A person who has a wound to their skin may experience pain. Opioid drugs can be taken to relieve the pain. Opioid receptors are located in the presynaptic membrane of a cholinergic synapse. Fig. 6.1 shows the action of an opioid drug on the presynaptic membrane. [Figure 6.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

1. [5] any five from:
2. [5] 1 opiod binds to receptor;
3. [5] 2 activates G protein;
4. [5] 3 blocks Ca²⁺ channels or no/less, Ca²⁺ enter presynaptic, knob / neurone ;
5. [5] 4 vesicles do not, move towards / fuse with, presynaptic membrane or fewer vesicles, move towards / fuse with, presynaptic membrane; A cell surface/plasma, membrane
6. [5] 5 no/less, Ach / acetylcholinesterase, released into (synaptic) cleft or no/less, exocytosis of ACh ;
7. [5] 6 no/less, Ach / acetylcholinesterase, binds to receptors on postsynaptic membrane ;
8. [5] 7 no/smaller, depolarisation of postsynaptic membrane ;
9. [2] automatic /involuntary / no conscious thought / brain not involved ;
10. [2] response always the same / stereotypic ;
11. [2] (sensory) dorsal root ganglion or (sensory) part way along (neurone) / AW ;
12. [2] and
13. [2] (motor) CNS / spinal cord or (motor) at end (of neurone);
14. [4] sensory
15. [4] 1 transmit, impulses / action potentials, from receptor;
16. [4] 2 to, relay / intermediate, neurone or CNS/spinal cord or motor neurone;
17. [4] motor
18. [4] 3 transmit, impulses / action potentials, from, CNS/spinal cord or relay / intermediate, neurone or sensory neurone ;
19. [4] 4 to effector / muscle / gland ;