Connections between the facial and trigeminal nerves: Anatomical basis for facial muscle proprioception

Sensorimotor regulation of facial expression - An untouched frontier

Facial expression is a critical form of nonverbal social communication which promotes emotional exchange and affiliation among humans. Facial expressions are generated via precise contraction of the facial muscles, guided by sensory feedback. While the neural pathways underlying facial motor control are well characterized in humans and primates, it remains unknown how tactile and proprioceptive information reaches these pathways to guide facial muscle contraction. Thus, despite the importance of facial expressions for social functioning, little is known about how they are generated as a unique sensorimotor behavior. In this review, we highlight current knowledge about sensory feedback from the face and how it is distinct from other body regions. We describe connectivity between the facial sensory and motor brain systems, and call attention to the other brain systems which influence facial expression behavior, including vision, gustation, emotion, and interoception. Finally, we petition for more research on the sensory basis of facial expressions, asserting that incomplete understanding of sensorimotor mechanisms is a barrier to addressing atypical facial expressivity in clinical populations.

The Feel of Speech: Multisystem and Polymodal Somatosensation in Speech Production

PURPOSE

The oral structures such as the tongue and lips have remarkable somatosensory capacities, but understanding the roles of somatosensation in speech production requires a more comprehensive knowledge of somatosensation in the speech production system in its entirety, including the respiratory, laryngeal, and supralaryngeal subsystems. This review was conducted to summarize the system-wide somatosensory information available for speech production.

METHOD

The search was conducted with PubMed/Medline and Google Scholar for articles published until November 2023. Numerous search terms were used in conducting the review, which covered the topics of psychophysics, basic and clinical behavioral research, neuroanatomy, and neuroscience.

RESULTS AND CONCLUSIONS

The current understanding of speech somatosensation rests primarily on the two pillars of psychophysics and neuroscience. The confluence of polymodal afferent streams supports the development, maintenance, and refinement of speech production. Receptors are both canonical and noncanonical, with the latter occurring especially in the muscles innervated by the facial nerve. Somatosensory representation in the cortex is disproportionately large and provides for sensory interactions. Speech somatosensory function is robust over the lifespan, with possible declines in advanced aging. The understanding of somatosensation in speech disorders is largely disconnected from research and theory on speech production. A speech somatoscape is proposed as the generalized, system-wide sensation of speech production, with implications for speech development, speech motor control, and speech disorders.

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

Proprioceptive innervation of the human lips

The objective of this study was to analyze the proprioceptive innervation of human lips, especially of the orbicularis oris muscle, since it is classically accepted that facial muscles lack typical proprioceptors, that is, muscle spindles, but recently this has been doubted. Upper and lower human lips (n = 5) from non-embalmed frozen cadavers were immunostained for detection of S100 protein (to identify nerves and sensory nerve formations), myosin heavy chain (to label muscle fibers within muscle spindles), and the mechano-gated ion channel PIEZO2. No muscle spindles were found, but there was a high density of sensory nerve formations, which were morphologically heterogeneous, and in some cases resemble Ruffini-like and Pacinian sensory corpuscles. The axons of these sensory formations displayed immunoreactivity for PIEZO2. Human lip muscles lack typical proprioceptors but possess a dense sensory innervation which can serve the lip proprioception.

El posible papel de la irritación del nervio trigémino en la fisiopatología de la parálisis de Bell. Un reporte de caso desde la Terapia Neural

Resumen Introducción. La parálisis de Bell o parálisis facial periférica es una lesión nerviosa que afecta la función motora y sensorial del nervio facial y se presenta de forma súbita con afectación de la mímica facial. Es una neuropatía que representa un riesgo anátomo-funcional del nervio facial. El presente reporte de caso tiene como objetivo presentar, desde la perspectiva neuralterapéutica, una posible hipótesis soportada por las leyes descritas por el nervismo acerca de la patogénesis de la parálisis de Bell. Presentación de caso. Mujer de 32 años procedente de Brasil, quien presentó parálisis facial de grado III en la parte derecha. Luego de 10 días de evolución sin respuesta al tratamiento, decidió acudir a Terapia Neural. Previo análisis cronológico de la historia de vida con enfoque neuralterapéutico, se apreció una afectación distrófica del nervio trigémino dado por focos odontológicos irritativos, los cuales fueron tratados, obteniendo una mejoría clínica a grado I. Conclusiones. La conexión neuroanatómica entre el nervio facial y el trigémino posibilita la hipótesis etiológica de la parálisis de Bell. Este caso muestra el papel terapéutico que desempeñaron la Terapia Neural y la odontología en la resolución de una parálisis de Bell al abordar las irritaciones del nervio trigémino.

Delineation and histological examination of the intramuscular innervation of the platysma: Application to botulinum neurotoxin injection

This study aimed to identify the whole innervation pattern of the platysma using the Sihler's staining, and the axonal composition profile of the sensory-motor anastomosis identified by immunofluorescence assays. The findings provide a comprehensive understanding of the neural anatomy of the platysma and facilitate efficient and safe manipulation for neurotoxin injection. Ten fixed and two fresh hemifaces were included in this study. Sihler's staining was used to the study 10 fixed hemifaces and two fresh hemifaces were used for immunofluorescence assays. In all cases, the cervical branch of facial nerve (Cbr) broadly innervated the platysma, and the marginal mandibular branch of facial nerve (MMbr) provided supplementary innervation to the uppermost part of the platysma. The transverse cervical nerve (TCN), great auricular nerve (GAN), and supraclavicular nerve (SCN) were observed in the lower half of the platysma. In 30% of all cases, there was a communicating loop between the Cbr and TCN. In 20% of all the cases, a communicating branch joined between the Cbr and GAN. For successful esthetic rejuvenation procedures, a clinician should consider the Cbr distribution to the overall platysma and additionally innervation by individual nerves (MMbr, GAN, TCN, and SCN) to the middle and lower portions of the platysma muscle.

Connections between postparotid terminal branches of the facial nerve: An immunohistochemistry study

It has been assumed that connections between the postparotid terminal branches of the facial nerve are purely motor. However, the nature of their fibers remains unexplored. The aim of this study is to determine whether these connections comprise motor fibers exclusively. In total 17 connections between terminal facial nerve branches were obtained from 13 different facial nerves. Choline acetyltransferase antibody (ChAT) was used to stain the fibers in the connections and determine whether or not all of them were motor. All connections contained ChAT positive and negative fibers. The average number of fibers overall was 287 (84-587) and the average proportion of positive fibers was 63% (37.7%-91.5%). In 29% of the nerves, >75% of the fibers were ChAT+ (strongly positive); in 52.94%, 50%-75% were ChAT+ (intermediately positive); and in 17.65%, <50% were ChAT+ (weakly positive). Fibers traveling inside the postparotid terminal cranial nerve VII branch connections are not exclusively motor.

Contribution of the Synkinesis Assessment Questionnaire and the Sunnybrook Facial Grading System to the evaluation of synkinesis after peripheral facial palsy: A STROBE observational study

OBJECTIVES

The main aim of the study was to determine whether the perception of synkinesis by patients with peripheral facial palsy (PFP) matched their clinician's severity assessment. Secondary objectives comprised: (1) to determine whether objective measurement of synkinesis matched the patient's perception; and (2) is to identify factors influencing patients' perceptions.

METHODS

This retrospective study took place from January to May 2020. Forty patients (8 per PFP grade, I-V/VI; 20 women, 20 men) filled out the Synkinesis Assessment Questionnaire (SAQ) and were assessed on the Sunnybrook Facial Grading System (SFGS). Photographs were analyzed on MEEI-Facegram software.

RESULTS

Perceived synkinesis (total SAQ) matched objective grades (SFGS) (Z=2.89; P=0.004), especially for smiling (Z=3.84; P<0.001) and lip protrusion (Z=3.79; P<0.001). Synkinesis on lip protrusion was a more sensitive indicator of perceived synkinesis than synkinesis on smiling (Z=2.96; P=0.003). Duration (ρ=0.5137; P<0.001) and grade of PFP (Chi²=13.82; P=0.008) heightened the perception of synkinesis.

CONCLUSION

Patient-reported outcome measures (PROMs) such as the SAQ are relevant for clinical evaluation.

The Anatomical Specificity Between Acupoints and the Facial Nerve: A Cadaveric Study

Objective

This research was conducted to increase precision of point location and standardize acupoint targets in relation to the facial nerve (CN VII).

Materials and Methods

A literature review, cadaver dissection, and electrostimulation of the CN VII were performed, focusing on the anatomical locations of the acupoints along the facial-nerve trajectory. The results were contrasted against established acupoint locations described in the 4th edition of Chinese Acupuncture and Moxibustion.

Results

Triple Energizer (TE)-17 targets the facial nerve at its cranial exit; Qian Zheng at the facial nerve trunk; Tai Yang at the frontotemporal branch; Gallbladder (GB)-3 at the zygomatic branch; Small Intestine (SI)-18 at the buccal branch; Stomach (ST)-5 at the marginal mandibular branch; SI-17 at the cervical branch; TE-18 at the posterior auricular branch; TE-19 at the posterior auricular nerve; TE-20 at the superior auricular nerve; and TE-22 at the anterior auricular nerve.

Conclusions

This study demonstrated the specificity with which acupuncture points are located in relation to the facial nerve. Standardization of facial acupuncture points to distinct branches of the facial nerve will facilitate reproducible research designs and interpractitioner reproducibility.

A case of migraine treatment in a patient with a clinical diagnosis of CHARGE syndrome using onabotulinum toxin A

CHARGE syndrome is a genetic disorder that affects multiple organ and sensory systems. Cranial nerve involvement is one of the key clinical diagnostic criteria. We present the case of an 8-year-old girl with CHARGE syndrome, associated right-sided facial palsy, and chronic severe migraines, that were intractable to medical treatment. At age 6, onabotulinum toxin A was used to weaken the contralateral non-paralyzed side of her face to address her stigmatizing asymmetry. Onabotulinum toxin A chemodenervation was performed on the left lower lip depressors to relax the muscles and improve left lower lip position. Coincidentally, it was noted that with these treatments, migraine symptoms resolved. As the chemodenervation subsided over the next 3-4 months, the severe migraines returned. Continued treatment with onabotulinum toxin A injections every 3 months has resulted in ongoing improvements in facial symmetry and migraine control. Onabotulinum toxin A is a well-known treatment of chronic migraine. Injections are usually directed to the occipitalis, frontalis, and corrugator muscles. The literature has no reports of injections to the lower lip depressors as a useful therapy for migraine, making the results from this case unique.

A direct transcutaneous approach to infraorbital nerve biopsy

Purpose: To describe a novel transcutaneous infraorbital nerve biopsy technique which can be performed to aid in the diagnosis of perineural invasion (PNI) of facial cutaneous squamous cell carcinoma (SCC).Methods: A single-center retrospective chart review was performed. Patients diagnosed with SCC with PNI via an infraorbital nerve biopsy between February 2019 and February 2020 were included. Data collected consisted of patient demographics, medical history, clinical presentation and exam, histologic and radiographic findings, treatment, and outcomes.Results: Four patients (3 male, 1 female) met inclusion criteria. The mean age at diagnosis was 79.5 years (range 66-85 years). Three of the four patients had a history of facial skin lesions, including actinic keratosis and SCC, involving the nose, cheek, or ear. One patient had no history of cutaneous malignancy. All patients presented with cranial neuropathies, including total V2 hypoesthesia. The most common presenting symptom was facial pain, followed by diplopia, unilateral facial weakness, and hypoesthesia in the V1 and/or V2 distribution. Transcutaneous infraorbital nerve biopsy in all patients revealed squamous cell carcinoma with no biopsy complications.Conclusion: Definitive diagnosis of PNI can be challenging but is important to minimize tumor-related morbidity. Infraorbital nerve biopsy can establish this diagnosis, especially in the context of negative or indeterminate imaging findings. This work comprises the first description of a transcutaneous approach to infraorbital nerve biopsy, which is a minimally invasive technique that can be performed in an outpatient procedure suite with limited to no sedation.

Integrative traditional Korean medicine management, including acupuncture and Chuna-manual therapy, for stroke-related central facial palsy: A study of three case reports

Central facial palsy, which is a sequela of stroke, is associated with decreased quality of life and psychosocial function. Integrative Korean medicine management, including acupuncture and Chuna-manual therapy, has been used to treat neurological diseases, including facial palsy. We report three cases of central facial palsy patients who had these symptoms over three months after a stroke. They had received rehabilitation treatment for the paralyzed upper and lower extremities. However, as their central facial palsy symptom did not improve, Chuna-manual therapy (SJS nonresistance technique) for facial palsy was started along with acupuncture. Oral region movement improved after four weeks of integrative acupuncture and Chuna-manual therapy. After ten to sixteen weeks of treatment, the facial nerve grading system 2.0 grades improved by one in two cases. In one case, although there was no significant change in the grade, she was satisfied with the results after adding Chuna-manual therapy to acupuncture. There were no adverse events. Integrative management, including acupuncture and Chuna-manual therapy, might be an effective treatment strategy for central facial palsy. Further prospective, controlled studies are warranted.

Role of cutaneous and proprioceptive inputs in sensorimotor integration and plasticity occurring in the facial primary motor cortex

KEY POINTS

Previous studies investigating the effects of somatosensory afferent inputs on cortical excitability and neural plasticity often used transcranial magnetic stimulation (TMS) of hand motor cortex (M1) as a model, but in this model it is difficult to separate out the relative contribution of cutaneous and muscle afferent input to each effect. In the face, cutaneous and muscle afferents are segregated in the trigeminal and facial nerves, respectively. We studied their relative contribution to corticobulbar excitability and neural plasticity in the depressor anguli oris M1. Stimulation of trigeminal afferents induced short-latency (SAI) but not long-latency (LAI) afferent inhibition of face M1, while facial nerve stimulation evoked LAI but not SAI. Plasticity induction was observed only after a paired associative stimulation protocol using the facial nerve. Physiological differences in effects of cutaneous and muscle afferent inputs on face M1 excitability suggest they play separate functional roles in behaviour.

ABSTRACT

The lack of conventional muscle spindles in face muscles raises the question of how sensory input from the face is used to control muscle activation. In 16 healthy volunteers, we probed sensorimotor interactions in face motor cortex (fM1) using short-afferent inhibition (SAI), long-afferent inhibition (LAI) and LTP-like plasticity following paired associative stimulation (PAS) in the depressor anguli oris muscle (DAO). Stimulation of low threshold afferents in the trigeminal nerve produced a clear SAI (P < 0.05) when the interval between trigeminal stimulation and transcranial magnetic stimulation (TMS) of fM1 was 15-30 ms. However, there was no evidence for LAI at longer intervals of 100-200 ms, nor was there any effect of PAS. In contrast, facial nerve stimulation produced significant LAI (P < 0.05) as well as significant facilitation 10-30 minutes after PAS (P < 0.05). Given that the facial nerve is a pure motor nerve, we presume that the afferent fibres responsible were those activated by the evoked muscle twitch. The F-wave in DAO was unaffected during both LAI and SAI, consistent with their presumed cortical origin. We hypothesize that, in fM1, SAI is evoked by activity in low threshold, presumably cutaneous afferents, whereas LAI and PAS require activity in (higher threshold) afferents activated by the muscle twitch evoked by electrical stimulation of the facial nerve. Cutaneous inputs may exert a paucisynaptic inhibitory effect on fM1, while proprioceptive information is likely to target inhibitory and excitatory polysynaptic circuits involved in LAI and PAS. Such information may be relevant to the physiopathology of several disorders involving the cranio-facial system.

Changes in Articulatory Control Pre- and Post-Facial Transplant: A Case Report

Purpose Facial transplantation involves partial or total replacement of neuromuscular and skeletal structures of the face, head, and neck using donor tissues and is among the most extensive facial reconstructive procedures. This case report compares changes in speech production and articulator movement in a 44-year-old man from pretransplant to a 13-month posttransplant period. Method Speech production and articulator movement data were examined at 5 time points, once pretransplant and 4 times posttransplant (4, 7, 10, and 13 months), and compared to 4 healthy controls. A motion capture system was used to track jaw and vertical/horizontal lip movement during nonspeech and speech tasks. Speech intelligibility, jaw displacement, lip aperture, and movement variability were measured. Results Speech intelligibility varied across the study period and was restored to control status by 7 months posttransplant. Jaw displacement and lip aperture in the vertical plane significantly increased over time for nonspeech and speech tasks. Changes in horizontal lip movements over time were minimal. Jaw and lip movement variability fluctuated over time and was greater than the controls by 13 months posttransplant. Discussion Findings quantify changes in articulator movement and contributions to improved speech production following facial transplant. Changes reflect the adaptability of the speech motor system and are discussed in relation to pretransplant speech motor control patterns.

An Integrative Model Accounting for the Symptom Cluster Triggered After an Acoustic Shock

Acoustic shocks and traumas sometimes result in a cluster of debilitating symptoms, including tinnitus, hyperacusis, ear fullness and tension, dizziness, and pain in and outside the ear. The mechanisms underlying this large variety of symptoms remain elusive. In this article, we elaborate on the hypothesis that the tensor tympani muscle (TTM), the trigeminal nerve (TGN), and the trigeminal cervical complex (TCC) play a central role in generating these symptoms. We argue that TTM overuse (due to the acoustic shock), TTM overload (due to muscle tension), and ultimately, TTM injury (due to hypoxia and "energy crisis") lead to inflammation, thereby activating the TGN, TCC, and cortex. The TCC is a crossroad structure integrating sensory inputs coming from the head-neck complex (including the middle ear) and projecting back to it. The multimodal integration of the TCC may then account for referred pain outside the ear when the middle ear is inflamed and activates the TGN. We believe that our model proposes a synthetic and explanatory framework to explain the phenomena occurring postacoustic shock and potentially also after other nonauditory causes. Indeed, due to the bidirectional properties of the TCC, musculoskeletal disorders in the region of the head-neck complex, including neck injury due to whiplash or temporomandibular disorders, may impact the middle ear, thereby leading to otic symptoms. This previously unavailable model type is experimentally testable and must be taken as a starting point for identifying the mechanisms responsible for this particular subtype of tinnitus and its associated symptoms.

The Use of Botulinum Toxin for Treatment of Depression

A series of clinical studies have shown that botulinum toxin can treat major depression. Subjects suffering from unipolar depression may experience a quick, strong, and sustained improvement in the symptoms of depression after a single glabellar treatment with botulinum toxin.Preliminary data suggest that botulinum toxin therapy may also be effective in the treatment of other mental disorders characterized by an excess of negative emotions, such as borderline personality disorder.The mood-lifting effect of botulinum toxin therapy is probably mediated by the interruption of a proprioceptive feedback loop from the facial musculature to the emotional brain.

Characterization of sensory neuronal subtypes innervating mouse tongue

The tongue is uniquely exposed to water-soluble environmental chemicals that may lead to injury or tumorigenesis. However, comparatively little research has focused on the molecular and functional organization of trigeminal ganglia (TG) afferent neurons innervating the tongue. The current study identified and characterized lingual sensory neurons based on a neuronal subtype classification previously characterized in the dorsal root ganglion (DRG) neurons. We employed immunohistochemistry on transgenic reporter mouse lines as well as single-cell PCR of known markers of neuronal subtypes to characterize neuronal subtypes innervating the tongue. Markers expressed in retrogradely labeled TG neurons were evaluated for the proportion of neurons expressing each marker, intensity of expression, and overlapping genes. We found that tongue-innervating sensory neurons primarily expressed CGRP, TRPV1, TrkC, 5HT3A and Parvalbumin. These markers correspond to peptidergic and a subgroup of non-peptidergic C-nociceptors, peptidergic A nociceptors, proprioceptors and myelinated low-threshold mechanoreceptors (LTMRs). Interestingly, as reported previously, we also found several differences between TG and DRG neurons indicating the need for single-cell sequencing of neuronal types based on tissue type within all TG as well as DRG neurons.

Improvement of Olfactory Function With High Frequency Non-invasive Auricular Electrostimulation in Healthy Humans

In past literature on animal models, invasive vagal nerve stimulation using high frequencies has shown to be effective at modulating the activity of the olfactory bulb (OB). Recent advances in invasive vagal nerve stimulation in humans, despite previous findings in animal models, used low frequency stimulation and found no effect on the olfactory functioning. The present article aimed to test potential effects of non-invasive, high and low frequency vagal nerve stimulation in humans, with supplementary exploration of the orbitofrontal cortex using near-infrared spectroscopy (NIRS). Healthy, male adult participants (n = 18) performed two olfactory tests [odor threshold test (OTT) and supra-threshold test (STT)] before and after receiving high-, low frequency vagal nerve stimulation and placebo (no stimulation). Participant's olfactory functioning was monitored using NIRS, and assessed with two behavioral olfactory tests. NIRS data of separate stimulation parameters were statistically analyzed using repeated-measures ANOVA across different stages. Data from olfactory tests were analyzed using paired parametric and non-parametric statistical tests. Only high frequency, non-invasive vagal nerve stimulation was able to positively modulate the performance of the healthy participants in the STT (p = 0.021, Wilcoxon sign-ranked test), with significant differences in NIRS (p = 0.014, post-hoc with Bonferroni correction) recordings of the right hemispheric, orbitofrontal cortex. The results from the current article implore further exploration of the neurocircuitry involved under vagal nerve stimulation and the effects of non-invasive, high frequency, vagal nerve stimulation toward olfactory dysfunction which showcase in Parkinson's and Alzheimer's Diseases. Despite the sufficient effect size (moderate effect, correlation coefficient (r): 0.39 for the STT) of the current study, future research should replicate the current findings with a larger cohort.