You do it. I do it. We all do it.
When we twist our ankle the very first response we have is to reach down and begin rubbing. We kiss our children’s boo boos for the same reason. We massage a hurting joint. But wouldn’t it make more sense to not touch an already sore part of our body? So why are we doing it? And does it really help or is it just some superstitious, learned behavior that doesn’t do a thing?
First let’s understand what pain actually is
Pain isn’t just too much of otherwise normal feelings, it’s something entirely different. Pain is transmitted to the brain along different nerves and isn’t really related to touch. There are many different nerves doing specific tasks, but as far as sensation goes there are two main players: large fiber and small fiber nerves. Large fiber nerves (A-alpha and A-beta) transmit touch, the fine sensations we discern with our fingertips, vibrations, and a general sense of where our body parts are located in space, something called proprioception. Specially, A-alpha nerves tell us where our body parts are located in space. A-beta nerves tell us what we’re touching. Pain is transmitted along small fiber nerves (A-delta and C-fiber), giving us a sense of pain, itch, and temperature.
Pain transmitters: The fast and the furious vs the slow and the small
Large and small fiber nerves transmit sensation to our brain at very different speeds. Small fiber nerves – like the A-delta and C fiber – are relatively slow, often taking their time to register pain in our brains. Those signals travel at a leisurely 2 miles per hour. In contrast, touch nerves travel at about 200 miles per hour! If you’ve ever stubbed your toe you probably noticed this disparity. Your toe rams into something, you immediately notice the sensation of your toe hitting – touch traveling along the large A-beta nerves – and a second or two later the pain arrives: Yow! What’s going on? The difference in transmission speed is caused by a hack that large fiber nerves use to speed up transmission along the axons (nerve fibers). Large A-alpha and A-beta nerves have a protein coating called myelin with occasional gaps. Nerve signals jump from gap to gap, basically skipping past the slow conducting nerves, and reach the brain almost instantly. Small fiber pain nerves don’t have myelin coats and so transmit at regular speed. The bottom line is that pain arrives a couple seconds after touch. A giraffe stubbing its toe wouldn’t feel the pain for over 6 seconds!
The need for speed
It’s a good thing that touch nerves transmit sensation quickly. Imagine if sensation and body position (proprioception) traveled along pain nerves. We’d try to walk around, our brain wouldn’t recognize where our foot was placed until 2 seconds after it hit the ground, and we’d tumble down confused. Large fiber nerves, with their specialized myelin sheathes, provide us almost an instant sense of body position and touch, allowing us to move and react quickly, with an almost instant sense of what we’re doing and where our body is.
Lanes merge ahead – why science backs up what we do automatically
Large and small fiber nerves travel in separate lanes, until they reach the spinal cord. Then they have to merge to pass through a virtual gate. Only one signal can pass through at a time, so when we’re feeling pain and touch at the same time the nerves compete for access to the gate. It’s this merger at the gate that is behind our rubbing behavior. Since only one nerve can send signals at a time, overloading one nerve type will tend to block the other. If your toe is feeling pain, rubbing the skin, vigorously stimulating the touch receptors and starting signals traveling along the large fiber nerves, creates a continuous signal along those large nerves. If you rub enough, those nerves tend to dominate the gate, like a lane of cars speeding through a merger without letting the other lane merge. The more aggressive drivers pass and the less aggressive drivers wait. That’s what’s behind our strange rubbing behavior. No voodoo and all science. Rub ahead!