Trigeminal somatosensation in the temporomandibular joint and associated disorders

Neuroplasticity and Pain Perception: Exploring the Complexities of Temporomandibular Disorders

Temporomandibular disorders (TMDs) are prevalent conditions affecting the temporomandibular joint (TMJ), masticatory muscles, and associated structures, leading to pain, restricted movement, and joint noises. These disorders are multifactorial in origin, involving structural, functional, and psychological components. This review delves into the neurophysiological mechanisms of pain perception in TMDs, focusing on peripheral and central processes, including the role of neural plasticity in chronic pain. Peripheral mechanisms involve nociceptors in the TMJ, activated by inflammatory mediators, mechanical stress, and tissue damage, leading to pain. Peripheral sensitization, driven by factors such as cytokines and neuropeptides, enhances nociceptor sensitivity, contributing to chronic pain states. The trigeminal nerve is pivotal in transmitting nociceptive information to the central nervous system (CNS), with C-fibers and A-delta fibers involved in pain perception. Central sensitization, a hallmark of chronic pain in TMDs, involves neuroplastic changes in the CNS, including wind-up and long-term potentiation (LTP), enhancing pain perception and facilitating pain persistence. Neuroplasticity, both central and peripheral, plays a critical role in the development of chronic pain. Central plasticity includes synaptic changes and alterations in brain connectivity, which were observed in functional imaging studies of TMD patients. Peripheral plasticity involves the upregulation of ion channels and neurotransmitters, sustaining pain signals. Additionally, neuroimmune interactions between microglia, astrocytes, and pain pathways are integral to central sensitization. Understanding these mechanisms is crucial for developing effective treatments targeting both peripheral and central pain processes. Emerging therapies, including transient receptor potential (TRP) channel blockers and neuroimmune modulators, offer new avenues for managing TMD pain, emphasizing the need for a multifaceted treatment approach.

Resistant and refractory migraine - two different entities with different comorbidities? Results from the REFINE study

BACKGROUND

Resistant and refractory migraine are commonly encountered in specialized headache centers. Several comorbidities, mostly psychiatric conditions, have been linked to migraine worsening; however, there is little knowledge of the comorbidity profile of individuals with resistant and refractory migraine.

METHODS

REFINE is a prospective observational multicenter international study involving individuals with migraine from 15 headache centers. Participants were categorized into three groups based on the European Headache Federation criteria: non-resistant and non-refractory (NRNRM), resistant (ResM), and refractory (RefM). We explored the prevalence of 20 comorbidities at baseline in the three groups.

RESULTS

Of the 689 included patients (82.8% women), 262 (38.0%) had ResM, 73 (10.4%) had RefM and 354 (51.4%) NRNRM. A higher prevalence of psychiatric comorbidities, trigger points, temporomandibular joint disorders, thyroiditis, and cerebrovascular diseases was observed in the RefM group, followed by ResM and NRNRM. Multiple comorbidities were more common in the RefM group, followed by the ResM group and by the NRNRM group (41.6% vs. 24.5% vs. 14.1% respectively; p < 0.001). At the sensitivity analysis, exploring participants with chronic migraine, significant differences among the NRNRM, ResM, and RefM groups were found in the prevalence of anxiety (p < 0.001), asthma and rhinitis (p = 0.013), bipolar and other psychiatric disorders (p = 0.049), cerebrovascular diseases (p < 0.001), depression (p < 0.001), obesity (p = 0.002), thyroiditis (p < 0.001), and trigger points (p = 0.008).

CONCLUSION

REFINE data indicate that individuals with ResM and RefM have a higher burden of comorbidities than those with NRNRM. It can be postulated that those comorbidities may have an impact on the progression of migraine from a form that is easy to treat to a form that is resistant or refractory to treatments. Longitudinal studies are needed to understand the direction of the association between ResM or RefM and those comorbidities and if proper treatment of comorbidities might help overcome treatment resistance or refractoriness.

Intradental mechano-nociceptors serve as sentinels that prevent tooth damage

Pain is the anticipated output of the trigeminal sensory neurons that innervate the tooth's vital interior 1,2 ; however, the contribution of intradental neurons to healthy tooth sensation has yet to be defined. Here, we employ in vivo Ca 2+ imaging to identify and define a population of myelinated high-threshold mechanoreceptors (intradental HTMRs) that detect superficial structural damage of the tooth and initiate jaw opening to protect teeth from damage. Intradental HTMRs remain inactive when direct forces are applied to the intact tooth but become responsive to forces when the structural integrity of the tooth is compromised, and the dentin or pulp is exposed. Their terminals collectively innervate the inner dentin through overlapping receptive fields, allowing them to monitor the superficial structures of the tooth. Indeed, intradental HTMRs detect superficial enamel damage and encode its degree, and their responses persist in the absence of either PIEZO2 or Na v 1.8 3,4 . Optogenetic activation of intradental HTMRs triggers a rapid, jaw opening reflex via contraction of the digastric muscle. Taken together, our data indicate that intradental HTMRs serve as sentinels that guard against mechanical threats to the tooth, and their activation results in physical tooth separation to minimize irreversible structural damage. Our work provides a new perspective on the role of intradental neurons as protective rather than exclusively pain-inducing and illustrates additional diversity in the functions of interoreceptors.