Interconnected neurons in the brain that coordinate the movement of the tongue and jaw keep the tongue safe from injury, according to a new study.
Researchers at the Duke University have used a sophisticated tracing technique in mice to map the underlying brain circuitry that keeps mealtime relatively painless.
The study, published in the journal eLife, could lend insight into a variety of human behaviours, from nighttime teeth grinding to smiling or complex vocalisations.
"Chewing is an activity that you can consciously control, but if you stop paying attention these interconnected neurons in the brain actually do it all for you," said Edward Stanek IV, lead study author and graduate student at Duke University School of Medicine.
Chewing requires a complex interplay between the tongue and jaw, with the tongue positioning food between the teeth and then moving out of the way every time the jaw clamps down to grind it up.
Researchers know that the movement of the muscles in the jaw and tongue are governed by special neurons called motoneurons and that these are in turn controlled by another set of neurons called premotor neurons.
Stanek used a special form of the rabies virus to trace the origins of chewing movements.
Stanek used a genetically disabled version of rabies that could only jump from the muscles to the motoneurons, and then back to the premotor neurons.
The virus also contained a green or red fluorescent tag, which enabled the researchers to see where it landed after it was done jumping.
Stanek injected these fluorescently labelled viruses into two muscles, the tongue-protruding genioglossus muscle and the jaw-closing masseter muscle.
He found that a group of premotor neurons simultaneously connect to the motoneurons that regulate jaw opening and those that trigger tongue protrusion.
Similarly, he found another group that connects to both motoneurons that regulate jaw closing and those responsible for tongue retraction.
The results suggest a simple method for coordinating the movement of the tongue and jaw that usually keeps the tongue safe from injury.