Norepinephrine is the main neurotransmitter released by most postganglionic neurons of the sympathetic nervous system.
The sympathetic system helps prepare the body for demanding or threatening situations. Norepinephrine released at many nerve endings increases heart rate and contractility, constricts selected blood vessels, and supports alertness. In the brain, it is also a major neurotransmitter involved in attention, arousal, and stress responses.
The adrenal medulla releases mostly epinephrine into the bloodstream and some norepinephrine as hormones. This distinction causes a common mix-up: norepinephrine is the principal transmitter at most sympathetic nerve endings, whereas epinephrine is the main adrenal-medulla hormone released during sympathetic activation.
Norepinephrine is synthesized from dopamine by the enzyme dopamine beta-hydroxylase. Neurons in the locus coeruleus are a major source of norepinephrine projections in the brain. Its effects depend on receptor type, tissue, and concentration, so it does not produce one universal response in every organ.