"$L3f",null,{"backHref":"/gcse/biology/edexcel/questions","basePath":"/gcse/biology/edexcel/the-brain-and-the-nervous-system/questions","formulaUrl":null,"initialIndex":2,"initialQuestionWindow":[{"id":"033585d4-3dd7-4bb7-9afb-815eb60d74f5","revisionId":"0d6dcff1-7269-4e66-ab77-dd28dfe5b010","questionNumber":null,"questionType":"LA","specification":"Botulinum toxin is a highly potent neurotoxin produced by the bacterium *Clostridium botulinum*. At a neuromuscular junction (the synapse between a motor neurone and a muscle cell), this toxin acts by preventing the release of neurotransmitters from the pre-synaptic vesicles.\n\n![Synapse Diagram](https://assets.mathsgenie.co.uk/aqa-qb/c4efcecb-6dfd-4cdd-a3a9-73d30bb76055.png)","difficulty":6,"difficultyBand":"medium","totalMarks":6,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"7bfc0d14-7f91-4c17-8e70-88809d2f2b88","label":null,"content":"Explain how an electrical impulse is normally transmitted across a synapse, and explain how botulinum toxin prevents muscle contraction.","markscheme":"- An electrical impulse reaches the pre-synaptic neurone terminal M1\n- This triggers the release of neurotransmitter molecules from synaptic vesicles A1\n- The neurotransmitters diffuse across the synaptic cleft B1\n- They bind to specific receptor molecules on the post-synaptic membrane to initiate a new electrical impulse C1\n- Botulinum toxin prevents the release of neurotransmitters from pre-synaptic vesicles M2\n- Without neurotransmitters binding to the post-synaptic receptors, no electrical signal/depolarisation occurs in the post-synaptic cell, preventing muscle contraction A2","marks":6,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"normal_transmission_and_toxin","label":"Trace the synaptic signal","steps":[{"type":"text","order":1,"title":"Start with the impulse","content":"An electrical impulse reaches the terminal of the pre-synaptic neurone. This is the starting event in the sequence and earns **M1**.","highlights":[{"text":"how an electrical impulse is normally transmitted across a synapse","location":"specification"}]},{"type":"text","order":2,"title":"Release the transmitter","content":"The arriving impulse triggers synaptic vesicles to release neurotransmitter molecules. A neurotransmitter is a chemical messenger. This precise link earns **A1**.","highlights":[{"text":"preventing the release of neurotransmitters from the pre-synaptic vesicles","location":"specification"}]},{"type":"text","order":3,"title":"Cross the synaptic cleft","content":"The neurotransmitter molecules **diffuse** across the synaptic cleft. Diffusion means particles spread from an area where there are more of them to an area where there are fewer. State that they cross the cleft by diffusion for **B1**.","highlights":[{"text":"synapse between a motor neurone and a muscle cell","location":"specification"}]},{"type":"text","order":4,"title":"Bind to receptors","content":"The neurotransmitter molecules bind to **specific receptor molecules** on the post-synaptic membrane. This initiates a new electrical impulse in the muscle cell, earning **C1**.","highlights":[{"text":"Explain how an electrical impulse is normally transmitted across a synapse","location":"specification"}]},{"type":"text","order":5,"title":"Identify the toxin block","content":"Botulinum toxin prevents neurotransmitter release from the pre-synaptic vesicles. Therefore, neurotransmitter cannot diffuse across the cleft or bind to receptors. This earns **M2**.","highlights":[{"text":"this toxin acts by preventing the release of neurotransmitters from the pre-synaptic vesicles","location":"specification"}]},{"type":"text","order":6,"title":"Link toxin to paralysis","content":"Because no neurotransmitter is released, none can bind to receptors on the muscle cell. Therefore no new electrical signal (depolarisation) is produced in that cell, so it cannot contract — this earns **A2**.","highlights":[{"text":"Explain how botulinum to

The brain and the nervous system

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The brain and the nervous system Questions

The brain and the nervous system

Question 2

The brain and the nervous system Questions