Intracortical modulation of cortical-bulbar responses for the masseter muscle

Sensorimotor integration in cranial muscles tested by short- and long-latency afferent inhibition

OBJECTIVE

To compressively investigate sensorimotor integration in the cranial-cervical muscles in healthy adults.

METHODS

Short- (SAI) and long-latency afferent (LAI) inhibition were probed in the anterior digastric (AD), the depressor anguli oris (DAO) and upper trapezius (UT) muscles. A transcranial magnetic stimulation pulse over primary motor cortex was preceded by peripheral stimulation delivered to the trigeminal, facial and accessory nerves using interstimulus intervals of 15-25 ms and 100-200 ms for SAI and LAI respectively.

RESULTS

In the AD, both SAI and LAI were detected following trigeminal nerve stimulation, but not following facial nerve stimulation. In the DAO, SAI was observed only following trigeminal nerve stimulation, while LAI depended only on facial nerve stimulation, only at an intensity suprathreshold for the compound motor action potential (cMAP). In the UT we could only detect LAI following accessory nerve stimulation at an intensity suprathreshold for a cMAP.

CONCLUSIONS

The results suggest that integration of sensory inputs with motor output is profoundly influenced by the type of sensory afferent involved and by the functional role played by the target muscle.

SIGNIFICANCE

Data indicate the importance of taking into account the sensory receptors involved as well as the function of the target muscle when studying sensorimotor integration, both in physiological and neurological conditions.

Pathophysiological mechanisms of oromandibular dystonia

Oromandibular dystonia (OMD) is a rare form of focal idiopathic dystonia. OMD was clinically identified at the beginning of the 20th century, and the main clinical features have been progressively described over the years. However, OMD has several peculiarities that still remain unexplained, including the high rate of oral trauma, which is often related to the onset of motor symptoms. The purpose of this paper was to formulate a hypothesis regarding the pathophysiology of OMD, starting from the neuroanatomical basis of the masticatory and facial systems and highlighting the features that differentiate this condition from other forms of focal idiopathic dystonia. We provide a brief review of the clinical and etiological features of OMD as well as neurophysiological and neuroimaging findings obtained from studies in patients with OMD. We discuss possible pathophysiological mechanisms underlying OMD and suggest that abnormalities in sensory input processing may play a prominent role in OMD pathophysiology, possibly triggering a cascade of events that results in sensorimotor cortex network dysfunction. Finally, we identify open questions that future studies should address, including the effect of abnormal sensory input processing and oral trauma on the peculiar neurophysiological abnormalities observed in OMD.

Changes of neuroplasticity in cortical motor control of human masseter muscle related to orthodontic treatment

BACKGROUND

Orthodontic treatment is a common clinical method of malocclusion. Studies have found that neurons in the sensorimotor cortex of the brain undergo adaptive remodeling in response to changes in oral behavior or occlusion.

OBJECTIVE

To explore whether orthodontic treatment could be sufficient to cause neuroplastic changes in the corticomotor excitability of the masseter muscle.

METHODS

Fifteen Angle Class II malocclusion patients who were receiving orthodontic treatment participated in the study. Cortical excitability was assessed by electromyographic activity changes evoked by transcranial magnetic stimulation. Four orthodontic time points were recorded, including baseline, day 1, day 7, and day 30. Motor evoked potentials (MEPs) were recorded in the masseter muscle and the first dorsal interosseous muscle (FDI) serving as a control. The data were analysed by stimulus-response curves and corticomotor mapping. Statistical analyses involved repeated measures analysis of variance, two-way ANOVA, and Tukey's post hoc tests.

RESULTS

Motor evoked potentials (MEPs) of the masseter muscle were significantly decreased during orthodontic treatment compared with those of the baseline (p < .001). MEPs of the masseter muscle were dependent on session and stimulus intensity (p < .001), whereas MEPs of FDI were only dependent on stimulus intensity (p = .091). Finally, Tukey's post hoc tests demonstrated that MEPs of the masseter muscle on days 1 and 7, with 70%-90% stimulus intensities, were higher than those of baseline values (p < .001).

CONCLUSIONS

The present study suggested that orthodontic treatment can lead to neuroplastic changes in the corticomotor control of the masseter muscle, which may add to our understanding of the adaptive response of subjects to changes of oral environment during the orthodontic treatment.

Predicting Resting Oral Commissure Tone Outcomes Following Masseter Nerve Transfer in Facial Reanimation

Objective: To quantify the degree of oral commissure resting tone improvement in patients undergoing masseter to facial nerve transfer. Methods: A retrospective cohort study was completed in a tertiary academic medical practice. Consecutive cases of masseter nerve transfer patients within a patient database were evaluated from 6/2012 to 9/2017. Inclusion criteria were patients >18 years of age, with complete unilateral paralysis, receiving a masseter to facial nerve transfer, with at least 12 months of recovery, and possessing complete pre- and postoperative data. Patients were excluded if a simultaneous adjunctive procedure was performed so that tone could not be attributed to masseter transfer alone. The main outcome measure was the facial asymmetry index (FAI): the measured difference in distance between the medial canthus and oral commissure of the healthy and paralyzed sides. Results: Twenty-nine patients met inclusion and exclusion criteria and were further analyzed for this study. The oral commissure symmetry improved from 4.7 ± 2.8 mm preoperatively to 2.2 ± 2.3 mm postoperatively. In multivariate analysis, the preoperative FAI was the only significant predictive factor for improvement in commissure symmetry at rest (r = 0.589). This suggests that for each 1.0 mm of worse preoperatively oral commissure asymmetry, the improvement postoperatively was 0.6 mm. Age, gender, body mass index, side of paralysis, duration of paralysis, and recipient branch of facial nerve were not significant predictors in a multivariate analysis. Conclusion: Masseter to facial nerve transfer yields an estimated 60% correction in the oral commissure asymmetry. This estimation may be helpful in determining if adjunctive procedures should be utilized.

Combination of jaw and tongue movement training influences neuroplasticity of corticomotor pathways in humans

Since humans in daily life perform multiple motor behaviors that often involve the simultaneous activation of both jaw and tongue muscles, it is essential to understand the effects of combined orofacial sensorimotor tasks on plasticity in corticomotor pathways. Moreover, to establish novel rehabilitation programs for patients, it is important to clarify the possible interrelationships in corticomotor excitability between jaw and tongue motor control. The aim of this study was to examine the effect of a combination of a repetitive tooth bite task (TBT) and a repetitive tongue lift task (TLT) on corticomotor excitability of the tongue and jaw muscles as assessed by transcranial magnetic stimulation (TMS). Sixteen healthy individuals participated in three kinds of training tasks consisting of 41-min TBT, 41-min TLT, and 82-min TBT + TLT. Motor-evoked potentials (MEPs) from the tongue muscle, masseter muscle, and first dorsal interosseous muscle were measured before and after the training tasks. The amplitude of tongue MEPs after training with TLT and TLT + TBT, and masseter MEPs after training with TBT and TLT + TBT, were significantly higher than before training (P < 0.05). Tongue MEPs and masseter MEPs were significantly higher after TLT + TBT than after TBT or TLT (P < 0.05). The present results suggest that a task combining both jaw and tongue movement training is associated with a greater degree of neuroplasticity in the corticomotor control of jaw and tongue muscles than either task alone.

Electrical Stimulation of Back Muscles Does Not Prime the Corticospinal Pathway

OBJECTIVES

To investigate whether peripheral electrical stimulation (PES) of back extensor muscles changes excitability of the corticospinal pathway of the stimulated muscle and synergist trunk muscles.

METHODS

In 12 volunteers with no history of low back pain (LBP), intramuscular fine-wire electrodes recorded electromyography (EMG) from the deep multifidus (DM) and longissimus muscles. Surface electrodes recorded general EMG from the erector spinae and abdominal muscles. Single- and paired-pulse transcranial magnetic stimulation (TMS) paradigms tested corticospinal excitability, short-interval intracortical inhibition (SICI-2 and 3 ms), and intracortical facilitation (ICF) optimized for recordings of DM. Active motor threshold (aMT) to evoke a motor-evoked potential (MEP) in DM was determined and stimulation was applied at 120% of this intensity. PES was provided via electrodes placed over the right multifidus. The effect of 20-min PES (ramped motor activation) was studied.

RESULTS

Mean aMT for DM was 42.7 ± 10% of the maximal stimulator output. No effects of PES were found on MEP amplitude (single-pulse TMS) for any trunk muscles examined. There was no evidence for changes in SICI or ICF; that is, conditioned MEP amplitude was not different between trials after PES.

CONCLUSION

Results indicate that, unlike previous reports that show increased corticospinal excitability of limb muscles, PES of back muscles does not modify the corticospinal excitability. This difference in response of the motor pathway of back muscles to PES might be explained by the lesser importance of voluntary cortical drive to these muscles and the greater role of postural networks. Whether PES influences back muscle training remains unclear, yet the present results suggest that potential effects are unlikely to be explained by the effects of PES at corticospinal level with the parameters used in this study.

Impact of sleep bruxism on training-induced cortical plasticity

PURPOSE

To investigate if sleep bruxism (SB) influences training-induced cortical plasticity and performance in terms of accuracy and precision of a tooth-clenching task (TCT).

METHODS

Thirty-eight participants were allocated into SB group (N=19) and control group (N=19) according to presence of SB based on a 2-week screening. The participants were instructed to perform a standardized TCT for 58min at three different force levels (10%, 20% and 40% of maximum voluntary contraction; MVC) in three series (first and third without visual-feedback and second with visual-feedback). Accuracy and precision of the TCT were calculated from actual bite force values. Transcranial magnetic stimulation was applied to elicit motor evoked potentials (MEPs) from the masseter and first dorsal interosseous muscle (FDI) before the TCT (pre-TCT-session) and 5-min after the TCT (post-TCT-session).

RESULTS

Accuracy was significantly dependent on the series and target force level (P<0.001), however, there was a significant decrease only in the control group at 10% MVC from first to third session (P<0.001). No significant differences between groups were observed for the precision of the TCT. Masseter MEPs in the SB group in the pre-TCT-session at 120% and 160% motor threshold were significantly lower than in the control group (P<0.05). Masseter MEPs of the control group in the post-TCT-session were significantly higher than the pre-TCT-session (P<0.05) but not SB. FDI MEPs were only dependent on stimulus intensity (P<0.001).

CONCLUSIONS

SB is associated with significant changes not only in excitability of corticomotor control but also motor learning of jaw movements and force control.

Cortical Regulation of Nociception of the Trigeminal Nucleus Caudalis

Pain perception is strongly influenced by descending pathways from "higher" brain centers that regulate the activity of spinal circuits. In addition to the extensively studied descending system originating from the medulla, the neocortex provides dense anatomical projections that directly target neurons in the spinal cord and the spinal trigeminal nucleus caudalis (SpVc). Evidence exists that these corticotrigeminal pathways may modulate the processing of nociceptive inputs by SpVc, and regulate pain perception. We demonstrate here, with anatomical and optogenetic methods, and using both rats and mice (of both sexes), that corticotrigeminal axons densely innervate SpVc, where they target and directly activate inhibitory and excitatory neurons. Electrophysiological recordings reveal that stimulation of primary somatosensory cortex potently suppresses SpVc responses to noxious stimuli and produces behavioral hypoalgesia. These findings demonstrate that the corticotrigeminal pathway is a potent modulator of nociception and a potential target for interventions to alleviate chronic pain.SIGNIFICANCE STATEMENT Many chronic pain conditions are resistant to conventional therapy. Promising new approaches to pain management capitalize on the brain's own mechanisms for controlling pain perception. Here we demonstrate that cortical neurons directly innervate the brainstem to drive feedforward inhibition of nociceptive neurons. This corticotrigeminal pathway suppresses the activity of these neurons and produces analgesia. This corticotrigeminal pathway may constitute a therapeutic target for chronic pain.

Decreased γ-aminobutyric acid levels in the brainstem in patients with possible sleep bruxism: A pilot study

BACKGROUND

An increasing number of studies have indicated that the central and autonomic nervous systems play roles in the genesis of sleep bruxism (SB). The role of specific neurochemicals in SB has been a subject of interest.

OBJECTIVE

In this study, we use proton magnetic resonance spectroscopy (¹ H-MRS) to determine whether the levels of γ-aminobutyric acid (GABA) and glutamate (Glu) are different in the brainstem and bilateral cortical masticatory area (CMA) between possible sleep bruxism (SB) patients and controls, and discuss whether the brainstem or cortical networks which may affect the central masticatory pathways are under the genesis of SB.

METHODS

Twelve possible SB patients and twelve age- and gender-matched controls underwent ¹ H-MRS using the "MEGA-Point Resolved Spectroscopy Sequence" (MEGA-PRESS) technique in the brainstem and bilateral CMA. Proton magnetic resonance spectroscopy data were processed using LCModel. Because the signal detected by MEGA-PRESS includes contributions from GABA, macromolecules (primarily proteins) and homocarnosine, the GABA signal is referred to as "GABA+". The glutamate complex (Glx) signal contains both glutamate (Glu) and glutamine (Gln), which mainly reflect glutamatergic metabolism.

RESULTS

Edited spectra were successfully obtained from the bilateral CMA in all subjects. There were no significant differences in neurochemical levels between the left and right CMA in possible SB patients and controls. In the brainstem, significantly lower GABA+ levels were found in possible SB patients than in controls (P = .011), whereas there was no significant difference (P = .307) in Glx levels between the 2 groups.

CONCLUSIONS

SB patients may possess abnormalities in the GABAergic system of brainstem networks.

Combination of Static Magnetic Fields and Peripheral Nerve Stimulation Can Alter Focal Cortical Excitability

For clinical application of transcranial static magnetic stimulation (tSMS), it is important to achieve a focal target cortical stimulation. Previous study suggested that the associative stimulation combining non-invasive stimulation of the motor cortex (M1) and the peripheral nerve stimulation (PNS) may be useful to produce cortical excitability change. To test this hypothesis, we measured the M1 excitability and intracortical circuits by using transcranial magnetic stimulation (TMS) before and after the tSMS of short duration (5 min) combined with PNS. Thirty-three normal volunteers were participated; tSMS+PNS (n = 11), sham+PNS (n = 11), and tSMS alone (n = 11). We found the transient suppression of the motor-evoked potential (MEP) of the right abductor pollicis brevis (APB) muscle, but not of the abductor digiti minimi (ADM) muscle, when combining tSMS with PNS over median nerve at the wrist. The lack of suppressive effect on APB in tSMS alone with short duration is in accord with the previous observation. In addition, the tendency of transient enhancement of the short-latency intracortical inhibition was observed immediately after intervention in the tSMS±PNS group. These findings show that the combination of tSMS and PNS can induce the cortical excitability change in target cortical motor area and potentiate the suppression effect.

Short-term effects of repetitive transcranial magnetic stimulation on sleep bruxism - a pilot study

The purpose of this study was to investigate the effects of repetitive transcranial magnetic stimulation (rTMS) on patients with sleep bruxism (SB). Twelve patients with SB were included in an open, single-intervention pilot study. rTMS at 1 Hz and an intensity of 80% of the active motor threshold was applied to the ‘hot spot' of the masseter muscle representation at the primary motor cortex bilaterally for 20 min per side each day for 5 consecutive days. The jaw-closing muscle electromyographic (EMG) activity during sleep was recorded with a portable EMG recorder at baseline, during rTMS treatment and at follow-up for 5 days. In addition, patients scored their jaw-closing muscle soreness on a 0–10 numerical rating scale (NRS). Data were analysed with analysis of variance. The intensity of the EMG activity was suppressed during and after rTMS compared to the baseline (P = 0.04; P = 0.02, respectively). The NRS score of soreness decreased significantly during and after rTMS compared with baseline (P < 0.01). These findings indicated a significant inhibition of jaw-closing muscle activity during sleep along with a decrease of muscle soreness. This pilot study raises the possibility of therapeutic benefits from rTMS in patients with bruxism and calls for further and more controlled studies.

The effects of impulsivity and proactive inhibition on reactive inhibition and the go process: insights from vocal and manual stop signal tasks

This study measured proactive and reactive response inhibition and their relationships with self-reported impulsivity. We examined the domains of both vocal and manual responding using a stop signal task (SST) with two stop probabilities: high and low probability stop (1/3 and 1/6 stops respectively). Our aim was to evaluate the effect stop probability would have on reactive and proactive inhibition. We tested 44 subjects and found that for the high compared to low probability stop signal condition, more proactive inhibition was evident and this was correlated with a reduction in the stop signal reaction time (SSRT). We found that reactive inhibition had a positive relationship with dysfunctional but not functional impulsivity in both vocal and manual domains of responding. These findings support the hypothesis that proactive inhibition may pre-activate the network for reactive inhibition.

Vocal response inhibition is enhanced by anodal tDCS over the right prefrontal cortex

Stopping outright (reactive inhibition) and slowing down (proactive inhibition) are types of response inhibition which have mainly been investigated in the manual effector system. This study compared reactive inhibition across manual and vocal effector systems, examined the effects of excitatory anodal transcranial direct current stimulation (anodal tDCS) over the right prefrontal cortex (right-PFC) and looked at the relationship between reactive and proactive inhibition. We hypothesised (1) that vocal reactive inhibition would be less effective than manual reactive inhibition as evidenced by longer stop signal reaction times; (2) that anodal tDCS would enhance both vocal and manual reactive inhibitions and (3) that proactive and reactive inhibitions would be positively related. We tested 14 participants over two sessions (one session with anodal tDCS and one session with sham stimulation) and applied stimulation protocol in the middle of the session, i.e. only during the second of three phases. We used a stop signal task across two stop conditions: relevant and irrelevant stop conditions in which stopping was required or ignored, respectively. We found that reactive inhibition was faster during and immediately after anodal tDCS relative to sham. We also found that greater level of proactive inhibition enhanced reactive inhibition (indexed by shorter stop signal reaction times). These results support the hypothesis that the right-PFC is part of a core network for reactive inhibition and supports previous contention that proactive inhibition is possibly modulated via preactivating the reactive inhibition network.

Cortical Silent Period Reveals Differences Between Adductor Spasmodic Dysphonia and Muscle Tension Dysphonia

BACKGROUND

The pathophysiology of adductor spasmodic dysphonia (AdSD), like other focal dystonias, is largely unknown.

OBJECTIVE

The purposes of this study were to determine (a) cortical excitability differences between AdSD, muscle tension dysphonia (MTD), and healthy controls; (b) distribution of potential differences in cranial or skeletal muscle; and (c) if cortical excitability measures assist in the differential diagnosis of AdSD and MTD.

METHODS

Ten participants with adductor spasmodic dysphonia, 8 with muscle tension dysphonia, and 10 healthy controls received single and paired pulse transcranial magnetic stimulation (TMS) to the primary motor cortex contralateral to tested muscles, first dorsal interosseus (FDI), and masseter. We tested the hypothesis that cortical excitability measures in AdSD would be significantly different from those in MTD and healthy controls. In addition, we hypothesized that there would be a correlation between cortical excitability measures and clinical voice severity in AdSD.

RESULTS

Cortical silent period duration in masseter and FDI was significantly shorter in AdSD than MTD and healthy controls. Other measures failed to demonstrate differences.

CONCLUSION

There are differences in cortical excitability between AdSD, MTD, and healthy controls. These differences in the cortical measure of both the FDI and masseter muscles in AdSD suggest widespread dysfunction of the GABAB mechanism may be a pathophysiologic feature of AdSD, similar to other forms of focal dystonia. Further exploration of the use of TMS to assist in the differential diagnosis of AdSD and MTD is warranted.

Differentiation of motor cortical representation of hand muscles by navigated mapping of optimal TMS current directions in healthy subjects

The precision of navigated transcranial magnetic stimulation (TMS) to map the human primary motor cortex may be effected by the direction of TMS-induced current in the brain as determined by the orientation of the stimulation coil. In this study, the authors investigated the effect of current directionality on motor output mapping using navigated brain stimulation. The goal of this study was to determine the optimal coil orientation (and, thus, induced brain current) to activate hand musculature representations relative to each subject's unique neuroanatomical landmarks. The authors studied motor output maps for the first dorsal interosseous, abductor pollicis brevis, and abductor digiti minimi muscles in 10 normal volunteers. Monopolar current pulses were delivered through a figure-of-eight-shaped TMS coil, and motor evoked potentials were recorded using electromyography. At each targeted brain region, the authors systematically rotated the TMS coil to determine the direction of induced current in the brain for induction of the largest motor evoked potentials. These optimal current directions were expressed as an angle relative to each subject's central sulcus. Consistency of the optimal current direction was assessed by repeating the entire mapping procedure on two different occasions across subjects. The authors demonstrate that systematic optimization of current direction as guided by MRI-based neuronavigation improves the resolution of cortical output motor mapping with TMS.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Intracortical circuits, sensorimotor integration and plasticity in human motor cortical projections to muscles of the lower face

Previous studies of the cortical control of human facial muscles documented the distribution of corticobulbar projections and the presence of intracortical inhibitory and facilitatory mechanisms. Yet surprisingly, given the importance and precision in control of facial expression, there have been no studies of the afferent modulation of corticobulbar excitability or of the plasticity of synaptic connections in the facial primary motor cortex (face M1). In 25 healthy volunteers, we used standard single- and paired-pulse transcranial magnetic stimulation (TMS) methods to probe motor-evoked potentials (MEPs), short-intracortical inhibition, intracortical facilitation, short-afferent and long-afferent inhibition and paired associative stimulation in relaxed and active depressor anguli oris muscles. Single-pulse TMS evoked bilateral MEPs at rest and during activity that were larger in contralateral muscles, confirming that corticobulbar projection to lower facial muscles is bilateral and asymmetric, with contralateral predominance. Both short-intracortical inhibition and intracortical facilitation were present bilaterally in resting and active conditions. Electrical stimulation of the facial nerve paired with a TMS pulse 5-200 ms later showed no short-afferent inhibition, but long-afferent inhibition was present. Paired associative stimulation tested with an electrical stimulation-TMS interval of 20 ms significantly facilitated MEPs for up to 30 min. The long-term potentiation, evoked for the first time in face M1, demonstrates that excitability of the facial motor cortex is prone to plastic changes after paired associative stimulation. Evaluation of intracortical circuits in both relaxed and active lower facial muscles as well as of plasticity in the facial motor cortex may provide further physiological insight into pathologies affecting the facial motor system.

Effects of volitional contraction on intracortical inhibition and facilitation in the human motor cortex

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 healthy subjects. Paired-pulse TMS was delivered to the left hemisphere at the following interstimulus intervals (ISIs): 2 and 3 ms for SICI, 10 and 15 ms for ICF and 1-5 ms for SICF. Motor-evoked potentials were recorded from the resting and active right FDI. The effects exerted on SICI and ICF by four intensities (60-90% of active motor threshold, AMT) of the conditioning stimulus (S1) and by three levels of muscle contraction (10%, 25%, 50% of maximal voluntary contraction, MVC) were evaluated. The effects exerted on SICF were evaluated with two intensities (90% and 70% of AMT) of the test stimulus (S2) and with the same levels of muscle contraction. Results showed that: (i) during 10% MVC, maximum SICI was observed with S1 = 70% AMT; (ii) the amount of SICI obtained with S1 = 70% AMT was the same at rest as during 10% MVC, but decreased at higher contraction levels; (iii) ICF was observed only at rest with S1 = 90% AMT; (iv) SICF was facilitated at 10% and 25% MVC, but not at 50% MVC. We conclude that during muscle activation, intracortical excitability reflects a balance between activation of SICI and SICF systems. Part of the reduction in SICI during contraction is due to superimposed recruitment of SICF. Low intensity (70% AMT) conditioning stimuli can test SICI independently of effects on SICF at low contraction levels.