexcel International A Level Biology
Action potentials and saltatory conduction, cholinergic synapses, the pupil reflex, drugs at synapses, rods and rhodopsin, and habituation.
Revision notes
7 short notes, in the order of the specification. Each one in short:
An action potential is a brief reversal of the potential difference across an axon membrane. Sodium channels open, so sodium ions enter and depolarise it; they then close and potassium channels open, so potassium ions leave and repolarise it. Local currents trigger the next region, so the impulse travels along the axon.
In a myelinated axon, the myelin sheath insulates the membrane so depolarisation can occur only at the nodes of Ranvier. Local circuits make the action potential jump from node to node, which is saltatory conduction. This is much faster than conduction along an unmyelinated axon.
A synapse is a junction between two neurones where a neurotransmitter carries the signal across the synaptic cleft. At a cholinergic synapse, calcium ions enter the presynaptic knob and acetylcholine is released, binds to receptors on the postsynaptic membrane and opens sodium channels. Acetylcholinesterase then breaks the acetylcholine down.
The pupil changes size because circular and radial muscles in the iris work antagonistically. In bright light, circular muscles contract and radial muscles relax, so the pupil constricts and protects the retina. In dim light, radial muscles contract and circular muscles relax, so the pupil dilates and lets in more light.
Drugs affect the nervous system by changing nerve impulse transmission. Nicotine mimics acetylcholine, lidocaine blocks voltage-gated sodium channels, cobra venom alpha toxin blocks acetylcholine receptors, L-DOPA is converted to dopamine in Parkinson’s disease, and MDMA increases the amount of serotonin in synapses.
Rod cells in the retina contain rhodopsin, made of opsin and retinal. In the dark, cation channels are open, Na⁺ enters and the rod is depolarised. Light splits rhodopsin, the channels close, the rod hyperpolarises and releases less transmitter, so a bipolar neurone triggers action potentials in the optic neurone.
Habituation
PracticalHabituation is a decrease in the response to a repeated, harmless stimulus. It happens because fewer calcium channels open in the presynaptic knob, so less neurotransmitter is released and the postsynaptic neurone is less likely to reach threshold. It lets an animal ignore irrelevant stimuli and respond to new, important ones.
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Quick check questions
What is the resting potential of an axon, approximately?
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About –70 mV (the inside is negative relative to the outside).
Which ion enters the presynaptic knob and causes vesicles to release acetylcholine?
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Calcium ions (Ca²⁺).
What does acetylcholinesterase do?
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It hydrolyses acetylcholine to choline and ethanoate in the synaptic cleft, ending the stimulation.
What is saltatory conduction?
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Conduction in which the action potential jumps from one node of Ranvier to the next along a myelinated axon.
What happens to a rod cell’s membrane potential when light strikes it?
Show answer
It becomes hyperpolarised, because rhodopsin splits and the cation channels close.
Frequently asked questions
How does an action potential work?
An action potential is a brief reversal of the potential across an axon membrane. Voltage-gated sodium channels open at threshold, so Na⁺ enters and the membrane depolarises. The sodium channels then close and potassium channels open, so K⁺ leaves and the membrane repolarises. Local circuits depolarise the next region, so the impulse travels.
Why do myelinated axons conduct impulses faster?
Myelin insulates the axon so ions cross the membrane only at the nodes of Ranvier. The action potential jumps from node to node by local circuits, which is called saltatory conduction. Less of the membrane has to depolarise, so conduction reaches about 100 m s⁻¹, much faster than the 0.5 to 2 m s⁻¹ in an unmyelinated axon.
How does acetylcholine transmit a signal across a synapse?
An impulse opens calcium channels in the presynaptic knob, and calcium ions make vesicles release acetylcholine by exocytosis. Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium channels. Sodium ions enter and depolarise it. Acetylcholinesterase then breaks the acetylcholine down.
How do rod cells detect light?
Light splits the pigment rhodopsin into opsin and retinal. This closes cation channels in the rod, so sodium ions stop entering and the rod hyperpolarises. It then releases less inhibitory transmitter, so a bipolar neurone is depolarised and passes the signal on, producing action potentials in the optic neurone.
What is habituation?
Habituation is a reduced response to a repeated stimulus that is harmless. Fewer calcium channels open in the presynaptic knob each time, so less neurotransmitter is released and the postsynaptic neurone is less likely to fire. It lets an animal ignore unimportant stimuli and stay alert to new, important ones.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026