Here’s some interesting pain neurology for you.
When you injure yourself you’ll find that the injured area becomes super sensitive.
Touch it, owww!
Ice it, owww!
Alcohol to clean it…. Jeez, owww!!!
This is called primary hyperalgesia.
Nerve endings at the injured are sensitised by the damage and inflammation so that they fire more readily.
Something that would normally be slightly uncomfortable becomes incredibly painful.
And for good reason. You don’t want to keep poking an injury.
But another thing happens too…
A little after the injury you’ll notice this sensitivity spreads to an area around the injury.
Pressure applied to those areas will more readily active nerve endings and result in pain where you might normally only notice minor discomfort.
This is called secondary hyperalgesia. Secondary because these areas are not injured, but they’re still sensitised as a result of the injury.
And this happens centrally, meaning in the brain and spinal cord.
Central mechanisms determine that this area needs protecting and so increase the sensitivity of surrounding nerve endings that detect dangerous amounts of pressure.
And it’s all quite clever.
It makes you very cautious around an injury.
You’re less likely to put weight through a sensitive ankle, or be careless waving around an injured limb, if the whole thing is sensitive.
It serves a purpose.
It’s what’s known as an adaptive response.
In our past, humans who protected injured areas were more likely to survive, so that response propagated and developed.
And for most people that response settles after a few days or weeks, depending on the injury.
As the tissues begin to heal the sensitivities die down and everything returns to normal.
But for some people, they don’t.
For some people the original injury heals up perfectly but the sensitivity persists.
They might have a genetic predisposition towards hypersensitivity.
Or they might be chronically stressed and sleep deprived and so primed for hypersensitivity at that point in time.
Or their injury lasts longer than they expected, they don’t ever get a clear understanding of it or are told unhelpful things by medical professionals and so the body never gets the message that the threat is over.
For whatever reason, that sensitivity can persist long after the injury has healed and morph into something different.
The area affected can spread, and nerves that once only registered pressure as dangerous can start registering gentle touch and other inputs as painful.
It’s not the whole picture, but these mechanisms help to make sense of some people’s experience of persistent pain.