Presynaptic neurone

 The presynaptic neurone is the neurone that releases neurotransmitter, across the synapse, to the post synaptic neurone.

When an action potential arrives at the axon terminal of the presynaptic membrane, Ca2+ ion channels open, allowing the ions to move into the cell down their concentration gradient. This in turn causes the neurotransmitter containing vesicles to translocate and fuse to the presynaptic membrane. The neurotransmitter diffuses across the synapse and starts a new action potential in the post synaptic neurone.