The Cholinergic Basal Forebrain: A Critical Role in Cortical Arousal

Abstract

Acetylcholine (ACh) has long been implicated in the regulation of arousal or wakefulness. However, the anatomical basis for this regulation had been missing because relatively little was known about the organization of central cholinergic pathways. During the last decade, however, specific immuno-histochemical markers became available, and by using these markers central cholinergic neurons have been mapped and their projections delineated (see Semba and Fibiger, 1989, for review). It is now well established that there are two major cholinergic projection systems in the CNS: cholinergic neurons in the basal forebrain project widely to the cerebral cortex, and those in the mesopontine tegmentum project heavily to the thalamus. Armed with these anatomical findings, researchers of behavioral state have begun to investigate the role of specific populations of central cholinergic neurons in the regulation of waking and sleep. One important conclusion which has emerged from such recent studies is that cholinergic neurons in the basal forebrain have a crucial role in cortical arousal. In the present paper, both anatomical and physiological evidence supporting this notion is discussed, and clues are explored as to how the activity of basal forebrain cholinergic neurons is regulated during different behavioral states.