Synaptic transmission 3.6.2.2 ( Organisms respond to changed in their internal and external environments 3.6)

Question 1

Describe the structure of a synapse

Answer 1

Pre synaptic neurone :
- axon
- vesicle containing neurotransmitter
- axon terminal
- voltage gated calcium ion channel
• Synaptic cleft
Post synaptic neurone :
- Receptor and sodium ion channel

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 presynaptic membrane causes opening of voltage gated Ca2+ channels
• Ca2+ diffuse into presynaptic neurone / knob
2) causing vesicles containing acetylcholine (ACh) to move and fuse with pre synaptic membrane
• releasing ACh into the synaptic cleft

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 to 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 synapse result in unidirectional nerve impulses

Answer 5

• neurotransmitter only made in / released from presynaptic neurone
• receptors only on postsynaptic membrane

Question 6

explain summation by synapses

Answer 6

• addition of a number of impulses converging on a single post synaptic neurone
• causing rapid build up of neurotransmitter
• so threshold more likely to be reached to generate an action potential

Question 7

what’s the importance of summation by synapses

Answer 7

low frequency action potentials release insufficient neurotransmitter to exceed threshold

Question 8

Describe spatial summation

Answer 8

• many presynaptic neurones share one post synaptic neurone
• collectively release sufficient neurotransmitter to reach threshold to trigger an action potential

Question 9

Describe temporal summation

Answer 9

• one presynaptic neurone releases neurotransmitter many times over a short time
• sufficient neurotransmitter to reach threshold to trigger an action potential

Question 10

Describe inhibition by inhibitory synapses

Answer 10

• inhibitory neurotransmitters hyperpolarise postsynaptic membrane as :
  • Cl- channels open so Cl- diffuse in
  • K+ channels open so K+ diffuse out
• This means inside of the 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 membrane

Question 11

what is the importance of inhibitory synapses

Answer 11

both excitatory and inhibitory neurone forming synapses with the same postsynaptic membrane gives control of whether it fires an action potential

Question 12

Describe the structure of a neuromuscular junction

Answer 12

Very similar to a synapse except :
- receptors are on muscle fibre sarcolemma instead of post synaptic membrane and there are more
- muscle fibres form clefts to store enzyme e.g. acetylcholinesterase to break down neurotransmitter

Question 13

Compare transmission across cholinergic synapses and neuromuscular junctions

Answer 13

In both : transmission is unidirectional

Cholingeric synapse :
- Neurone to neurone
- 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 14

Use examples to explain the effect of drugs on a synapse

Answer 14

• some drugs stimulate the nervous system , leading to more action potentials e.g
  • similar shape to neurotransmitter
  • stimulate release of more neurotransmitter
  • inhibit enzymes that break down neurotransmitter - na+ continues to enter
• Some drugs inhibit the nervous system , leading to fewer action potentials e.g.
  • inhibit release of neurotransmitter e.g prevent opening of calcium ion channels
  • block receptors by mimicking shape of neurotransmitter