Abstract
Norepinephrine (NE) neurons project throughout the neuraxis and convey a variety of functions. The nucleus locus coeruleus (LC) gives rise to a highly branched set of axonal projections that innervate the entire cerebral cortex, hippocampus, thalamus, and other forebrain areas. This system is involved in arousal, attention, and behavioral flexibility. Neurons in the medulla provide strong NE innervation of the hypothalamus, midbrain, basal forebrain, and amygdala, and function in visceral autonomic processes as well as in anxiety and stress. Actions of NE at target neurons indicate both ionotropic and neuromodulatory, gain-enhancing effects, affording these brain NE systems network-level actions that underlie these important behavioral functions.