6.2.7 Cholinergic Synapses

Key Features of a Cholinergic Synapse

1. Presynaptic Neurone:

• Contains vesicles filled with acetylcholine (ACh).
• When an action potential arrives at the synaptic knob, voltage-gated calcium ion channels open, allowing $Ca²⁺$ ions to diffuse into the knob.

2. Synaptic Cleft:

• A gap (20–30 nm) between the presynaptic and postsynaptic membranes.
• Acetylcholine is released into this cleft by exocytosis and diffuses across to the postsynaptic membrane.

3. Postsynaptic Neurone:

• The postsynaptic membrane contains specific receptors that bind acetylcholine.
• Binding opens ligand-gated sodium ion channels, causing Na⁺ ions to enter and depolarise the membrane.

4. Enzyme Action:

• Acetylcholine is broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate).
• These products are reabsorbed by the presynaptic neurone to resynthesise acetylcholine using ATP.

The Process of Transmission at a Cholinergic Synapse

5. Arrival of Action Potential:

• An action potential reaches the presynaptic neurone, opening voltage-gated $Ca²⁺$ channels.
• $Ca²⁺$ ions enter the synaptic knob.

6. Release of Acetylcholine:

• The influx of $Ca²⁺$ causes vesicles containing acetylcholine to move to the presynaptic membrane.
• Acetylcholine is released into the synaptic cleft by exocytosis.

7. Binding to Receptors:

• Acetylcholine diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
• This opens sodium ion channels, allowing Na⁺ ions to diffuse into the postsynaptic neurone, leading to depolarisation.

8. Generation of a New Action Potential:

• If the depolarisation reaches the threshold, a new action potential is generated in the postsynaptic neurone.

9. Recycling of Acetylcholine:

• Acetylcholinesterase breaks down acetylcholine into choline and acetate, preventing continuous stimulation.
• Choline and acetate are reabsorbed by the presynaptic neurone and recombined into acetylcholine using ATP.

Importance of Cholinergic Synapses

• Excitatory or inhibitory responses:
  • Acetylcholine can either stimulate an action potential (excitatory) or inhibit one (inhibitory) depending on the receptor type.
• Prevention of overstimulation:
  • The breakdown of acetylcholine ensures the response is brief and prevents overstimulation of the postsynaptic neurone or effector.