Our ability to sense gentle touch is known to develop early and to remain ever-present in our lives but it is the least understood of the senses scientifically, because, unlike with vision or taste, scientists have not known the identity of the molecules that mediate it.
Researchers at the University of California, San Francisco (UCSF) identified the exact subset of nerve cells responsible for communicating gentle touch to the brains of Drosophila larvae - called class III neurons.
Scientists stroked the soft body of a newborn fruit fly larva ever-so-gently with a freshly plucked eyelash, and it responded to the tickle by altering its movement.
The researchers also uncovered a particular protein called NOMPC, which is found abundantly at the spiky ends of the nerves and appears to be critical for sensing gentle touch in flies.
Without this key molecule, the team discovered, flies are insensitive to any amount of eyelash stroking, and if NOMPC is inserted into neurons that cannot sense gentle touch, those neurons gain the ability to do so.
"NOMPC is sufficient to confer sensitivity to gentle touch," said Yuh Nung Jan, a professor of physiology, biochemistry and biophysics and a Howard Hughes Medical Institute investigator at UCSF.
Jan added that while the new work reveals much, many unanswered questions remain, including the exact mechanism through which NOMPC detects mechanical force and the identity of the analogous human molecules that confer gentle touch sensitivity in people.
Scientists generally feel that, like those other senses, the sense of touch is governed by peripheral nerve fibres stretching from the spine to nerve endings all over the body.
Special molecules in these nerve endings detect the mechanical movement of the skin surrounding them when it is touched, and they respond by opening and allowing ions to rush in.
The nerve cell registers this response, and if the signal is strong enough, it will fire, signalling the gentle touch to the brain.
The new finding is a milestone in that it defines the exact nerves and uncovers the identity of the NOMPC channel, one of the major molecular players involved – at least in flies, researchers said in a statement.