A neuronal system that is responsible for the synthesis, storage, and release of the neurotransmitter norepinephrine. Norepinephrine, also known as noradrenalin, consists of a single amine group and a catechol nucleus (a benzene ring with two hydroxyl groups) and is therefore referred to as a monoamine or catecholamine. It exists in both the central and peripheral nervous systems. In the autonomic system in the periphery, norepinephrine is the primary neurotransmitter released by the sympathetic nervous system. The sympathetic nervous system mediates the “fight or flight” reaction, preparing the body for action by affecting cardiovascular function, gastrointestinal motility and secretion, bronchiole dilation, glucose metabolism, and so on. Within the central nervous system, norepinephrine has been associated with several brain functions, including sleep, memory, learning, and emotions. The major clustering of norepinephrine-producing neuron cell bodies in the central nervous system is in the locus coeruleus. This center, located in the pons with extensive projections throughout the brain, produces more than 70% of all brain norepinephrine.