Chewing your food may appear to be an effortless task but it is actually more complicated than it seems. Your tongue is in a vulnerable position when you chew your food but it seldom happens that you bite your tongue even when you are in a hurry to finish eating. Another amazing thing about the task is that you can either control it or not pay attention to it and you still always manage to grind the food in your mouth without risking injury to your tongue.
In an effort to understand how this complex biological task works, researchers from the Duke University in the United Kingdom conducted an experiment with mice. For the study, which was funded by the National Institutes of Health and published in eLife on June 3, Edward Stanek IV, a graduate student at the Duke University School of Medicine, and colleagues wanted to map the neural circuits responsible for the coordination of the tongue and jaw muscles.
The researchers are already aware that the jaw and tongue muscles are regulated by a set of neurons known as motoneurons, which in turn are controlled by a different group of neurons known as the premotor neurons. To better understand how these neural circuits work, they used a special tracing technique that involves a modified form of rabies virus that contains either a red or a green fluorescent dye, which functions as a visual identifier that would allow them to track the origins of the chewing movements.
"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," Stanek said. "We were interested in understanding how this all works, and the first step was figuring out where these neurons reside."
By injecting dye-c containing rabies virus to the genioglossus, the major muscle involved in the sticking out of the tongue, and the masseter muscle that control that closing of the jaws, the researchers observed that a set of premotor neurons instantaneously connect to the motoneurons that are involved in tongue retraction and those that control the closing of the jaw revealing a simple method that ensures the harmonized movements of the jaw and tongue that protect the latter from getting injured in a chewing movement.
Stanek said that controlling several muscles with shared premotor neurons may be one of the features of the motor system.
"Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination," the researchers wrote.
