It’s one of the most innocuous of injuries, but also one of the most painful – a paper cut from the filing you’ve just done or the papers you’ve neatly ordered. So why do they hurt so much?
Before we explore the answer, it is important to point out that there is very little scientific evidence as to why these tiny lacerations are so incredibly painful. Perhaps pain researchers have bigger questions to concern themselves with than a cut you need a magnifying glass to see.
But taking a closer look at the structure of skin, and the network of nerves and pain receptors that lie within it, does offer a logical explanation.
The topmost layer of skin, known as the epidermis, contains cells that form a protective barrier around your body.
Beneath this is the dermis, where the sweat glands and hair follicles reside, and most importantly, nerve endings also known as nociceptors.
When you receive a normal cut from something sharp, the object will penetrate both these layers, driving into the lower subcutaneous fat layer and onwards into deeper tissue if the cut is very severe. This cut will cause damage to the local nociceptors and expose their sensitive endings to the outside world, triggering a wave of pain (danger) signals.
To make things worse, fingers have a particularly high number of these pain receptors. This is why injuries to your hands tend to hurt a lot more than injuries to other parts of your body, such as back and chest.
When you’re cut, blood rushes in, coagulates and seals the wound so the nerves are once again shielded from the outside environment. The initial pain then begins to subside as tissue repair begins under the cover of a newly formed scab.
Paper cuts, though, do not generally penetrate beyond the dermal layer. The fact that only the top layers of the skin are breached means that the blood flow to the area is heavily reduced, preventing a clot and scab from forming and leaving the sensitive nerves exposed.
This is why when you get a paper cut, it’s a good idea to tape it shut with a sticking plaster, preferably after applying a dab of antimicrobial ointment. Otherwise, you’ll keep feeling a sharp pain whenever the movement of your fingers opens the cut and agitates the exposed pain receptors – not to mention the agonising sting as soap sneaks into the cut the next time you wash your hands.
The level of pain associated with paper cuts is not just down to anatomy, but also the microscopic structure of the paper itself.
Looking at the edge of a piece of paper under a microscope (see right) reveals a jagged surface that acts more like a saw than a sharp knife. The serrated edge does not cleanly break the skin’s surface and so causes a lot more cellular and tissue damage.
Jake Port contributes to the Cosmos explainer series.
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