Although chemical transmission in the nervous system is characterized by an amazing complexity, classical neurotransmitter systems involve:
-inhibitory neurotransmitters: aminoacidergic [γ-aminobutyric acid (GABA) and glycine
-excitatory neurotransmitters: aspartate and glutamate
-cholinergic neurotransmission : acetylcholine
-monoaminergic neurotransmission: mainly adrenaline, noradrenaline, dopamine, and serotonin
-purinergic neurotransmission: adenosine and adenosine mono-, di-, and triphosphat).
The addition of neuropeptides (the largest family of signalling molecules in the nervous system) and channels (that modulate neurotransmitter actions) will likely reclassify neurotransmitter systems and disorders in the future.