synaptic transmission

Question 1

Describe the structure of a synapse

Answer 1

axon
voltage gated calcium ion channel
synaptic cleft
vesicle containing neurotransmitter
axon terminal
receptor and sodium ion channel

see flash card

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 Ca2+ channels
○ Ca2+ 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

Importance - 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 NT
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 NT 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
● More Na+ required for depolarisation
● Reduces likelihood of threshold being met / action potential formation
at post-synaptic membranes

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

Question 10

Describe the structure of a neuromuscular junction

Answer 10

● Receptors are on muscle fibre 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

In both, transmission is unidirectional

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