ABSTRACT
In primates, cholinergic and monoaminergic axons that innervate the cerebral cortex originate almost exclusively from subcortical nuclei in the brainstem and basal forebrain. These projections are thought to modulate cortical activity during arousal, attention and memory formation. Physiological and anatomical evaluations of these ascending projections suggest that they have overlapping but somewhat distinctive synaptic targets in the cortex. This review compares the anatomical organization of acetylcholine-, dopamine-, norepinephrine-, and serotonin-containing axon systems in the monkey and human cerebral cortex. Analysis of the distributions of axons, receptors, and synapses suggests that each system in likely to have a differential role in modulating cortical function.