How do two neurones interact at synapse?

When an action potential reaches the synaptic knob of the pre synaptic neurone, this causes calcium ion channels to open. This leads to an influx of calcium ions ( Ca2+) into the synaptic knob. Presence of Ca2+ leads to synaptic vesicles (containing neurotransmitter) to move towards and fuse with the synaptic membrane, the neurotransmitter is released into the synaptic cleft via exocytosis. The neurotransmitter may then diffuse across the synaptic cleft and bind to specific receptors embedded in the membrane of the post synaptic neurone, leading to opening of ion channels leading to a response. The response is dependent on the type of neurotransmitter ie, excitatory or inhibitory.
Once the appropriate response in the post synaptic neurone has been propagated the neurotransmitter must be removed from the synaptic cleft, this is done via the use of enzymes. The most common example is the enzyme acetylcholinesterase for the removal of the neurotransmitter acetylcholine. Acetylcholine is hydrolysed into choline and acetic acid (ethanoic acid.) These products are returned to the presynaptic neurone and recombined into acetylcholine which is packaged into vesicles.