Why do stinging nettles sting? This is a question I ruefully thought after falling backwards into a bed of nettles. Each nettle is covered in tiny hairs. Some are longer than others, and these are called trichomes (from the Greek τρίχωμα meaning ‘hair’). Each hair is actually a singular cell, which has elongated to form this extension. It’s shape is closer to a hollow tube than an actual hair. The walls of this hair-like structure are composed of silica. Glass is also made of silica, so essentially this hair is a minuscule glass vial.
At the tip of the needle, there is a small bobble, and directly below this there is a point of weakness. Thus, when an animal brushes against this needle, the tip breaks off, leaving a dagger-like spike capable of piercing the skin.
The source of the real pain can be found at the base of the hair. Here, toxins such as formic acid, histamine, acetylcholine and 5-hydroxytryptamine (serotonin) collect in a sac. Once the tip breaks off, this concoction of chemicals is injected into the skin through the tiny glass syringe. The liquid inside the sac is under pressure, so when the tip is snapped, the solution shoots out much like a shaken bottle of coke. To exacerbate this injecting of liquid, when the hairs are touched they are at the same time compressed downwards. In this way, you are theoretically creating even more pressure which forces the liquid out to a greater extent. The easiest analogy to imagine is pushing down on the plunger of a syringe.
So what do the chemicals do? Natural histamine is responsible for inflammation, which is why your skin turns red and inflamed on touching a stinging nettle. Acetylcholine and serotonin are both neurotransmitters, and formic acid is an acid which will burn the skin. All of these substances together trigger pain receptors responsible for the pain you feel.