Vesicles containing neurotransmitters are found in the swollen end of the presynaptic neurone - the synaptic knob. Large amounts of mitochondria are needed for the synthesis of neurotransmitters.Neurotransmitters can be excitatory and bring about depolarisation (e.g. acetylcholine) or they can be inhibitory and prevent an action potential being triggered (e.g. GABA or gamma-aminobutyric acid found in the brain).