Why We Don't Bite Our Tongues

If you're munching on a snack during this episode, ponder why you rarely bite your tongue when you eat. We should know more about this intricate mechanism, but it's something we're slowly uncovering.

Two types of neurons work together to coordinate muscle movements of the jaw and tongue, the motoneurons which in turn are controlled by the premotor neurons. But exactly which premotor neurons connect to which motoneurons and the precise muscles have remained a mystery.

To find out, scientists used an engineered, disabled rabies virus. They chose this virus because of its ability to migrate up peripheral neurons toward the central nervous system. By adding a fluorescent green or red tag to the virus, they could track it on its journey through the circuitry that controls chewing.

Researchers injected the virus into two muscles, the genioglossus muscle that controls tongue protrusion and the masseter muscle involved in jaw closing. They saw that one group of premotor neurons connect to both muscle groups, while another set of premotor neurons regulate just the opposite movements, tongue retraction and jaw opening. This means multiple muscle groups, the jaw and tongue, are controlled by the same set of premotor neurons.

It's an elegantly simple system designed so the body can not automatically close the jaw without also retracting the tongue. Since there are at least ten other muscles active while we chew, drink, and speak, this revelation only touches on the mechanism of the movements of our mouth. We'll need far more studies to map all the neurons involved in the complex orchestrated motions necessary for what are seemingly simple and routine tasks.