Why We Don't Bite Our Tongues When We Chew

Chewing is a very complex process, and half the time we are not consciously trying to keep our tongue away from our chompers. So why is it that it is relatively rare to bite your own tongue? New neural mapping may have the answer.

According to a study published in the peer-reviewed journal eLife, researchers from Duke University have successfully mapped the underlying brain circuitry that helps mammals painlessly chew.

"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," Edward Stanek IV, lead study author, said in a statement. "We were interested in understanding how this all works, and the first step was figuring out where these neurons reside."

According to the study, Stanek's team accomplished this by tagging various muscle-neuron interactions with a florescent and genetically disarmed version of the rabies virus.

The rabies virus traditionally works by jumping along neurons until it has infected the entire brain of its victim, However, Stanek's team used harmless forms of this virus genetically altered so that they would continuously jump along neurons associated with specific motor pathways.

These viruses were injected into the tongue muscles and jaw muscles of lab mice, who were then observed as they ate. The resulting scans showed that a group of premotor neurons simultaneously connect to the motoneurons that regulate jaw opening and those that stick out the tongue. Another group of premotor neurons were found connected to both motoneurons that close the jaw and those responsible for tongue retraction. These findings suggest a simple forced action method for keeping the tongue safe, where the body cannot automatically close the mouth without simultaneously retracting the tongue.

According to Stanek, this finding may represent a general pattern for neurons in the body.

"Using shared premotor neurons to control multiple muscles may be a general feature," he said, adding that it will be important to remember that "individual neurons can have effects in multiple downstream areas" in later studies.

The study was published in eLife on April 30.