Cholinergic synaptic transmission

A cholinergic synapse is a junction where one neurone communicates with another using the neurotransmitter acetylcholine. The two neurones do not touch: a narrow synaptic cleft separates the presynaptic membrane of the signalling neurone from the postsynaptic membrane of the receiving neurone.

The presynaptic terminal contains vesicles filled with acetylcholine and mitochondria that supply energy for processes in the terminal. The postsynaptic membrane contains receptors specific to acetylcholine.

Acetylcholine release and the postsynaptic response

1. An action potential arrives at the presynaptic terminal and opens voltage-gated calcium ion channels in its membrane.
2. Calcium ions ($$\ce{Ca^{2+}}$$) enter the terminal. This causes acetylcholine-containing vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the cleft by exocytosis.
3. Acetylcholine diffuses across the cleft and binds to its receptors on the postsynaptic membrane. This opens ligand-gated sodium ion channels.
4. Sodium ions ($$\ce{Na+}$$) enter the postsynaptic neurone, causing a local depolarisation called an excitatory postsynaptic potential. If the depolarisation reaches threshold, an action potential can be generated in the postsynaptic neurone.

Ending the acetylcholine signal

The enzyme acetylcholinesterase hydrolyses acetylcholine in the synaptic cleft. Acetylcholine therefore stops acting on its receptors, ending the signal.

Choline from the breakdown is taken back into the presynaptic terminal and used to resynthesise acetylcholine. The acetylcholine is then packaged into vesicles for release again.

Transmission is unidirectional: acetylcholine is released from the presynaptic terminal, while its receptors are located on the postsynaptic membrane.