Transmission across a Cholinergic Synapse 5.3.9

Question 1

What is the key neurotransmitter used throughout the nervous system?

Answer 1

Acetylcholine

Question 2

What are synapses called that use ACh?

Answer 2

Cholinergic synapses

Question 3

State the detailed process of transmission of ACh across a synapse

Answer 3

1. Arrival of action potential at presynaptic causes depolarisation of membrane
2. Stimulates voltage gated calcium ion channels to open
3. Calcium ions diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse into the synaptic knob
4. Stimulates ACh containing vesicles to fuse with the presynaptic membrane, releasing ACh by exocytosis
5. ACh diffuses across the synaptic cleft and temporarily bind to cholinergic receptors in the postsynaptic membrane
6. This causes sodium ion channels to open
7. Sodium ions diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone
8. Sodium ions cause depolarisation of the post synaptic membrane - initiating the action potential
9. ACh molecules are broken down and recycled by acetylcholinesterase into acetate and choline
10. Choline is absorbed back into the presynaptic neurone and reacts with acetyl coenzyme A to form ACh

Question 4

Describe the unidirectionality of synapses

Answer 4

• Synapses ensure one way transmission of impulses
• Impulses can only pass in one direction because neurotransmitter is released on one side and the receptors are on the other

Question 5

Why is a single impulse sometimes insufficient to generate an action potential?

Answer 5

As only a small amount of ACh is released, meaning only a small amount of gated ion channels are opened in the axon membrane and threshold value is not exceeded

Question 6

What is summation?

Answer 6

The effect of multiple impulses can be added together

Question 7

What are the benefits of summation?

Answer 7

• Allows for the effect of a stimulus to be magnified
• A combination of different stimuli can trigger a response
• It avoids the nervous system being overwhelmed by impulses

Question 8

What are the two types of summation?

Answer 8

• Temporal
• Spatial

Question 9

What is temporal summation?

Answer 9

If multiple impulses arrive with quick succession the effect of the impulses can be added together to generate an action potential

Question 10

What is spatial summation?

Answer 10

Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential

Question 11

What are excitatory neurotransmitters?

Answer 11

They stimulate the generation of an action potential in a post synaptic neurone.
This is done by opening sodium ion channels in the post synaptic membrane which causes depolarisation if a threshold is reached

Question 12

What are inhibitory neurotransmitters?

Answer 12

They prevent the generation of an action potential in a post synaptic neurone
They do this by opening potassium ion channels in the post synaptic membrane which causes hyperpolarisation of the membrane