5.3.9 Transmission Across a Cholinergic Synapse

Question 1

What are cholinergic synapses?

Answer 1

Synapses that use the neurotransmitter acetylcholine (ACh)

Question 2

What is the process of synaptic transmission using ACh?

Answer 2

1. Arrival of action potential at presynaptic membrane causes depolarisation of membrane
2. Stimulates voltage-dated calcium ion channel proteins to open
3. Ca2+ diffuse down electrochemical gradient from tissue fluid surrounding synapse into presynaptic knob
4. ACh containing vesicles stimulated to fuse with presynaptic membrane releasing ACh molecules into synaptic cleft by exocytosis
5. ACh molecules diffuse across synaptic cleft and temporarily bind to cholinergic receptors in post synaptic membrane
6. Na+ channels open
7. Na+ diffuses down electrochemical gradient from synaptic cleft to cytoplasm of postsynaptic neurone
8. Na+ causes depolarisation of postsynaptic membrane restarting electrical impulse once threshold of action potential is reached
9. ACh molecules broken down and recycled (preventing permanent depolarisation of postsynaptic membrane. Acetylcholinesterase catalyses hydrolysis of ACh into acetate and choline
10. Choline absorbed back into presynaptic membrane and reacts with acetyle coenzyme A forming ACh, which is packaged into vesicles ready to be used when another action potential arrives

Question 3

What are some important features of synapses?

Answer 3

• Unidirectionality
• Temporal Summation
• Spatial Summation

Question 4

What is unidirectionality in synapses?

Answer 4

• Impulse can only travel in 1 direction
• Neurotransmitter released on 1 side, receptors on the other
• So chemical transmission cannot occur in other direction
• This way impulses cannot travel wrong way

Question 5

What is temporal summation?

Answer 5

• If multiple impulses arrive in quick succession the effect of the impulses can be added to generate action potential
• Lots of ACh released into synaptic cleft
• Lots of gated ion channels open
• Lots of Na+ passes through membrane

Question 6

What is spatial summation?

Answer 6

• Multiple impulses arriving simultaneously at different synaptic knobs on the same cell body
• Multiple impulses result in lots of ACh released into synaptic cleft
• Lots of gated ion channels open
• Lots of Na+ passes through membrane

Question 7

What are excitatory neurotransmitters?

Answer 7

• Excitatory neurotransmitters stimulate generation of action potential in post synaptic neurone
• Na+ channels open in post synaptic membrane causing depolarisation if action potential is reached

Question 8

What are inhibitory neurotransmitters?

Answer 8

• Prevent generation of action potential in post synaptic neurone
• Open K+ channels causing hyperpolarisation