Describe and explain how a nerve impulse is transmitted across a cholinergic synapse.

An action potential from the presynaptic neurone arrives at the presynaptic knob. This process causes voltage-gated Calcium ion channels in the presynaptic membrane to open. Calcium ions then diffuse into the presynaptic knob. The influx of calcium ions causes vesicles (containing the neurotransmitter acetylcholine) to move towards the presynaptic membrane. The vesicles fuse with the presynaptic membrane and release the neurotransmitter acetylcholine into the synaptic cleft. Acetylcholine then diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. This causes sodium ion channels in the postsynaptic membrane to open. An influx of sodium ions into the postsynaptic neurone causes depolarisation across the postsynaptic membrane. If threshold is reached, an action potential is initiated in the postsynaptic neurone. To prevent continual action potentials the acetylcholine neurotransmitter is broken down by an enzyme called acetylcholinesterase. The products are taken back up across the presynaptic membrane and are used to synthesise more acetylcholine.