Neurotransmitters
They may be small molecules or peptides (although their mechanisms and sites of synthesis are different):
Molecules
mall molecule transmitters are synthesized at terminals, packaged into small clear-core vesicles (often referred to as synaptic vesicles)
Peptides
Peptides, or neuropeptides are synthesized in the endoplasmic reticulum and transported to the synapse, sometimes they are processed along the way. Neuropeptides are packaged in large dense-core vesicles
Release
$Ca^{2+}$ is necessary for release
Experiments show, that transmission does not work every time (stochiastic) and is therefore not completely reliable. The sizes of EPP is not always the same (some bigger/ smaller) and sometimes there are spontaneous EPP’s (without axon stimulation). ➔ Quantum transmission