3.6.2.2 Synaptic transmission

Question 1

Describe the structure of a synapse.

Answer 1

A synapse consists of:

-presynaptic neuron,
-a synaptic cleft,
-and a postsynaptic neuron.

-The presynaptic neuron has synaptic vesicles containing neurotransmitters,

-and the postsynaptic neuron has specific receptor proteins on its membrane.

Question 2

Describe the structure of a neuromuscular junction.

Answer 2

-A neuromuscular junction is the synapse between a motor neuron and a muscle fibre.

-It includes a presynaptic neuron with synaptic vesicles containing acetylcholine, a synaptic cleft, and a postsynaptic membrane (sarcolemma) with acetylcholine receptors.

Question 3

Explain the sequence of events in transmission across a cholinergic synapse.

Answer 3

1. Action potential arrives at the presynaptic neuron, causing voltage-gated calcium ion channels to open.
2. Calcium ions diffuse into the presynaptic neuron.
3. Synaptic vesicles fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis.
4. Acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.
5. Sodium ion channels open, causing sodium ions to diffuse into the postsynaptic neuron, leading to depolarisation.
6. If the threshold is reached, an action potential is generated in the postsynaptic neuron.

Question 4

How does unidirectionality occur at a synapse?

Answer 4

Unidirectionality occurs because neurotransmitters are released only from the presynaptic neuron, and receptors are located only on the postsynaptic membrane.

Question 5

What is temporal summation, and how does it work?

Answer 5

-Temporal summation occurs when multiple impulses arrive at the same presynaptic neuron in rapid succession.

-This increases the release of neurotransmitter, leading to enough depolarisation to reach the threshold in the postsynaptic neuron.

Question 6

What is spatial summation, and how does it work?

Answer 6

-Spatial summation occurs when multiple presynaptic neurons release neurotransmitters simultaneously onto the same postsynaptic neuron.

-This combined effect increases depolarisation, potentially reaching the threshold.

Question 7

How do inhibitory synapses prevent action potentials?

Answer 7

-Inhibitory synapses release neurotransmitters that bind to receptors on the postsynaptic membrane, opening channels for chloride ions to diffuse in or potassium ions to diffuse out.

-This hyperpolarises the postsynaptic membrane, making it less likely to reach the threshold.

Question 8

Compare transmission across a cholinergic synapse and a neuromuscular junction.

Answer 8

Both involve acetylcholine as the neurotransmitter and rely on similar mechanisms of release and receptor binding. However, a cholinergic synapse occurs between two neurons, while a neuromuscular junction occurs between a motor neuron and a muscle fibre.

At a neuromuscular junction, the response is always excitatory and leads to muscle contraction, whereas a cholinergic synapse may be excitatory or inhibitory.

Question 9

How can drugs affect synaptic transmission?

Answer 9

Drugs can affect synapses by mimicking neurotransmitters (agonists), blocking receptors (antagonists), inhibiting neurotransmitter breakdown enzymes, or affecting the reuptake of neurotransmitters.

For example, an agonist increases the likelihood of an action potential in the postsynaptic neuron, while an antagonist decreases it.

Question 10

Describe the sequence of events involved in transmission across a cholinergic synapse.

Answer 10

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 11

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

Answer 11

1. (Inside of postsynaptic) neurone becomes more
   negative/hyperpolarisation/inhibitor postsynaptic potential;
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 likel

Question 12

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

Answer 12

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);