6.2.8 Transmission Across a Cholinergic Synapse

Steps in Transmission Across a Cholinergic Synapse

1. Arrival of Action Potential:

• An action potential travels along the presynaptic neurone to the synaptic knob.
• The depolarisation causes voltage-gated calcium ion $(Ca²⁺)$ channels to open.

2. Influx of Calcium Ions:

• $Ca²⁺$ions diffuse into the synaptic knob from the synaptic cleft.
• The influx of calcium triggers the movement of synaptic vesicles towards the presynaptic membrane.

3. Release of Acetylcholine (ACh):

• The vesicles fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft via exocytosis.

4. Binding to Postsynaptic Receptors:

• Acetylcholine diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
• This binding causes ligand-gated sodium ion $(Na⁺)$ channels to open.

5. Depolarisation of the Postsynaptic Membrane:

• Sodium ions diffuse into the postsynaptic neurone, causing depolarisation.
• If the depolarisation reaches the threshold potential, an action potential is generated in the postsynaptic neurone.

6. Enzymatic Breakdown of Acetylcholine:

• The enzyme acetylcholinesterase in the synaptic cleft breaks down acetylcholine into choline and acetate.
• This prevents continuous stimulation of the postsynaptic neurone.

7. Recycling of Acetylcholine Components:

• Choline is reabsorbed by the presynaptic neurone and combined with acetate to resynthesise acetylcholine, using ATP.

Key Features of Transmission

• Unidirectionality:
  • Transmission occurs in one direction only as neurotransmitter vesicles are found only in the presynaptic neurone, and receptors are on the postsynaptic membrane.
• Discrete Transmission:
  • The breakdown of acetylcholine ensures the response is temporary, allowing the synapse to reset for the next signal.
• Summation:
  • Spatial summation: Multiple presynaptic neurones release neurotransmitter to exceed the threshold.
  • Temporal summation: A single presynaptic neurone releases neurotransmitter repeatedly in quick succession to reach the threshold.