Facilitation

An action potential travels down the neuron and reaches the presynaptic knob.

This causes the Calcium ion channels to open and allow an influx of calcium into the knob.

The increased concentration of calcium causes the secretory vesicles within the knob to bind to the outer membrane and release their neurotransmitter (e.g. ACh) into the synaptic cleft.

Leaves the presynaptic neuron, activates ion channel

a neuron

Presynaptic neurons release the neurotransmitter in response to an action potential. Postsynaptic neurons receive the neurotransmitter (and can however become presynaptic to the next nerve cell, if the neurotransmitter has stimulated the cell enough).