The Body's Emergency Button
There are stories of a parent lifting part of a car when their child is trapped underneath. Feats of great strength like this, often in a crisis, have been linked to one chemical: adrenaline. It is both a hormone and a medication.
Most of it is made by the adrenal glands, and a little comes from a small number of nerve cells in a part of the brain called the medulla oblongata. It plays a key role in the fight-or-flight response. It sends more blood to the muscles, makes the heart pump out more blood, widens the pupils and raises blood sugar.
Humans are not the only ones who have it. Adrenaline is found in many animals and even in some single-celled organisms. It has also been isolated from a plant, Scoparia dulcis, found in Northern Vietnam. Its main triggers are stresses such as a physical threat, excitement, noise, bright lights, and high or low temperatures.
Fuel for a Fast Getaway
Adrenaline acts on nearly every tissue in the body by locking onto adrenergic receptors. What happens next depends on the tissue. In the airways, high levels relax the muscle and open them up. In the walls of most small arteries, the same hormone does the opposite and makes the muscle tighten.
It also unlocks the body's energy stores. Adrenaline tells liver cells to break down glycogen and release glucose into the blood. It speeds up the breakdown of fat and holds back insulin from the pancreas. Together, these changes raise blood sugar and fatty acids, ready fuel for cells everywhere.
Unlike many hormones, adrenaline has no negative feedback to slow its own production. Blood levels can rise about 10 times during exercise and 50 times or more during stress. The exercise link was first shown in an unusual way: by measuring the pupil of a cat on a treadmill.
Why Scary Moments Stick
Fear is the emotion most studied in relation to adrenaline. In one experiment, people given adrenaline watched frightening films. Compared with a control group, they made more negative faces, felt more intense fear and had stronger negative memories. Yet they did not find a funny film any funnier, or feel angrier during an anger film.
Part of the reason may be the body itself. Adrenaline raises the heart rate and can make the knees shake, and people may read those signals as fear, no matter how scary the film really was. It also helps with memory consolidation. When something emotional or stressful happens, adrenaline helps lock in the memory, making it stronger in proportion to how important the event was.
Oddly, adrenaline does not easily cross the blood-brain barrier, so its effect on memory starts, at least partly, with receptors outside the brain. Adrenaline junkies, people who crave extreme excitement such as risky sports, may not be driven by adrenaline itself. Stress also switches on the brain's reward system, which drives behavior.
One Hormone, Two Names
Adrenaline was the first hormone ever identified. In 1895, Polish physiologist Napoleon Cybulski made the first extracts from adrenal glands. That same year, George Oliver and Edward Albert Schäfer showed that something from the gland's inner part raised blood pressure and heart rate. Secretin is often called the first hormone, but its effect was only observed in 1902.
Then came the naming fight. In 1897, pharmacologist John Jacob Abel made an extract he named epinephrine. In 1900, Japanese chemist Jōkichi Takamine and his assistant Keizo Uenaka purified a version about 2,000 times more active, called adrenaline, and it was patented in 1901. Partly because Adrenaline was a registered trademark in the United States, Americans adopted epinephrine as the generic name.
Funnily, both names mean the same thing. Adrenaline comes from Latin for on the kidney, while epinephrine comes from Ancient Greek for upon the kidney. Today, European doctors mostly say adrenaline, while American ones mostly say epinephrine. The first lab-made adrenaline arrived in 1904, created separately by Friedrich Stolz and Henry Drysdale Dakin.
A Front-Line Lifesaver
Adrenaline is also a crucial medicine. It remains a first-line treatment for cardiac arrest and for severe allergic reactions, known as anaphylaxis. It is also used to treat bleeding at the surface of the body. It can be given into a vein, into a muscle, just under the skin, or breathed in.
Breathed in, it can ease the symptoms of croup, and it may help with asthma when other treatments do not work. It can even treat a type of glaucoma, because it helps fluid flow out of the eye and lowers the pressure inside. Before 20 April 1896, the American eye doctor William H. Bates had already found a use for it in eye surgery.
Like many strong medicines, it has side effects. Common ones include shakiness, anxiety and sweating, and it can bring on a fast heart rate and high blood pressure. It is a neat twist: the same chemical that makes a scary film feel scarier is one of the first things doctors turn to when a heart stops.
