Noradrenaline and Adrenaline

The fight-or-flight chemicals: noradrenaline from the locus coeruleus tunes alertness and attention in the brain, while adrenaline from the adrenal glands readies the body for action.

Noradrenaline (called norepinephrine in the US) and adrenaline (epinephrine) are catecholamines, made from the amino acid tyrosine by the same pathway that makes dopamine — noradrenaline is made from dopamine, and adrenaline from noradrenaline.

The blue spot

Most of the brain's noradrenaline comes from the locus coeruleus ("blue spot"), a tiny cluster of neurons — about tens of thousands on each side in humans — in the pons, named for its bluish pigment. Its axons reach nearly the entire brain. Its activity tracks arousal: it is quiet during sleep (almost silent in REM sleep), moderately active when awake and focused, and highly active under stress or when something surprising happens.

Arousal, attention and the sweet spot

Noradrenaline helps the brain switch between focused attention and scanning for new information, sharpens responses to important stimuli and strengthens emotional memories. Performance often follows an inverted U — too little arousal and you're sluggish, too much and you're scattered — echoing the century-old Yerkes–Dodson observation. Drugs that boost noradrenaline are used in ADHD and depression; drugs that reduce its effects, such as clonidine and prazosin, are used for high blood pressure and sometimes for PTSD nightmares.

Fight or flight in the body

In the body, the sympathetic nervous system uses noradrenaline at most of its target organs, and the inner part of the adrenal glands releases adrenaline (with some noradrenaline) into the blood as a hormone. Together they raise heart rate and blood pressure, widen the airways, release glucose from the liver, dilate the pupils and divert blood to the muscles. Walter Cannon described this as the "fight or flight" response in the early 20th century.

Adrenaline injections (auto-injectors) are the emergency treatment for severe allergic reactions, because adrenaline reverses airway constriction and blood-pressure collapse.