Researchers at Karolinska Institute in Sweden have discovered a new sensory organ that can detect painful mechanical damage such as pricks and impacts. These octopus-like cells surround the pain-sensing nerves and extend into the outer layer of skin. The discovery raises hopes that it could lead to the development of new painkilling drugs.

“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. The discovery changes our understanding of the cellular mechanisms of physical sensation,” Prof Patrik Ernfors, a co-author of the research from the Karolinska Institute in Sweden, told the ScienceDaily.

Pain causes suffering and almost one in every five people experience chronic pain. However, sensitivity to pain is also required for survival. Without it, we could damage our bodies without even being aware of it. New findings provide fresh insight into pain and could help answer questions that have been puzzling scientists for years.

“The major question for us now is whether these cells are actually the cause for certain kinds of chronic pain disorders,” Ernfors told the Guardian.

The new findings were published in the journal Science; they describe a type of Schwann cell, an octopus-shaped cell, that wraps around and engulfs nerve cells and helps to keep them alive. The body of the cells sits below the outer layer of the skin, and long extensions wrap around the ends of pain-sensing nerve cells that extend up into the epidermis. The finding differs from previous beliefs that the endings of nerve cells in the epidermis are bare or unwrapped.

“In the pain field, we talk about free nerve endings that are responsible for pain sensation. But actually they are not free,” said Ernfors.

The most important finding is that Schwann cells can sense pain and send a signal to the brain through nerve cells. Researchers used optogenetics, a technique that involves the use of light to control neurons in living tissue. Neurons have to be genetically modified to express light-sensitive ion channels.

Researchers have genetically modified mice so Schwann cells in the skin of their feet produced a protein that could absorb light. When cells detect light, this protein changes, affecting the membrane and producing a shift in the cells’ electrical charge or stimulation.

When the light was shone on these cells, the mice lifted their feet, they were licking, shaking and guarding of their paws. As the pulses of light increased in duration, the number of nearby nerve cells firing increased. Stimulation of Schwann cells caused pain in these mice.

The team exposed the feet of the mice to other stimuli like heat, cold, pinpricks and looked at their behaviour. Also, they tested the animals’ responses when the light was used to activate Schwann cells, and when they were deactivated. For all three types of stimuli, mice showed a stronger pain response after the cells were activated by light. Furthermore, it seems Schwann cells react stronger to mechanical harm.

What all of these findings implicate is that the special Schwann cells are important in sensing pain. Those cells and the nerves form a mesh-like network, a new pain-sensing organ.

“It is a two-cell receptor organ: the nerve and Schwann cell together,” said Ernfors.

According to Prof Peter McNaughton, an expert in sensory systems from King’s College London, many questions remain open. One of the most important mysteries is how the Schwann cells transmit the signal to the pain-sensitive nerve cells.

“If borne out by subsequent studies, this paper will be a paradigm shift showing that pain-sensitive nerve cell terminals are not in fact always directly driven by a painful stimulus but instead can be driven by associated [Schwann] cells,” said McNaughton.

This interesting and radical study has just scratched the surface. For patients with chronic pain, more research must be carried out on the topic.

Learn more about pain in the video below:  

By Andreja Gregoric, MSc