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. Norepinephrine is the primary neurotransmitter released by the sympathetic nervous system, which 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.
After synthesis, the majority of norepinephrine is transported into synaptic vesicles in the nerve terminals, where it remains until needed. When the nerve terminal is activated by depolarization, calcium flows into it, leading to the release of norepinephrine into the synaptic cleft. Once released into the synaptic cleft, norepinephrine is free to bind to specific receptors located on the presynaptic or postsynaptic terminal, which initiates a chain of events (the effector system) in the target cell that can be mediated by a number of different second messenger systems. The exact effect is determined by the identity of the receptor activated. See Epinephrine, Second messenger, Sympathetic nervous system, Synaptic transmission
Termination of norepinephrine occurs by a reuptake mechanism in the presynaptic membrane. Once transported back into the presynaptic terminal, norepinephrine can be stored in vesicles for future use or enzymatically degraded by monoamine oxidase.
Certain medications achieve their effect by altering various stages of synthesis, storage, release, and inactivation of norepinephrine. The behavioral manifestations of these alterations have led to a better understanding of norepinephrine's role in various psychiatric disorders. See Affective disorders, Monoamine oxidase, Psychopharmacology, Schizophrenia, Stress