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).