3.6.4: Synaptic Transmission

Question 1

Describe the sequence of events involved
in transmission across a cholinergic
synapse.
Do not include details on the breakdown
of acetylcholine in your answer.

Answer 1

1. Depolarisation of presynaptic membrane;
   Accept action potential for depolarisation.
2. Calcium channels open and calcium ions enter (synaptic knob);
   Accept Ca2+
   .
3. (Calcium ions cause) synaptic vesicles move to/fuse with
   presynaptic membrane and release acetylcholine/neurotransmitter;
   Accept abbreviations for acetylcholine as term is in the question.
4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
   Accept abbreviations for acetylcholine as term is in the question.
5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;
6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 2

Describe how the influx negatively
charged ions can inhibit postsynaptic
neurones.

Answer 2

1. (Inside of postsynaptic) neurone becomes more
   negative/hyperpolarisation/inhibitory postsynaptic potential;
   Ignore K+
   Accept -75mV or any value below this as equivalent to more negative
   Accept ‘decrease in charge’
2. More sodium ions required (to reach threshold)
   OR
   Not enough sodium ions enter (to reach threshold);
   Accept Na+ for sodium ions
3. For depolarisation/action potential;
   Context must covey idea that depolarisation / action potential is less
   likely

Question 3

Explain how blocking the calcium ion channels
at some synapses can reduce impulses at the
post synaptic membrane.

Answer 3

1. No/fewer calcium ions enter synaptic knob OR No/less calcium enter synaptic knob via
   calcium ion channels;
2. No/fewer synaptic vesicles move to/fuse with presynaptic membrane and no/less
   glutamate is released;
3. No/less neurotransmitter diffuses across (synaptic cleft);
4. No/less (neurotransmitter attaches) to receptors on the postsynaptic membrane;
5. No/fewer sodium ions enter (postsynaptic neurone) so no/ fewer impulses (sent to
   brain);