OxfordAQA International A Level Biology 3.4.3 Nerve Impulses and Synaptic Transmission Exam Questions

About OxfordAQA International A Level Biology 3.4.3 Nerve Impulses and Synaptic Transmission

This Unit 4 topic explains how nerve impulses are generated and passed between cells. You need to describe the structure of a myelinated motor neurone, explain the resting potential and the events of an action potential, including depolarisation, repolarisation, the all-or-nothing principle and the refractory period. You also need to explain how myelination, axon diameter and temperature affect the speed of conduction. The synapse section covers cholinergic synapses and neuromuscular junctions, unidirectionality, temporal and spatial summation and inhibitory synapses. You should be able to predict the effects of drugs on synapses from information given. It is one of the most heavily examined Unit 4 topics.

What you need to know

• Resting potential The resting potential is about -70 mV. Sodium-potassium pumps actively move three sodium ions out for every two potassium ions in, and the membrane is more permeable to potassium, which diffuses out, so the inside is negative.
• Action potential A stimulus opens voltage-gated sodium channels, sodium ions diffuse in and the membrane depolarises to about +40 mV. Sodium channels then close and potassium channels open, so potassium ions diffuse out and the membrane repolarises, with a brief hyperpolarisation.
• All-or-nothing and refractory period If the threshold is reached an action potential of the same size is produced; a stronger stimulus increases the frequency of action potentials. The refractory period makes impulses separate and unidirectional and limits their frequency.
• Speed of conduction Myelination allows saltatory conduction, where depolarisation occurs only at nodes of Ranvier. Wider axons and higher temperatures (up to a point) also increase speed.
• Cholinergic synapse An action potential opens calcium channels in the presynaptic knob, calcium ions enter and vesicles release acetylcholine by exocytosis. Acetylcholine diffuses across the cleft, binds to receptors and opens sodium channels in the postsynaptic membrane. Acetylcholinesterase then hydrolyses it.
• Summation and inhibition Spatial summation is when several presynaptic neurones together release enough neurotransmitter to reach threshold; temporal summation is when one neurone fires repeatedly. Inhibitory synapses cause chloride ions to enter or potassium ions to leave, hyperpolarising the postsynaptic membrane.

Common mistakes in 3.4.3 Nerve Impulses and Synaptic Transmission

• Saying the sodium-potassium pump causes depolarisation. Depolarisation is caused by sodium ions diffusing in through voltage-gated channels.
• Saying a bigger stimulus gives a bigger action potential. Action potentials are all-or-nothing; a bigger stimulus gives more frequent impulses.
• Saying the impulse jumps along the myelin sheath. Depolarisation jumps from node to node, where there is no myelin.
• Saying acetylcholine enters the postsynaptic neurone. It binds to receptors on the membrane and is then broken down.

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