On the other hand, people who can’t feel pain, due to a rare genetic defect, also lack the sense of smell. At least this seems to be the case according to a new small scale study just published in Nature. The unexpected discovery shows that nerves that detect pain and odors rely on the same protein to transmit information to the brain.
Researchers examined three people with mutations in the SCN9A gene which means they can’t feel pain. All those studied had suffered multiple broken bones without feeling any pain, and two had gone through childbirth birth painlessly. However they weren’t aware that they also couldn’t smell a thing.
None of the study participants could distinguish balsamic vinegar, orange, mint, coffee or perfume from plain water, even when researchers poured on so much perfume and vinegar that the scents were unbearable to people with a normal sense of smell.
It may not be so strange that none of the people realized that they lacked a sense of smell. “If this was a genetic defect from birth they wouldn’t even know what they were missing,” says Graeme Lowe, a neurophysiologist at the Monell Chemical Senses Center in Philadelphia who was not involved in the study.
Researchers had previously shown that SCN9A controls pain sensitivity in people. The gene makes a sodium channel that lets sodium pass in through a nerve cell’s membrane when the nerve detects something painful. That flood of sodium sends an electrical signal racing toward the brain.
In the new study, the team discovered that odor-detecting nerve cells have the same sodium channel.
Because the sodium channel is missing in people with SCN9A mutations, the messages sent by pain and odor-sensing nerves never actually make it to the brain.
“It was completely surprising that these two sensory systems would use the same sodium channel,” says Frank Zufall, a neurophysiologist at the University of Saarland School of Medicine in Homburg, Germany. “But it’s clearly not needed for all senses.” None of the people with the faulty gene had hearing or vision problems. The researchers next plan to test whether those people have a sense of taste, and whether taste cells also use the sodium channel to communicate.
These findings are particularly interesting given that some drug companies are working on painkilling drugs that block the sodium channel’s activity. The results of this study suggest that such drugs could have the side effect of eliminating smell, and could also compromise people’s ability to taste.