Norepinephrine is a neurotransmitter and hormone that helps control alertness, arousal, and the body’s response to stress. In the brain it is released by small groups of neurons whose axons reach widely through the cortex and limbic system.

As a neurotransmitter it increases vigilance and readiness for action. As a hormone released by the adrenal medulla it contributes to the fight-or-flight response, raising heart rate and blood pressure and mobilizing energy stores. Levels fluctuate with the sleep-wake cycle and rise sharply during acute stress or threat.

Many antidepressant medications influence norepinephrine signaling, either by blocking its reuptake or by other mechanisms. Imbalances have been implicated in depression, anxiety, and attention disorders. The same chemical system that sharpens focus in a crisis can, when chronically overactive, contribute to sustained tension and physiological strain.

When the body and brain shift into a state of heightened readiness, norepinephrine is one of the principal agents of that shift.