The sense of touch is more of a mystery than that of hearing, smell, sight or taste. Perhaps that’s why more songs have been written about it (“Smell Me in the Morning” just doesn’t have the same ring.) But now scientists have unlocked one secret about how it works using only a fruit fly and an eyelash.
Researchers at the University of California in San Francisco have identified the precise subset of nerve cells responsible for transmitting gentle touch to the brains of Drosophila, or fruit fly, larvae. They’re called class III neurons. At the spiky end of these nerve cells one particular protein, NOMPC, apparently dominates. This molecule appears to be critical in communicating the type of touch a nerve senses.
Tactile perception has long been the most complicated of the senses to study, yet it’s a strong candidate for being our most important sense. Research into orphans suggests that babies who are never touched can never really form bonds with other humans. And even before birth, before a fetus responds to visual or auditory stimuli, he or she responds to touch. Ultrasounds show apparently insensate little creatures squirming away from pokes to the belly or needles taking a sample of amniotic fluid.
Gentle touch, or stroking, is particularly important. Doctors now know that premature babies need to be gently caressed to thrive, despite their fragility. Researchers have suggested that everything from grooming behaviors among other primates to the the popularity of face creams among ours can be traced to living beings’ need for a tender touch. Michelangelo, after all, did not paint God hearing Adam into being on the ceiling of the Sistine chapel.
But touch is a very complicated sense to study. How does the body know, for example, to flinch away from a pinprick or a hot kettle, but not from a soft caress? “Our understanding of touch is lagging behind vision, olfaction or taste,” says UCSF physiology professor Yuh-Nung Jan, lead author of the new study. “This is in part due to the difficulty in firmly identifying the sensor molecules for touch. The sensor molecules have been found for vision (rhodopsins), olfaction (G-protein coupled receptors) and taste for some time now.” Studies have shown that in humans gentle touch is carried along certain nerves known as C-tactile fibers, that are stripped of insulating proteins. But it’s less clear exactly what chemicals are at play here, since C-fibers react to various stimuli: temperature, pressure or even chemical changes.
The UCSF researchers found that fruit flies that were genetically engineered to have no NOMPC did not respond to gentle touch. When they tickled the NOMPC-deficient flies with an eyelash, they got no reaction. And no, that’s not E.Y.E.L.A.S.H., an acronym for some complicated piece of technical equipment. That’s an eyelash. “Any old eyelash would do,” says Jan. “Eyelashes happens to have the right size and stiffness for tickling fly larvae.” The Drosophilae grubs with the molecule wriggled; those without didn’t. When they added NOMPC to the nerves without it, the larvae moved again.
Why should we care about annoying insects that spoil our produce? Fruit flies have 75% of the same genes that cause disease with humans. They’re a favorite research subject because they breed super fast, don’t need much looking after and have salivary glands with big chromosomes, comparatively speaking, which makes genetic studies of them relatively easy. The bad news is humans don’t have NOMPC. “It is possible that there are proteins which are distantly related to NOMPC that function as touch sensors in humans,” says Jan, who led the study with his wife and UCSF collegue, Lily Jan. “It is also possible that an entirely different type of molecule serves that function in humans.”
You don’t have to wait for scientists to figure out which of these is true, however, to benefit from the effects of gentle touch. As long as it’s among appropriate and willing recipients.