You have probably experienced the agonizing pain of bumping your toes against furniture, haven’t you? And I bet that you instinctively rubbed your aching toes to lessen the pain. Am I right? And did it help? It probably did.

It has long been know that touch can reduce pain. This fact is actually linked to one of the most influential theories in pain research, the gate-control theory, published almost 50 years ago. Simply put, this theory states that the transmission of pain from the peripheral nervous system is subjected to modulation in the spinal cord. The major cell type controlled by this gate are the wide dynamic range (WDR) neurons of the dorsal horn of the spinal cord, the region to which sensory information is conveyed. One of the elements of this modulation is the input from large sensory nerve fibers activated by touch. Through an inhibitory circuit in the spinal cord, these fibers can inhibit the firing of WDR neurons and “close the gate”, therefore inhibiting the transmission of nociceptive inputs.

There are some specificities in this touch-induced pain relief. Studies have shown that tactile inputs can have an inhibitory effect on pain if delivered to the same dermatome where a painful input arises. A dermatome is an area of the skin innervated by one spinal nerve, i.e., whose nerve fibers converge to the same segment of the spinal cord. A recent study has further investigated this effect by evaluating how the distance between a tactile and a nociceptive stimulus within a dermatome can influence the perception of pain.

This study included two experiments. In the first experiment, volunteers received a painful laser stimulus and a tactile stimulus. The painful stimulus could either be a “medium” or a “high” intensity stimulus and participants had to be able to discriminate between them. Each trial consisted on either the application of a medium or high intensity painful stimulus alone or the application of both a painful and a tactile stimulus, with the tactile stimulus starting before the painful. In this first experiment, the tactile stimulus was delivered at a fixed distance from the painful stimulus.

The second experiment aimed at determining where within a dermatome is the analgesic effect of touch higher and if there is a relation between the distance of the tactile stimulus and its analgesic effect. This experiment followed a similar protocol to the first experiment, but the distance between the nociceptive and the tactile stimulus was variable. In each trial, the participants had to identify the painful stimulus and rate their pain perception. With these experiments, the authors confirmed that touch concomitant with a painful stimulation can have a significant analgesic effect. It was also shown that there is a spatial relation between touch and its effect on the painful stimulus, with tactile analgesia being stronger when the tactile stimulus was closer to the nociceptive stimulus. Thus, the main outcome of this study was the experimental demonstration that the location of a tactile stimulus matters for its effect in pain relief.

So, even though our everyday empirical knowledge has already taught us that if we bump our toes it’s pointless to rub our knee, there is now scientific evidence proving that we are right!

References

Dickenson AH (2002). Gate control theory of pain stands the test of time. British journal of anaesthesia, 88 (6), 755-7 PMID: 12173188

Kakigi R, & Watanabe S (1996). Pain relief by various kinds of interference stimulation applied to the peripheral skin in humans: pain-related brain potentials following CO2 laser stimulation. Journal of the peripheral nervous system : JPNS, 1 (3), 189-98 PMID: 10970109

Le Bars D (2002). The whole body receptive field of dorsal horn multireceptive neurones. Brain research. Brain research reviews, 40 (1-3), 29-44 PMID: 12589904

Mancini F, Nash T, Iannetti GD, & Haggard P (2014). Pain relief by touch: A quantitative approach. Pain, 155 (3), 635-42 PMID: 24361816

Melzack R, & Wall PD (1965). Pain mechanisms: a new theory. Science (New York, N.Y.), 150 (3699), 971-9 PMID: 5320816

Mendell LM (2014). Constructing and deconstructing the gate theory of pain. Pain, 155 (2), 210-6 PMID: 24334188

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