Why Don’t We Bite Our Tongues?

Dec. 26, 2014

By Medical Discovery News

Why We Don't Bite Our Tongues

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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Shining a Light on Cavities

Oct. 3, 2014

By Medical Discovery News

For all those who cringe at the thought of going to the dentist or hearing the word cavity, there is hope. Apparently, when low-power laser light is focused onto damaged teeth, it stimulates the regrowth of dentin to correct the damage. The laser light stimulates the stem cells that are already in teeth to differentiate and repair damage from within, so that someday dentists can repair or even regrow teeth without fillings.

Teeth consist of four different tissues, three of which are harder than bone (enamel, dentin, and cementum) while one (dental pulp) is soft. Enamel, the hardest material in the body, is the outer surface of the crown of a tooth. Once enamel has completely formed it cannot be repaired, but it can remineralize. It allows teeth to withstand large amounts of stress, pressure, and temperature differences.

Dentin lies beneath enamel and forms the main portion of a tooth through numerous microscopic channels called dentin tubules. These tubules house dentinal fibers, which are the trouble-makers responsible for transmitting pain stimuli. Cementum is a thin layer of tissue surrounding the root of a tooth. Within the center of the tooth is the pulp, which provides nutrition to the tooth and mediates dentin repair. The pulp contains nerves, blood vessels, lymph vessels, connective tissue, cells that produce dentin, and stem cells.

By adding specific molecules, stem cells are coaxed into regenerating or repairing tissues. Growth factors or chemicals, among others, stimulate them to differentiate into the types of cells that make up tissues. It is a challenge to stimulate stem cells in the body without them growing uncontrollably. As a result, most approaches to stem cells involve removing them from the body, manipulating them in the lab, and then returning them. However, scientists have found that lasers promote regeneration in the heart, skin, lung, and nervous tissues. The idea was that since teeth contain stem cells, laser light might be able to stimulate them to regenerate tooth tissue and repair damaged teeth.

To test this theory, scientists drilled holes in the dentin in the teeth of rats and then shined a non-ionizing, low-power laser on the damaged area and the pulp just above the stem cells. They then capped the damaged teeth to keep the animals comfortable and healthy. With just a single five-minute treatment, new dentin formed in the damaged area in 12 weeks. The laser seems to create micro-injuries and induce highly reactive oxygen species, which indirectly activate stem cells.

They also proved that dentin production could be stimulated with lasers in cultured human dental stem cells. However, this treatment still needs some work before it could benefit people, since the stem cells that produce enamel are not present in mature teeth. And dentists would still play a role in repairing damaged teeth.

Before this experiment, results of laser treatments have generally been inconsistent, making these results that much more significant. It is the first time scientists have been able to determine how low-power laser treatment works on the molecular level. Scientists aim to advance this study into human clinical trials and even use this approach to regenerate other tissues.

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