SynapsesEdexcel A-Level Biology A: Mind map

Exam questions on Synapses

1. A farm worker is exposed to an organophosphate pesticide, a chemical that inhibits acetylcholinesterase. Within hours she develops uncontrollable muscle twitching, because the cholinergic synapses at her neuromuscular junctions are being stimulated continuously.
   Explain why the pesticide causes the muscle fibres to be stimulated continuously.2 marks
2. Botulinum toxin is injected in very small doses to treat painful muscle spasm. It prevents the vesicles in the presynaptic knobs of motor neurones from fusing with the presynaptic membrane, so acetylcholine is not released.
   Explain how the toxin causes the muscle to relax.2 marks
3. Researchers investigating a neuromuscular junction removed calcium ions from the fluid surrounding an isolated motor neurone and muscle fibre. When the neurone was stimulated, an action potential still reached the end of the neurone but the muscle fibre no longer contracted. Contraction returned when calcium ions were restored to the fluid.
   Explain why removing calcium ions from the fluid prevented the muscle fibre from contracting.3 marks