bite your tongue
June 5, 2014

What Process Keeps You From Biting Your Tongue?

Alan McStravick for - Your Universe Online

We have all experienced it. While enjoying a delicious meal, the flavors and textures of food are delightful as we practice the mastication necessary for the beginning of the digestive process. Then, out of nowhere, our tongue fails to get out of the way of our teeth and we clamp down on the fleshy muscle, eliciting pain and, almost certainly, a few choice words. Thankfully this situation occurs with relative infrequency and, thanks to researchers from Duke University, we now have a better understanding of how our tongue and jaw typically work so well together.

The entire process of eating is, not unlike your heartbeat or your ability to breathe, a basic biological task. The tongue and jaw perform a well-choreographed ballet where the tongue positions your food for the teeth and recedes to its ready position just in time for the jaw to close down onto it.

Researchers at the university have been able to locate the underlying brain circuitry responsible for coordinating this complex action, via a sophisticated tracking technique in mice. Were there no such autonomic process directed from the brain, it would mean we would have to consciously work our tongue during meals, nighttime teeth grinding, smiling and other day-to-day functions.

"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, in a recent statement. "We were interested in understanding how this all works, and the first step was figuring out where these neurons reside."

[ Watch the Video: Neural Circuits Help You Avoid Biting Your Tongue ]

Prior to this most recent research, brain mapping efforts of the chewing control center were basically inconclusive. Scientists had a basic knowledge that the jaw and tongue movements fell under the purview of special neurons called motoneurons. Motoneurons, in turn, are controlled by another set of neurons called premotor neurons. And that is where their understanding came to an abrupt halt. Researchers were unable to define just which premotor neurons connected to which motoneurons.

The study, under the guidance of Fan Wang, PhD, associate professor of neurobiology and a member of the Duke Institute for Brain Sciences, allowed Stanek to utilize a special form of the rabies virus to help trace the origins of chewing movements. Fang has been working with mice to map neural circuits for many years.

The choice of the modified rabies virus for this study was arrived at because of the way the virus affects the brains of those infected. It naturally jumps backwards across neurons until infection of the entire brain has been achieved. The modified strain of the virus was limited in how it worked in that it could only jump from the muscles to the motoneurons and then to the premotor neurons of the subject animal. Another component of the modified virus was that it contained a green or red fluorescent tag. This visual identifier allowed researchers to see exactly where the virus landed each time it jumped.

After having injected the tongue-protruding genioglossus muscle and the jaw-closing masseter muscle, Stanek found that a group of premotor neurons connected simultaneously to the motoneurons that regulate both jaw opening and tongue protrusion. The actions of jaw closing and tongue retraction had similar corresponding premotor neurons and motoneurons as well. These results, he suggests, offer an explanation for a simple method of coordination of movement between the tongue and jaw that usually keeps the tongue safe from injury.

"Using shared premotor neurons to control multiple muscles may be a general feature of the motor system," said Stanek. "For other studies on the rest of the brain, it is important to keep in mind that individual neurons can have effects in multiple downstream areas."

This study, say the researchers, is but the first step in delving even further into the mouse brain. Using this novel technique, they hope to eventually map the brain circuitry all the way up to the cortex. The first step, they claim, is to gain a fuller understanding of the connections between premotor and motoneurons.

"This is just a small step in understanding the control of these orofacial movements," Stanek said. "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 these other muscles, and only then can we get a complete picture of how these all work as a unit to coordinate this behavior," said Stanek.

Supported by grants from the National Institutes of Health, this study was published in this month's edition of the journal eLife.