Anatomical Study of the Innervation of the Masseter Muscle and Its Correlation with Myofascial Trigger Points

Trigeminal somatosensation in the temporomandibular joint and associated disorders

The temporomandibular joint (TMJ) consists of bone, cartilage, ligaments, and associated masticatory muscles and tendons that coordinate to enable mastication in mammals. The TMJ is innervated by the trigeminal nerve (CNV), containing axons of motor and somatosensory neurons. Somatosensation includes touch, temperature, proprioception, and pain that enables mammals to recognize and react to stimuli for survival. The somatosensory innervation of the TMJ remains poorly defined. Disorders of the TMJ (TMD) are of diverse etiology and presentation. Some known symptoms associated with TMD include facial, shoulder, or neck pain, jaw popping or clicking, headaches, toothaches, and tinnitus. Acute or chronic pain in TMD stems from the activation of somatosensory nociceptors. Treatment of TMD may involve over- the-counter and prescription medication, nonsurgical treatments, and surgical treatments. In many cases, treatment achieves only a temporary relief of symptoms including pain. We suggest that defining the sensory innervation of the temporomandibular joint and its associated tissues with a specific focus on the contribution of peripheral innervation to the development of chronic pain could provide insights into the origins of joint pain and facilitate the development of improved analgesics and treatments for TMD.

Anatomical Bases of the Temporal Muscle Trigger Points

Method

Temporal muscles of 14 adult cadavers were studied. The muscle bellies were divided into six areas, three superior (1.2 and 3) and three inferior areas (4, 5, and 6) lower, according to a Cartesian plane to analyze and describe the entry points of the branches of the deep temporal nerves into the muscle. The branching distribution was analyzed using Poisson log-linear tests with Bonferroni post hoc tests for comparison between groups (sextants) (p < 0.05).

Results

Deep temporal nerve entry points were found in the temporal muscle in all areas. Most of the branches were observed in areas 2 and 5, which coincide with the muscle fibers responsible for mandible elevation and related to the previously described MTPs. Fewer branches were found in areas 1 and 6, where contraction produces mandible retraction.

Conclusion

There is an anatomical correlation between the branching pattern of the deep temporal nerve and temporal muscle trigger points. Adequate knowledge of the innervation of the temporal muscle may help elucidate the pathophysiology of myofascial syndromes and provide a rational basis for interventional or conservative approaches and help surgeons avoid iatrogenic lesions to the deep temporal nerve lesion.

Neurovascular Compression-Induced Intracranial Allodynia May Be the True Nature of Migraine Headache: an Interpretative Review

PURPOSE OF REVIEW

Surgical deactivation of migraine trigger sites by extracranial neurovascular decompression has produced encouraging results and challenged previous understanding of primary headaches. However, there is a lack of in-depth discussions on the pathophysiological basis of migraine surgery. This narrative review provides interpretation of relevant literature from the perspective of compressive neuropathic etiology, pathogenesis, and pathophysiology of migraine.

RECENT FINDINGS

Vasodilation, which can be asymptomatic in healthy subjects, may produce compression of cranial nerves in migraineurs at both extracranial and intracranial entrapment-prone sites. This may be predetermined by inherited and acquired anatomical factors and may include double crush-type lesions. Neurovascular compression can lead to sensitization of the trigeminal pathways and resultant cephalic hypersensitivity. While descending (central) trigeminal activation is possible, symptomatic intracranial sensitization can probably only occur in subjects who develop neurovascular entrapment of cranial nerves, which can explain why migraine does not invariably afflict everyone. Nerve compression-induced focal neuroinflammation and sensitization of any cranial nerve may neurogenically spread to other cranial nerves, which can explain the clinical complexity of migraine. Trigger dose-dependent alternating intensity of sensitization and its synchrony with cyclic central neural activities, including asymmetric nasal vasomotor oscillations, may explain the laterality and phasic nature of migraine pain. Intracranial allodynia, i.e., pain sensation upon non-painful stimulation, may better explain migraine pain than merely nociceptive mechanisms, because migraine cannot be associated with considerable intracranial structural changes and consequent painful stimuli. Understanding migraine as an intracranial allodynia could stimulate research aimed at elucidating the possible neuropathic compressive etiology of migraine and other primary headaches.

Nerve entry points - The anatomy beneath trigger points

INTRODUCTION

Myofascial trigger points (MTrPs) have been the subject of considerable scientific research for almost forty years. In their seminal paper, Travell and Simons described a model based on the presence of highly irritable, palpable nodules within taut bands of muscle. Since then, a significant number of studies have increased our understanding of the phenomenon, which has, in turn, resulted in refutation of the original model. Alternative models have explained certain properties of MTrP but fail to provide an explanation of their spatial distribution. The aim of this paper was to propose a hypothesis connecting MTrPs and distinct points along the course of the nerve called nerve entry points (NEPs). A literature review was performed in order to identify studies to support hypothesis development.

METHODS

Literature search of digital databases.

RESULTS

A total of 4631 abstracts were screened; 72 were selected for further review. Four articles made a direct connection between MTrPs and NEPs. Another fifteen articles provided high-quality data regarding the distribution of NEPs, thus strengthening the hypothesis.

CONCLUSIONS

There is sufficient evidence to hypothesise that NEPs are the anatomical basis for MTrPs. This presented hypothesis addresses one of the crucial issues in diagnosing trigger points, which is the lack of repeatable and reliable diagnostic criteria. By connecting subjective phenomenon of trigger points with objective anatomy, this paper provides a novel and practical foundation for identifying and treating pain conditions associated with MTrPs.

The effects of Gram-positive and Gram-negative bacterial toxins (LTA & LPS) on cardiac function in Drosophila melanogaster larvae

The effects of Gram negative and positive bacterial sepsis depend on the type of toxins released, such as lipopolysaccharides (LPS) or lipoteichoic acid (LTA). Previous studies show LPS to rapidly hyperpolarize larval Drosophila skeletal muscle, followed by desensitization and return to baseline. In larvae, heart rate increased then decreased with exposure to LPS. However, responses to LTA, as well as the combination of LTA and LPS, on the larval Drosophila heart have not been previously examined. This study examined the effects of LTA and a cocktail of LTA and LPS on heart rate. The combined effects were examined by first treating with either LTA or LPS only, and then with the cocktail. The results showed a rapid increase in heart rate upon LTA application, followed by a gradual decline over time. When applying LTA followed by the cocktail, an increase in the rate occurred. However, if LPS was applied before the cocktail, the rate continued declining. These responses indicate the receptors or cellular cascades responsible for controlling heart rate within seconds and the rapid desensitization are affected by LTA or LPS and a combination of the two. The mechanisms for rapid changes which are not regulated by gene expression by exposure to LTA or LPS or associated bacterial peptidoglycans have yet to be identified in cardiac tissues of any organism.

Comparative Efficacy of Platelet-Rich Plasma and Dry Needling for Management of Trigger Points in Masseter Muscle in Myofascial Pain Syndrome Patients: A Randomized Controlled Trial

AIMS

To compare the efficacy of platelet-rich plasma (PRP) injection vs dry needling (DN) for management of trigger points in the masseter muscle in myofascial pain syndrome (MPS) patients.

METHODS

This randomized controlled trial included 30 clinically confirmed cases of myofascial trigger points (MTrPs) in the masseter muscle who were randomly and equally (1:1) assigned to the test (PRP) and control (DN) groups. Both groups were evaluated for pain (visual analog scale [VAS]), range of functional movements, need for pain medication, patient satisfaction (Likert scale), and sleep (VAS) at baseline and 2-week, 1-month, and 3-month follow-ups. VAS pain and Likert score were also obtained at 6-month intervals.

RESULTS

The use of PRP solution in MTrPs in MPS patients had a better effect on pain and patient satisfaction compared to DN.

CONCLUSION

PRP appears to be a more effective treatment modality compared to DN in the management of MTrPs in MPS patients.

Exosomal mediated signal transduction through artificial microRNA (amiRNA): A potential target for inhibition of SARS-CoV-2

Exosome trans-membrane signals provide cellular communication between the cells through transport and/or receiving the signal by molecule, change the functional metabolism, and stimulate and/or inhibit receptor signal complexes. COVID19 genetic transformations are varied in different geographic positions, and single nucleotide polymorphic lineages were reported in the second waves due to the fast mutational rate and adaptation. Several vaccines were developed and in treatment practice, but effective control has yet to reach in cent presence. It was initially a narrow immune-modulating protein target. Controlling these diverse viral strains may inhibit their transuding mechanisms primarily to target RNA genes responsible for COVID19 transcription. Exosomal miRNAs are the main sources of transmembrane signals, and trans-located miRNAs can directly target COVID19 mRNA transcription. This review discussed targeted viral transcription by delivering the artificial miRNA (amiRNA) mediated exosomes in the infected cells and significant resources of exosome and their efficacy.

Identification of Trigeminal Sensory Neuronal Types Innervating Masseter Muscle

Understanding masseter muscle (MM) innervation is critical for the study of cell-specific mechanisms of pain induced by temporomandibular disorder (TMDs) or after facial surgery. Here, we identified trigeminal (TG) sensory neuronal subtypes (MM TG neurons) innervating MM fibers, masseteric fascia, tendons, and adjusted tissues. A combination of patch clamp electrophysiology and immunohistochemistry (IHC) on TG neurons back-traced from reporter mouse MM found nine distinct subtypes of MM TG neurons. Of these neurons, 24% belonged to non-peptidergic IB-4+/TRPA1- or IB-4+/TRPA1+ groups, while two TRPV1+ small-sized neuronal groups were classified as peptidergic/CGRP+ One small-sized CGRP+ neuronal group had a unique electrophysiological profile and were recorded from Nav1.8- or trkC+ neurons. The remaining CGRP+ neurons were medium-sized, could be divided into Nav1.8-/trkC- and Nav1.8low/trkC+ clusters, and showed large 5HT-induced current. The final two MM TG neuronal groups were trkC+ and had no Nav1.8 and CGRP. Among MM TG neurons, TRPV1+/CGRP- (somatostatin+), tyrosine hydroxylase (TH)+ (C-LTMR), TRPM8+, MrgprA3+, or trkB+ (Aδ-LTMR) subtypes have not been detected. Masseteric muscle fibers, tendons and masseteric fascia in mice and the common marmoset, a new world monkey, were exclusively innervated by either CGRP+/NFH+ or CGRP-/NFH+ medium-to-large neurons, which we found using a Nav1.8-YFP reporter, and labeling with CGRP, TRPV1, neurofilament heavy chain (NFH) and pgp9.5 antibodies. These nerves were mainly distributed in tendon and at junctions of deep-middle-superficial parts of MM. Overall, the data presented here demonstrates that MM is innervated by a distinct subset of TG neurons, which have unique characteristics and innervation patterns.

Topographic anatomical localization of the motor nerve entry points (MEPs) of the masseter muscle

PURPOSE

The masseteric nerve, which is a branch of the mandibular nerve, passes lateral to the mandibular notch and then spreads in the muscle to achieve motor innervation. The muscle entry points of these motor branches are the target points of minimally invasive interventions preferred in the treatment of masseter hypertrophy. The aim of this study was to reveal the areas where the motor entry points are concentrated in the muscle by dividing the muscle into topographic regions using reliable anatomic landmarks.

METHODS

Bilateral 20 masseter muscles (40 in total) belonging to 20 formalin-fixed cadavers (10 female and 10 male) were examined. The distribution of the nerve in the muscle and its motor entry points were demonstrated and marked on the muscle surface. The masseter muscle was divided into six areas by lines passing through reliable anatomical landmarks (Areas 1-6).

RESULTS

The total number of MEPs was 231.The mean distance of the MEPs from the Line-1 was 27.4 ± 11 mm, and the same distance from the Line-6 was 30.32 ± 7.2 mm. Most of the MEPs (123/231) were located in Area-4. Area-6 was the second (82/231) and Area-5 (19/231) was the third.

CONCLUSION

We suggest that interventions in Area-4 (especially in the middle part) may have less complications as a result of less relationship with surrounding anatomical structures and more effective with high MEP number.