Szechuan pepper taps at nerve fibers

As many Asian chefs know, a dash of Szechuan peppers delivers a lip-rattling experience. This fizzy, tingly sensation comes courtesy of specialized nerve fibers that detect physical vibrations, a new study suggests.

Szechuan peppers aren’t spicy, but chefs often pair them with hot chilies to elicit a devastating tingly hot culinary sensation. Although scientists had a good idea how Szechuan peppers affect mice, the new results are the first to describe how the peppers work in people, says Diana Bautista of the University of California, Berkeley. What’s more, she says, the pepper-induced tingle might be a good way to approximate painful conditions in which nerves tingle, such as carpal tunnel syndrome and diabetic neuropathy.

Study coauthor Patrick Haggard of University College London and his colleagues enrolled adventurous eaters to probe the sensation of the peppers in the lab. Sure enough, the 12 volunteers’ mouths began to tingle after researchers dabbed pepper extract on their lower lips. And this tingling felt very specific. Szechuan pepper elicited the sensation of a 50-hertz vibration, a frequency detected by a class of nerve fibers called RA1, Haggard and colleagues report September 10 in Proceedings of the Royal Society B.

The researchers measured the tingle’s frequency by introducing another source of buzzing. A mechanical vibrator jiggled the volunteers’ index fingers while their lips tingled with Szechuan pepper.  People then judged whether the lips or the finger were buzzing faster.

These vibrations reveal a culinary illusion, one that’s created by a mismatch between the stimulus and the sensation, says Haggard. The pepper stays still but the sensation oscillates, suggesting that an ingredient in the peppers hijacks nerve fibers that detect mechanical vibrations, he says.

The results are not conclusive, cautions neuroscientist Sliman Bensmaia of the University of Chicago. Multiple fibers could help carry the signal, he says, and tingling can be a confused reaction of nerve fibers to a variety of stimuli. “For example, if you put your tongue on a 9-volt battery, not recommended, you get a tingling sensation because the brain doesn’t know how to interpret this nonsensical input.”