Describe the process by which an action potential is propagated across a cholinergic synapse

A cholinergic synapse is one which uses acetycholine (ACh) as it's neurotransmitter. When the action potential reaches the synaptic knob at the presynaptic cell it leads to voltage gated Ca²⁺channels in the presynaptic membrane opening. This allows an influx of Ca²⁺ions into the presynaptic cell via facilitated diffusion. Facilitated diffusion occurs when ions move from a region of higher concentration to an area of lower concentration, as in simple diffusion. However in facilitated diffusion this movement only occurs via a transmembrane protein, in this case the voltage gated channels. These Ca²⁺ions that have entered the presynaptic cell cause vesicles containing acetylcholine (our neurotransmitter) to move to the presynaptic membrane and fuse with it. Acetylcholine is then released into the synaptic cleft whereby it diffuses across the cleft beofre binding to specific receptors on the postsynaptic membrane. These ACh receptors are bound to Na⁺ channels which open once ACh has bound. Na⁺ions can now move into the cell and cause a depolarisation, where the membrane potential becomes more positive, leading to an action potential being propagated in the postsynaptic cell. It is important to remember that a depolarisation is an all or nothing event that only occurs when the threshold membrane potential has been reached (usually around -55mv).