6.3 - Synaptic transmission

Question 1

Define synapse

Answer 1

junction between 2 neurones

Question 2

Describe the role of synapses

Answer 2

• prevent A.P travelling in wrong direction: neurotransmitter only made in presynaptic neurone, receptors only on postsynaptic
• summation: amplify effects of low frequency A.Ps
• can be inhibitory or excitatory

Question 3

Describe the 2 types of summation in synapses

Answer 3

temporal: 1 presynaptic neurone releases neurotransmitter many times over a short period

spatial: multiple presynaptic neurones release neurotransmitter

(to reach threshold value)

Question 4

Describe the transmission of an action potential across a synapse

Answer 4

• A.P arrives in presynaptic neurone, causing membrane to depolarise
• Ca2+ voltage gated channels open allowing Ca2+ ions to diffuse into presynaptic neurone down conc gradient
• causes vesicles containing neurotransmitter to move towards and fuse with presynaptic membrane
• neurotransmitter released into synaptic cleft
• diffuses across and binds to receptors on postsynaptic membrane
• stimulates opening of Na+ ion channels on postsynaptic membrane
• Na+ ions diffuse into postsynaptic neurone from presynaptic neurone
• if threshold reached, A.P generated in postsynaptic neurone
• acetylcholinesterase hydrolyses acetylcholine -> choline + acetyl, which diffuses back across synaptic cleft into presynaptic neurone to be reassembled and reused

Question 5

What is the benefit of acetylcholine hydrolysis and return to presynaptic neurone?

Answer 5

prevents continuous generation of A.P in postsynaptic neurone

Question 6

How do inhibitory synapses work?

Answer 6

• Cl- ion channels open: cause Cl- ions to enter postsynaptic neurone
• K+ ion channels open: cause K+ ions to leave
• hyperpolarisation occurs, axon potential below resting potential so threshold not met so A.P unlikely
• more Na+ required for depolarisation

Question 7

Describe the structure of a neuromuscular junction

Answer 7

• motor neurone
• muscle

Question 8

Compare and contrast neuromuscular junctions and cholinergic synapses

Answer 8

NM:
- unidirectional
- excitatory only
- connects motor neurone to muscles
- end point for A.P
- acetylcholine binds to receptors on muscle fibre membranes (sarcolemma) and A.P travels down T-tubules
- at end, Ca2+ ions diffuse out of sarcoplasmic reticulum

CS:
- unidirectional
- excitatory or inhibitory
- connects 2 neurones
- new A.P generated in next neurone
- acetylcholine binds to receptors on post-synaptic membrane of neurone

Question 9

Describe the transmission of an action potential across a neuromuscular junction

Answer 9

• depolarisation in axon terminal as A.P arrives
• Ca2+ voltage gated channels open allowing Ca2+ ions to diffuse into axon terminal down conc gradient
• causes vesicles containing neurotransmitter to move towards and fuse with axon terminal membrane
• neurotransmitter released into synaptic cleft
• diffuses across and binds to receptors on sarcolemma
• stimulates opening of Na+ ion channels on sarcolemma
• Na+ ions diffuse into muscle fibre from axon terminal
• muscle fibre depolarised which moves down T-tubules
• if threshold reached, generator potential produces A.P causing muscle contraction
• Ca2+ ions diffuse out of sarcoplasmic reticulum etc.