6.2.2 Synaptic transmission

Question 1

Describe the structure of a synapse

Answer 1

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Question 2

What are cholinergic synapses?

Answer 2

Synapses that use the neurotransmitter acetylcholine (ACh)

Question 3

Describe transmission across a cholinergic synapse

Answer 3

At pre-
synaptic
neurone :

1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca

2+ channels

○ Ca
2+ diffuse into pre-synaptic neurone / knob

2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane
   ○ Releasing ACh into the synaptic cleft (by exocytosis)At pre-
   synaptic

neurone

1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca

2+ channels

○ Ca
2+ diffuse into pre-synaptic neurone / knob

2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane
   ○ Releasing ACh into the synaptic cleft (by exocytosis)

At post-
synaptic neurone :

3. ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
4. Causing Na

+ channels to open
○ Na
+ diffuse into post-synaptic knob causing depolarisation
○ If threshold is met, an action potential is initiated

Question 4

Explain what happens to acetylcholine after synaptic transmission

Answer 4

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

Question 5

Explain how synapses result in unidirectional nerve impulses

Answer 5

● Neurotransmitter only made in / released from pre-synaptic neurone
● Receptors only on post-synaptic membrane

Question 6

Explain summation by synapses

Answer 6

● 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 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 time
● Sufficient neurotransmitter to reach
threshold to trigger an action
potential

Question 9

Describe inhibition by inhibitory synapses

Answer 9

● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ Cl
- channels open → Cl
- diffuse in

○ K
+ channels open → K

+ 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

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

Question 10

Describe the structure of a neuromuscular junction

Answer 10

● 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 11

Compare transmission across cholinergic synapses and neuromuscular
junctions

Answer 11

Cholinergic synapse :
- Neurone to neurone (or effectors, glands)
- Neurotransmitters can be excitatory or inhibitory
- Action potential may be initiated in postsynaptic neurone

Neuromuscular junction
- (Motor) neurone to muscle
- Always excitatory
- Action potential propagates along sarcolemma down T tubules

Question 12

Use examples to explain the effect of drugs on a synapse

Answer 12

● Some drugs stimulate the nervous system, leading to more action potentials, eg.:
○ Similar shape to neurotransmitter
○ Stimulate release of more neurotransmitter
○ Inhibit enzyme that breaks down neurotransmitter → Na
+ continues to enter

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