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Previously, it was believed that the sensory neurons within the tooth primarily convey pain signals to the brain; however, a recent study indicates that these neurons have multiple functions, including initiating a jaw-opening reflex that almost instantaneously prevents harm and additional injury to teeth.

The reflex that opens the lower jaw was a widely recognized craniofacial reflex, but prior to this study, the cellular sources of this phenomenon remained unidentified.

Researchers at the University of Michigan specializing in sensory neuroscience, dentistry, and mechanical engineering discovered the source using advanced live imaging, behavior-tracking tools, and murine molars to reveal the neurons’ supplementary function of monitoring the inner tooth and outer enamel.

This revelation and comprehension of this additional role highlight the significance of healthy, functioning nerves in preserving dental integrity.

“We had a hunch that tooth nerves might serve a more fundamental purpose,” said Joshua Emrick, the study’s senior author and an assistant professor at the U-M School of Dentistry. “When considering regenerating dental pulp, it’s crucial to restore the nerves.”

Emrick’s research team examined how nerve cells responded to stimuli from the molar teeth of mice in real-time. Their experiments unveiled a newly defined, protective function for intradental High-Threshhold Mechanoreceptors, specialized sensory neurons that react to tooth injury. These HTMRs identify perilous threats and promptly communicate signals to the brain for immediate action.

“Our research challenges the previous assumption that nerves within the tooth primarily serve to provoke pain, driving us directly to the dentist for assistance,” Emrick remarked. “If you’ve ever inadvertently bitten down on your fork, you’ve likely felt a sudden jolt, yet you managed to avert fracturing your teeth. You might owe that to these intradental HTMRs.”

This reflex essentially revolves around self-preservation.

“We believe the safeguarding of the teeth through this jaw-opening reflex is highly conserved among mammals that lack the capacity to replace teeth—like humans and in the molar teeth of mice,” Emrick explained. “Our findings indicate a potential to utilize these neurons to also elicit pain, opening avenues for devising new strategies for alleviating toothaches at the dentist’s office.”

To elaborate, the study, published in Cell Reports, demonstrated that when enamel or dentin sustains damage, the neurons elicit a response. Subsequent experiments examined the outcomes following the activation of HTMRs. As previously established, the team found that they incite acute pain; however, more intriguingly, they also observed a rapid jaw-opening reflex occurring within 5 to 15 milliseconds of activation.

While the authors concentrated their efforts on deciphering how the HTMRs operate within the tooth, this significant subclass of sensory neurons may also shield other oral and bodily structures from injury.
Elizabeth Ronan, postdoctoral fellow at the School of Dentistry and principal author of the study, remarked that the findings represent the initial step toward deeper insights.

“Although we usually associate sensation with our perceived external experiences, sensory neurons are equally vital in safeguarding and maintaining our tissues throughout our lives,” she stated. “Much remains to be uncovered concerning how sensory neurons operate within specific tissues, particularly internal ones like the teeth.”

In addition to Ronan and Emrick, co-authors included: Akash Gandhi, Karin Uchima Koecklin, Yujia Hu, Shuhao Wan, Brian Constantinescu, Mak Guenther, Ling-Yu Liu, Aditi Jha, Leen Dakhilalla, Kaitlyn Blumberg, Isaac Berthaume, Tomer Stern, Kevin Pipe, Bing Ye, Peng Li, all affiliated with U-M; and Maximilian Nagel from the National Center for Complementary and Integrative Health.

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