If you were to put your finger directly over an open flame, you would probably pull it away almost immediately after feeling the burning sensation on your skin. Then, to soothe the persisting pain from the burn, you might shake your finger through the air, or put it in your mouth.
For decades, scientists believed those two reactions to the pain — the immediate withdrawal reflex and the coping response to the lasting suffering — were triggered by nerve signals passing through the same nerve pathway. But now, new research at Harvard University shows that each response actually has its own nerve pathway.
“The [reason] for the existence of two sensor pathways is evolution,” said Qiufu Ma, senior author of the study, which was published Dec. 10 in the journal Nature. “It is a very good idea for protecting animals. It’s for protecting and monitoring your own body to reduce suffering.”
By performing tests on mice, researchers identified a specific kind of neuron, called Tac1, as cells that play a key role in the transmission of signals that contribute to the sensation of prolonged pain or sustained suffering, according to an article on Harvard’s news website.
The team found that mice with disabled Tac1 neurons withdrew from acute painful stimuli but did not engage in coping responses when their paws were pinched. Meanwhile, the mice with intact Tac1 neurons withdrew from the painful stimuli and tried to minimize the pain they felt, licking their paws to soothe the sustained pain, according to the article.
“The initial sharp pain or the sharpness percepts that cause quick reflexive defensive reactions is likely mediated by neurons independent of Tac1 neurons,” Ma said in an e-mail.
The lack of a coping response to prolonged painful stimuli exhibited by the Tac1-disabled mice parallels the loss of sensitivity to pain experienced by people who have suffered a stroke, or damage to the thalamus, the part of the brain responsible for pain perception, according to the article.
The researchers think there is a second pathway that must carry the pain that causes the immediate withdrawal reflex, though they haven’t mapped it out yet, Ma said.
The researchers are hoping that these findings will pave the way for the development of new medicines for people who experience chronic pain. The goal is for these new treatments to actively target one type of pain-signaling pathway over the other to more effectively attack the appropriate pain signal relaying mechanism, Ma said.