Dec. 26, 2014

By Medical Discovery News

Why don’t we normally bite our tongues when we eat? A recent study found that two types of cells in brain, the premotor neurons and motoneurons, work together to coordinate the movements of the jaw and tongue, so that you do not usually bite your own tongue.

We can control chewing consciously, but otherwise it works automatically. Coordination of jaw and tongue muscles during eating is one of the most intricate mechanisms of the motor system in animals and humans. The coordination concerns both the timing and the sequence of muscle activation, in order to achieve the smooth and effective motions required when eating.

Three basic systems must be coordinated when eating. First, activity of the left and right jaw muscles must be symmetrical. Second, the tongue must be coordinated to position food between the teeth while the jaw moves the teeth to break down the food during chewing. Finally, jaw opening and tongue protrusion must be coordinated with jaw closing and tongue retraction to prevent the tongue from getting in between rows of sharp teeth.

Muscles in the jaw and tongue are controlled by brain cells called motoneurons, and those are then controlled by premotor neurons. The previously unsolved mystery was exactly which premotor neurons connect to which motoneurons, which then control muscles. To find the answer, scientists engineered a rabies virus to map the signals that control chewing. The bullet-shaped rabies virus was useful for this study because it infects muscle cells and peripheral neurons and moves rapidly up the nerves to the central nervous system, where it replicates in the brain.

Scientists at Duke University in North Carolina took advantage of the rabies virus’s ability to migrate up peripheral neurons toward the central nervous system, so they could map the circuitry that controls chewing. The disabled rabies virus migrated from muscles to motoneurons and then to premotor neurons. They also added a fluorescent green or red tag to the virus so scientists could track the virus on its journey.

Scientists injected this virus into two muscles: the genioglossus muscle that controls tongue protrusion and the masseter muscle that is involved in jaw closing. They discovered that one group of premotor neurons connect to both these muscles. A separate group of premotor neurons regulates tongue retraction and jaw opening. Sharing premotor neurons to control multiple muscles is an elegantly simple system to coordinate the movements of the tongue and the jaw to protect the tongue from a painful bite. The body cannot close the jaw automatically without also retracting the tongue.

This study was conducted using only mice, and it is only the beginning of understanding how chewing is controlled. At least 10 other muscles are active while chewing, drinking, and speaking. Additional studies will be needed to map all the motoneurons and premotor neurons involved in the complex, orchestrated movements that accomplish what are seemingly simple and routine tasks.

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