are collections of neurons in the central nervous system that have been demonstrated by histochemical fluorescence to contain one of the neurotransmitters serotonin, dopamine, norepinephrine ( Fuxe-1970 ) or epinephrine ( Kitahama-1994 ). They are classified respectively as serotonergic cell groups, dopaminergic cell groups, noradrenergic cell groups and adrenergic cell groups.