Adrenaline · Grey Matter

Adrenaline (epinephrine) is the hormone the adrenal glands pour into the blood within seconds of a threat or effort, preparing the body to act (faster heart, more glucose, redirected blood flow), and it reaches the brain only indirectly, through nerves.

Adrenaline. Adrenaline (epinephrine) is the hormone the adrenal glands pour into the blood within seconds of a threat or effort, preparing the body to act (faster heart, more glucose, redirected blood flow), and it reaches the brain only indirectly, through nerves.

It is made in the adrenal medulla, the inner part of the adrenal gland, from noradrenaline by one more enzymatic step, and released when sympathetic nerves drive the gland. Through adrenergic receptors around the body it raises heart rate and the force of contraction, opens the airways, releases glucose from the liver and fat stores and moves blood toward the muscles. The response starts within seconds and fades within minutes as the hormone is cleared.

It does not enter the brain. Adrenaline is a charged molecule that crosses the blood-brain barrier poorly, so its effects on the mind come through the body: it excites the sensory fibres of the vagus nerve, which reach the brainstem and from there the locus coeruleus and its noradrenaline.

That relay strengthens memory. In animals and humans, adrenaline after a learning episode improves later recall of it, an effect that depends on the vagus and on noradrenaline acting in the amygdala; it is one reason emotional events are remembered well.

It is the fast arm of stress. Cortisol follows over minutes and acts for hours; adrenaline is over before cortisol peaks.

Adrenaline speaks to the brain through the body.

Barred from the brain by its barrier, it still marks a moment as important by exciting the nerves that report the body's state upward.

Questions: What is the difference between adrenaline and noradrenaline? Chemically they differ by one methyl group: adrenaline is made from noradrenaline by one more enzymatic step, and both act on the same family of adrenergic receptors. The difference is where they are released. Noradrenaline is a neurotransmitter, released by sympathetic nerves onto organs and, inside the brain, by the locus coeruleus onto almost every region. Adrenaline is mainly a hormone, released into the blood by the adrenal medulla, and it barely crosses the blood-brain barrier, so the brain's arousal runs on its own noradrenaline while adrenaline acts on the body. How do the fast and the slow stress responses differ? The fast response is nervous: within seconds the sympathetic system drives the adrenal medulla to release adrenaline, and with noradrenaline from sympathetic nerves it raises heart rate, blood glucose and alertness, fading within minutes. The slow response is hormonal: the hypothalamus releases CRH, the pituitary releases ACTH, and the adrenal cortex makes cortisol, which peaks some tens of minutes after the event. Cortisol is a steroid that enters cells and changes gene expression, so its effects last hours, and it ends the response by feeding back on the hypothalamus and pituitary. The first prepares for action now, the second reorganises metabolism, immunity and the brain for what follows. How can adrenaline strengthen memories if it does not cross the blood-brain barrier? Adrenaline released during an emotional or stressful event binds beta-adrenergic receptors on the sensory fibres of the vagus nerve. The vagus carries the signal to the nucleus of the solitary tract in the brainstem, which activates the locus coeruleus, and noradrenaline is then released in the amygdala and other regions involved in storing memories. In the work of McGaugh and colleagues, adrenaline given after training improves later memory in rats and humans, and blocking beta receptors in the amygdala or cutting the vagal route removes the effect. The body's alarm thus marks a moment as worth keeping without the hormone ever entering the brain.