Explain how an action potential is transmitted across a cholinergic synapse.

The action potential arrives at the synaptic knob of the presynaptic neurone. The action potential stimulates the opening of voltage-gated calcium ion channels in the presynaptic neurone, allowing calcium ions to diffuse into the synaptic knob. The influx of calcium causes synaptic vesicles containing acetyl choline (ACh) to move towards the presynaptic membrane. These vesicles fuse with the membrane and release ACh into the synaptic cleft by exocytosis. ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane. This causes sodium ion channels to open on the postsynaptic neurone. Sodium diffuses into the postsynaptic neurone, causing depolarisation. If there is a sufficient influx of sodium, the depolarisation will be sufficient to reach the threshold potential and an action potential will be generated on the postsynaptic membrane. ACh is broken down in the synaptic cleft by Acetylcholinesterase (AChE) and the products are reabsorbed into the presynaptic neurone to make more ACh.