3.6.2.2 - Synaptic transmission

Question 1

What are cholinergic synapses?

Answer 1

Synapses that use the neurotransmitter acetylcholine (ACh)

Question 2

Describe transmission across a cholinergic synapse

Answer 2

> At pre-synaptic
neurone
1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ ion channels
○ Ca2+ ions diffuse into pre-synaptic neurone / knob
2. Causing vesicles containing Acetyl choline to move and fuse with pre-synaptic membrane
○ Releasing Acetyl choline into the synaptic cleft (by exocytosis)

> At post-synaptic neurone
3. Acetyl choline diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
4. Causing Na+ ion channels to open
○ Na+ ions diffuse into post-synaptic knob causing depolarisation
○ If threshold is met, an action potential is initiated

Question 3

Explain what happens to acetylcholine after synaptic transmission

Answer 3

● It is hydrolysed by acetylcholinesterase
● Then the Products are reabsorbed by the presynaptic neurone
● To stop overstimulation - if not removed it would keep binding to receptors, causing continuous depolarisation

Question 4

Explain how synapses result in unidirectional nerve impulses

Answer 4

● Neurotransmitter only made in / released from pre-synaptic neurone

● Receptors only on post-synaptic membrane

Question 5

Explain summation by synapses

Answer 5

● Addition of a number of impulses converging on a single post-synaptic neurone
● Causing rapid buildup of neurotransmitter (NT)
● So threshold more likely to be reached to generate an action potential

Question 6

Importance of summation by synapses

Answer 6

low frequency action potentials release insufficient neurotransmitter to exceed threshold

Question 7

Describe spatial summation

Answer 7

● Many pre-synaptic neurones share
one synaptic cleft / post-synaptic neurone
● Collectively release sufficient neurotransmitter to reach threshold
to trigger an action potential

Question 8

Describe temporal summation

Answer 8

● One pre-synaptic neurone releases
neurotransmitter many times over a short period of time
● So Sufficient neurotransmitter is released to reach
threshold to trigger an action potential

Question 9

Describe inhibition by inhibitory synapses

Answer 9

● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ Cl- ion channels open
→ allowing Cl- ions to diffuse in
○ K+ ion channels open
→allowing K+ ions to diffuse out

● This means inside of axon has a more negative charge relative to outside / below resting potential

● So more Na+
required to enter for depolarisation

● This reduces likelihood of threshold being met / action potential formation at post-synaptic membranes

Question 10

Importance of inhibition by inhibitory synapses

Answer 10

• both excitatory and inhibitory neurones forming synapses with the same post-synaptic membrane gives control of whether it ‘fires’ an action potential

Question 11

How does a neuromuscular junction differ from a synapse

Answer 11

Very similar to a synapse except:

● Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter

Question 12

Compare transmission across cholinergic synapses and neuromuscular
junctions

Answer 12

> Both:

• transmission is unidirectional

> differences

• Transmission across cholinergic synapse is from Neurone to neurone (or effectors, glands),
• Transmission across neuromuscular junction is from (Motor) neurone to muscle
• In the Transmission across cholinergic synapse, Neurotransmitters can be excitatory or inhibitory,
• In the transmission across a neuromuscular junction, Neurotransmitters are always excitatory.
• In the Transmission across cholinergic synapse, Action potential may be initiated in postsynaptic neurone,
• In the transmission across a neuromuscular junction, Action potential propagates along sarcolemma down T tubules

Question 13

Use examples to explain the effect of drugs on a synapse

Answer 13

● Some drugs stimulate the nervous system, leading to more action potentials, eg by:
○ Having Similar shape to neurotransmitter
○ Stimulating release of more neurotransmitter
○ Inhibiting enzyme that breaks down neurotransmitter
→ so Na+ continues to enter

● Some drugs inhibit the nervous system, leading to fewer action potentials, eg by:
○ Inhibiting release of neurotransmitter eg: by preventing opening of calcium ion channels
○ Block receptors by mimicking the shape of neurotransmitter