An organ that controls the way we sense pain without nerve cells has been identified, a reminder of just how much there is we don't know about our own bodies. The discovery will help advance our incomplete understanding of the workings of pain. Far down the track, it could even help us control some forms of chronic pain without resorting to opioids or other drugs of dependence.
The skin needs not only to report sources of damage as pain but to distinguish them so the brain can judge the best response. Telling the difference between a burn, a blunt impact, or a prick with a sharp object, for example, requires a diversity of cells and we have yet to find all of them, even though they are literally under our noses and other body parts.
Professor Patrik Ernfors of the Karolinska Institute, Sweden, has identified an organ formed from a mesh of long glial cell protrusions within the skin of mice. It detects mechanical forces such as pressure or the pricking of sharp objects. Glial cells are not neurons, but nevertheless play multiple essential roles in the nervous system. The protective layer of glial cells that surround sensory nerves had been thought to end at the skin's outer layer, but Ernfors and his co-authors found that these instead extend almost to the surface, where they respond to external pressure.
In Science, Ernfors describes using optogenetic stimulation to trigger responses in these cells in the footpads of mice, while leaving other sensors unaffected. The mice reacted as if they were in mild pain, retracting the foot and licking the stimulated spot. By blocking the transmission of signals from the as-yet unnamed organ, Ernfors reduced sensitivity to mechanical skin stimulation, but it had no effect on their response to cold, confirming the organ's role.
“Our study shows that sensitivity to pain does not occur only in the skin's nerve fibres, but also in this recently-discovered pain-sensitive organ,” Ernfors said in a statement.
The scientists have yet to confirm pain reception works the same way in humans but consider this likely since other senosary organs have the same functions across mammals.
The causes and transmission of sensation are so complex and so poorly understood that we have great trouble blocking pain that serves no purpose. Existing painkillers work well against some sources of pain, but inadequately against others, which leads people to take larger doses in the hope one more pill will ease their torment. The consequences can be deadly and are only likely to be addressed by understanding the different ways in which our cells detect and transmit pain.