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What stops us from biting our tongue? Complex neural circuitry

Were it not for some nifty neural intervention, we would be chewing up more tongue than food every time we eat. This is because, in the act of eating, the tongue receives food, places it between the jaws and steps out of harm’s way quickly before the jaws come clamping down to chew.

The question is: How does the tongue get away unscathed despite having to flirt so dangerously with the jaws hundreds and thousands of times through our lives?

A recent study has shown that a series of well-coordinated moves involving teeth and tongue managed by a complex neural network makes this possible.

To find out how the brain orchestrates the precisely coordinated oral dance, Duke University researchers deployed a sophisticated tracing technique, which mapped the brain circuitry in mice that prevents the tongue from getting slaughtered!

“While it is possible to consciously control chewing, should your mind be off the act, a group of interconnected neurons kicks in and does it all for you,” said Edward Stanek of the Duke University School of Medicine. “It is like switching to the auto-pilot mode.”

To understand how this happens the researchers had to first figure out the location of these neurons. Previous mapping attempts had shown that the jaw and tongue are able to waltz together because of two sets of specialized neurons called motoneurons and premotor neurons. But it was not known which premotor neurons connect to which motoneurons.

Under the guidance of senior study author Fan Wang, Ph.D., associate professor of neurobiology and a member of the Duke Institute for Brain Sciences, Stanek deployed a genetically disabled form of the rabies virus to get down to the bottom of the chewing story.

In its natural form the rabies virus jumps backwards across neurons until the entire brain of a victim has been infected. The version used by Stanek could only jump from the muscles to the motoneurons, and then back to the premotor neurons. The researchers embedded green or red fluorescent tags on the virus to allow them to locate where these modified viruses landed after they had done with their jumping.

These fluorescent viruses were injected into the tongue-protruding genioglossus muscle and the jaw-closing masseter muscle. This showed that motoneurons that regulate jaw opening and the ones that trigger tongue protrusion are connected simultaneously.

They also found a group that connects to both motoneurons that regulate jaw closing and those that cause tongue retraction. In other words, a surprisingly simple method that coordinates the movement of the tongue and jaw usually keeps the tongue safe.

The researchers are now drilling even deeper and may eventually jump even further back in the mouse brain to map the circuitry all the way up to the cortex.

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