How to Not Bite Your Tongue: Complex Brain Circuitry Underlies Chewing

Most people have accidentally bit their tongue at some point or other, whether it was after receiving a sudden jolt from an impact or otherwise. Yet we'd probably bite our tongues a lot more often if it weren't for a complex neural circuit. Scientists have found that there's some pretty sophisticated underlying brain circuitry that keeps us from chewing on our tongues.

When we eat, our tongue positions itself in a way to keep us from biting it. Precisely coordinated, this chewing action keeps us from biting more tongue than food.

"Chewing is an activity that you can consciously control, but if you stop paying attention these interconnected neurons in the brain actually do it for you," said Edward Stanek IV, one of the researchers, in a news release. "We were interested in understanding how this all works, and the first step was figuring out where these neurons reside."

In order to learn a bit more about the neural circuits, the researchers actually looked at a genetically disabled version of the rabies virus. The rabies virus works naturally by jumping backwards across neurons until it infects the entire brain. In this case, the modified rabies could only jump from the muscles to the motoneurons and then back to the premotor neurons. In addition, the virus either had a green or red fluorescent tag to allow the scientists to see where it landed once it was done jumping.

The researchers injected this modified virus into two muslces: the tongue-protruding genioglossus muscle and the jaw-closing masseter muscle. In the end, the scientists found that a group of premotor neurons simultaneously connect to the motoneurons that regulate jaw opening and those that trigger tongue protrusion. In addition, another group connects to both motoneurons that regulate jaw closing and those responsible for tongue retraction.

"This is just a small step in understanding the control of these orofacial movements," said Stanek in a news release. "We only looked at two muscles and there are at least 10 other muscles active during chewing, drinking and speech. There is still a lot of work to look at this other muscles, and only then can we get a complete picture of how these all work as a unit to coordinate this behavior."

The findings are published in the journal eLife.