Transmission Across a Cholinergic Synapse

Question 1

What happens when an action potential arrives at the presynaprtic neurone?

Answer 1

Calcium ion protein channels open, allowing calcium ions to enter the synaptic knob b y facilitated diffusion

Question 2

What does the influx of calcium ions cause?

Answer 2

It triggers synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft

Question 3

What happens when acetylcholine binds to receptors on the postsynaptic membrane?

Answer 3

Sodium in channels open, allowing sodium ions to diffuse in rapidly, triggering depolarisation

Question 4

How is acetylcholine removed from the synapse?

Answer 4

It is broken down by acetylcholinesterase into choline and ethanoic acid (acetyl), which diffuse back into the presynaptic neurone

Question 4

How does acetylcholine cross the synaptic cleft?

Answer 4

It diffuses rapidly due to the short diffusion pathway and binds to receptor sites on sodium ion protein channels in the postsynaptic membrane

Question 5

How is a new action potential generated in the postsynaptic neurone?

Answer 5

The influx of sodium ions depolarises the membrane, reaching the threshold value and triggering an action potential

Question 6

Why is acetylcholine broken down after transmission?

Answer 6

To prevent continuous stimulation of the presynaptic neurones, ensuring discrete nerve impulses

Question 7

What happens to choline and ethanoic acid after being recycled?

Answer 7

ATP from mitochondria is used to recombine them into acetylcholine, which is then stored in synaptic vesicles for future use

Question 8

What happens to sodium ion channels when acetylcholine is removed?

Answer 8

They close, stopping sodium ions influx and restoring the resting potential of the postsynaptic neurone

Question 9

Outline the stages of transmission across a cholinergic synapse

Answer 9

1. Arrival of action potential at the end of presynaptic neurone causes calcium ion protein channels to open and calcium ions to enter the synaptic knob by facilitated diffusion
2. The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
3. Acetylcholine molecules diffuse across the narrow synaptic cleft. It then binds to receptor sites on sodium ion protein channels in the membrane of the postsynaptic neurone, which causes sodium ion protein channels to open allowing sodium ions to diffuse rapidly along a concentration gradient
4. Influx of sodium ions generates a new action potential in the postsynaptic neurone
5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuses back across the synaptic cleft intot he presynaptic neurone.
   This rapid breakdown prevents acetylcholine from continuously generating a new action potential in the postsynaptic neurone and leads to discrete transfer of information across synapses
6. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion protein channels close in the absence of acetylcholine in the receptor sites