The term ascending reticular activating system refers to a set of neurons and pathways that are defined by their connections and functions. It overlaps but is not precisely coincident with the reticular formation (histological) of the brainstem ( Carpenter-1983, Anthoney-1994 ). Once believed to be a 'diffuse arousal network', a source of nonspecific activation of forebrain functions, subsequent research has indicated it to be composed of a large number of individual cell groups devoted to very specific motor and autonomic functions ( Buettner-Ennever-2004 ).