Everyone has had the experience at least once, where a slip or misjudgment lead to a painful stubbing of your elbow or toe. Usually when this happens though, there’s a near reflexive behavior we exhibit: we begin to rub the injured area knowing this will help the pain! Could there be an actual biological purpose behind this? Well according to a 20th century theory from Neuroscientists Ronald Melzack and David Wall, pain and touch may actually compete for perception from your elbow to the brain. This revolutionary concept is known as the Gate Control Theory of Pain, and is still a dominant theory of the interactions of touch and pain today.
So how could rubbing your whacked elbow actually dull out the pain? The theory is based on the possible pathways of two receptors: mechanoreceptors, which transmit touch as changes in pressure, vibration and movement on the skin; and nociceptors, which transmit pain from damage or potential damage to the skin. Both receptors send nerve signals through different pathways to a region of the spinal cord known as the Substantia Gelatinosa (SG), which is full of transmission cells that seem to send pain and touch signals to the brain.
However, pain and touch nerve fibres travel to the spinal cord through different pathways, which affects how they interact at the SG’s ‘gate’. Touch sensations reach the spinal cord through A-beta fibres, which are very fast due to their wide, myelinated axons. The sharp pain of nociception travels through slightly-slower myelinated A-delta fibres, and that dull, throbbing pain we feel occurs from a separate, slower C fibre.
The three theoretical states of the Pain Control gate. Via HowStuffWorks.com (References)
What Melzack and Wall’s Gate Control Theory proposes is that if touch and pain both meet in the SG, then touch will have an inhibitory effect on the transmission of the sensation of pain (left). So relating this back to that painfully-stubbed elbow, at first you’d likely feel a sharp pain followed by a dull, throbbing one (S). However, if you rub at the injured area afterwards, then the fast A-beta touch fibres (L) may cause an inhibitory effect on the pain transmission through the SG, exchanging the perception as touch instead! So perhaps rubbing that elbow really does make a difference in the end. Worth noting though is there is a reason this theory has received scrutiny; it is a rather simple theory to explain an entire range of somatosensation, and recent physiological work is shown that the transmissions of pain and touch are more complicated than once thought. Regardless, it may be the only somatosensory theory that can explain the many observed interactions between pain and touch, including why rubbing the skin of that stubbed elbow seems to mask the pain so well.
References
Wolfe, J. M., Kluender, K. R., Levi, D. M., Bartoshuk, L. M., et al. (2009). Sensation and Perception. (2nd ed.). Sinauer Associates, Inc.
http://www.drgordongadsby.talktalk.net/page13.htm
http://science.howstuffworks.com/environmental/life/human-biology/pain4.htm