Cholinergic synapses, drugs and toxins
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 3 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The gap between cells
- Structure of a synapse
- Transmission at a cholinergic synapse
- Removing the transmitter
- A past-paper example
Read the transcript of the teaching part
Last time
Before we start: last time, in The kidney, ultrafiltration and ADH, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The gap between cells
An action potential cannot jump from one neurone to the next, because there is a tiny gap between them. So the electrical signal is converted into a chemical one, and then back again. The chemical here is acetylcholine, and many drugs and poisons work by interfering with it at just one step.
Structure of a synapse
A synapse is a junction between two neurones, or between a neurone and an effector. A gap of about 20 nanometres, the synaptic cleft, separates the cells. The presynaptic knob has many mitochondria and vesicles full of neurotransmitter. It has voltage gated calcium channels in its membrane. The postsynaptic membrane has receptors for the neurotransmitter, joined to sodium ion channels.
Transmission at a cholinergic synapse
An action potential arrives at the presynaptic knob and opens the voltage gated calcium channels. Calcium ions diffuse in. Calcium causes vesicles of acetylcholine to fuse with the membrane and release it into the cleft by exocytosis. Acetylcholine diffuses across and binds to receptors on the postsynaptic membrane. Sodium channels open and sodium ions enter, depolarising the membrane. If the threshold is reached, an action potential starts in the postsynaptic neurone.
Removing the transmitter
The enzyme acetylcholinesterase in the cleft breaks acetylcholine into choline and ethanoic acid. The sodium channels close, and the response ends. Choline is taken back into the presynaptic knob, and resynthesised into acetylcholine using A T P from the mitochondria. This allows precise control, with each action potential causing a single, short response.
A past-paper example
May June 2023, Paper 43, question 9. Dopamine is released at some synapses in the brain, in a similar way to acetylcholine. Describe how its release is caused. An action potential opens voltage gated calcium channels. Calcium ions enter, and vesicles fuse with the presynaptic membrane, releasing the neurotransmitter. The neurotransmitter then diffuses across the cleft and binds to receptors.
Chapter 15: Coordination
- Stomata, auxin and gibberellin 6 min
- Receptors, neurones and the Venus fly trap 6 min
- Resting potential, action potentials and myelin 6 min
- Cholinergic synapses, drugs and toxins 6 min
- The endocrine system compared with the nervous system 6 min
- Striated muscle and the sliding filament model 5 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.