Effect of motor tasks on the cortical topography of the human masseter muscle

A 5-day course of rTMS before pain onset ameliorates future pain and increases sensorimotor peak alpha frequency

Repetitive transcranial magnetic stimulation (rTMS) has shown promise as an intervention for pain. An unexplored research question is whether the delivery of rTMS prior to pain onset might protect against a future episode of prolonged pain. The present study aimed to determine i) whether 5 consecutive days of rTMS delivered prior to experimentally-induced prolonged jaw pain could reduce future pain intensity and ii) whether any effects of rTMS on pain were mediated by changes in corticomotor excitability (CME) and/or sensorimotor peak alpha frequency (PAF). On each day from Day 0-4, forty healthy individuals received a single session of active (n = 21) or sham (n = 19) rTMS over the left primary motor cortex. PAF and CME were assessed on Day 0 (before rTMS) and Day 4 (after rTMS). Prolonged pain was induced via intramuscular injection of nerve growth factor (NGF) in the right masseter muscle after the final rTMS session. From Days 5-25, participants completed twice-daily electronic dairies including pain on chewing and yawning (primary outcomes), as well as pain during other activities (e.g. talking), functional limitation in jaw function and muscle soreness (secondary outcomes). Compared to sham, individuals who received active rTMS subsequently experienced lower pain on chewing and yawning. Although active rTMS increased PAF, the effects of rTMS on pain were not mediated by changes in PAF or CME. This study is the first to show that rTMS delivered prior to pain onset can protect against future pain and associated functional impairment. Thus, rTMS may hold promise as a prophylactic intervention for persistent pain.

The reliability of two prospective cortical biomarkers for pain: EEG peak alpha frequency and TMS corticomotor excitability

BACKGROUND

Many pain biomarkers fail to move from discovery to clinical application, attributed to poor reliability and an inability to accurately classify at-risk individuals. Preliminary evidence has shown that high pain sensitivity is associated with slow peak alpha frequency (PAF), and depression of corticomotor excitability (CME), potentially due to impairments in ascending sensory and descending motor pathway signalling respectively NEW METHOD: The present study evaluated the reliability of PAF and CME responses during sustained pain. Specifically, we determined whether, over several days of pain, a) PAF remains stable and b) individuals show two stable and distinct CME responses: facilitation and depression. Participants were given an injection of nerve growth factor (NGF) into the right masseter muscle on Day 0 and Day 2, inducing sustained pain. Electroencephalography (EEG) to assess PAF and transcranial magnetic stimulation (TMS) to assess CME were recorded on Day 0, Day 2 and Day 5.

RESULTS

Using a weighted peak estimate, PAF reliability (n = 75) was in the excellent range even without standard pre-processing and ∼2 min recording length. Using a single peak estimate, PAF reliability was in the moderate-good range. For CME (n = 74), 80% of participants showed facilitation or depression of CME beyond an optimal cut-off point, with the stability of these changes in the good range.

COMPARISON WITH EXISTING METHODS

No study has assessed the reliability of PAF or feasibility of classifying individuals as facilitators/depressors, in response to sustained pain. PAF was reliable even in the presence of pain. The use of a weighted peak estimate for PAF is recommended, as excellent test-retest reliability can be obtained even when using minimal pre-processing and ∼2 min recording. We also showed that 80% of individuals exhibit either facilitation or depression of CME, with these changes being stable across sessions.

CONCLUSIONS

Our study provides support for the reliability of PAF and CME as prospective cortical biomarkers. As such, our paper adds important methodological advances to the rapidly growing field of pain biomarkers.

Is bruxism associated with changes in neural pathways? A systematic review and meta-analysis of clinical studies using neurophysiological techniques

This study aimed to systematically review the literature to identify clinical studies assessing neuroplasticity changes induced by or associated with bruxism or a tooth-clenching task using neurophysiological techniques. Searches were performed in five electronic databases (PubMed, EMBASE, Scopus, Web of Science, and Google Scholar) in April 2020. This review included clinical studies using neurophysiological techniques to assess neuroplasticity changes in healthy participants before and after a tooth-clenching task or comparing bruxers and non-bruxers. The quality assessment was performed with the Joanna Briggs Institute tool and Grading of Recommendations Assessment, Development, and Evaluation. Meta-analyses were conducted with studies reporting similar comparisons regarding masseter motor evoked potential amplitude and signal change outcomes. Of 151 articles identified in the searches, nine were included, and five proceeded to meta-analysis. Included studies presented moderate to very low methodological quality. From these included studies, eight evaluated bruxers and non-bruxers, of which five of them observed brain activity differences between groups, and three found no differences. Even so, all studies have suggested distinct difference in the central excitability between bruxers and non-bruxers, the meta-analysis revealed no statistically significant differences (P > 0.05). It appears that bruxism seems, indeed, to be associated with distinct differences in the neural pathways related to the control of the jaw-closing muscles, but that considerable variability in terms of classification of bruxism and assessment of neuroplasticity hamper a definite conclusion. Future research projects should take these concerns into consideration in order to further the understanding of bruxism physiology and pathophysiology.

The effects of continuous oromotor activity on speech motor learning: speech biomechanics and neurophysiologic correlates

Sustained limb motor activity has been used as a therapeutic tool for improving rehabilitation outcomes and is thought to be mediated by neuroplastic changes associated with activity-induced cortical excitability. Although prior research has reported enhancing effects of continuous chewing and swallowing activity on learning, the potential beneficial effects of sustained oromotor activity on speech improvements is not well-documented. This exploratory study was designed to examine the effects of continuous oromotor activity on subsequent speech learning. Twenty neurologically healthy young adults engaged in periods of continuous chewing and speech after which they completed a novel speech motor learning task. The motor learning task was designed to elicit improvements in accuracy and efficiency of speech performance across repetitions of eight-syllable nonwords. In addition, transcranial magnetic stimulation was used to measure the cortical silent period (cSP) of the lip motor cortex before and after the periods of continuous oromotor behaviors. All repetitions of the nonword task were recorded acoustically and kinematically using a three-dimensional motion capture system. Productions were analyzed for accuracy and duration, as well as lip movement distance and speed. A control condition estimated baseline improvement rates in speech performance. Results revealed improved speech performance following 10 min of chewing. In contrast, speech performance following 10 min of continuous speech was degraded. There was no change in the cSP as a result of either oromotor activity. The clinical implications of these findings are discussed in the context of speech rehabilitation and neuromodulation.

Locating and Outlining the Cortical Motor Representation Areas of Facial Muscles With Navigated Transcranial Magnetic Stimulation

BACKGROUND

Navigated transcranial magnetic stimulation (nTMS) has become established as an accurate noninvasive technique for mapping the functional motor cortex for the representation areas of upper and lower limb muscles but not yet for facial musculature.

OBJECTIVE

To characterize the applicability and clinical impact of using nTMS to map cortical motor areas of facial muscles in healthy volunteers and neurosurgical tumor patients.

METHODS

Eight healthy volunteers and 12 patients with tumor were studied. The motor threshold (MT) was determined for the abductor pollicis brevis and mentalis muscles. The lateral part of the motor cortex was mapped with suprathreshold stimulation intensity, and motor evoked potentials were recorded from several facial muscles. The patient protocol was modified according to the clinical indication.

RESULTS

In all healthy subjects, motor evoked potentials were elicited in the mentalis (mean latency, 13.4 milliseconds) and orbicularis oris (mean latency, 12.6 milliseconds) muscles. At 110% of MT of the mentalis, the motor evoked potentials of facial muscles were elicited mainly in the precentral gyrus but also from one gyrus anterior and posterior to it. The cortical areas applicable for mapping were limited by an artifact attributable to direct peripheral nerve stimulation. The mapping protocol was successful in 10 of 12 tumor patients at locating the representation area of the lower facial muscles. The MT of the facial muscles was significantly higher than that of the abductor pollicis brevis.

CONCLUSION

nTMS is an applicable and clinically beneficial noninvasive method to preoperatively map the cortical representation areas of the facial muscles in the lower part of the face. Instead of using the MT of the abductor pollicis brevis, the stimulus intensity during mapping should be proportioned to the MT of a facial muscle.

Influence of voluntary control of masticatory side and rhythm on cerebral hemodynamics

The aim of this study was to investigate the influence on cerebral hemodynamics of voluntary control of masticatory side and rhythm during gum chewing. Blood flow velocity in the middle cerebral artery was measured using transcranial Doppler ultrasonography to evaluate cerebral circulation in healthy volunteers. Heart rate and masseter muscle activity were recorded simultaneously. Volunteers performed three tasks: (1) free gum chewing, (2) gum chewing in which mastication was limited to the right side, and (3) gum chewing in which mastication was limited to the right side and rhythm was set at 1.0 Hz. Changes in cerebral circulation during pre-task, on-task, and post-task periods were analyzed using random effects model, and differences in cerebral circulation and muscle activity between tasks were analyzed using the Friedman test. In all tasks, on-task cerebral circulation was greater than pre-task. Muscle activity and masticatory rhythm varied between tasks, whereas the rate of increase in cerebral circulation did not differ significantly among tasks. These results suggest that cerebral circulation is activated during gum chewing, irrespective of voluntary control of masticatory side and rhythm.

Cortical topography of human first dorsal interroseus during individuated and nonindividuated grip tasks

Neural activity in the motor cortex and its descending projections is modulated in a task-related manner. Several TMS studies have shown that when normal human subjects execute different manual tasks requiring similar contraction levels in first dorsal interroseous (FDI) there is a task-related modulation of the amplitude of FDI motor evoked potentials (MEPs). Not all studies of task-related changes show the same pattern of results, however. One reason for this might be methodological. Studies have assessed task-related changes by stimulating a single site, which can provide information about task-related changes in the excitability of the cortex at that site, but which is not sensitive to excitability changes throughout the muscle's cortical representation. We investigated how the execution of an individuated versus a nonindividuated isometric grasping task affected the excitability of FDI's entire cortical representation. We examined FDI MEP amplitudes while subjects grasped an object between their thumb and index finger, or when they grasped the same object between their thumb and all four fingers, keeping the background level of EMG in FDI constant for the two tasks. We found no overall task-related change in the excitability of FDI or its cortical topography, possibly due to behavioral differences of individual subjects. The stability of FDI's cortical representation during two different manual tasks expands the possibilities for studying cortical reorganization in the context of active muscle contraction, which will enable us to better understand whether changes in the motor system observed when muscles are at rest are also present during voluntary muscle recruitment.

Insights into the bilateral cortical control of human masticatory muscles revealed by transcranial magnetic stimulation

In this brief review I describe details of the functional organisation of the bilateral corticobulbar projections to the trigeminally innervated masticatory muscles, as revealed by transcranial magnetic stimulation of the human brain. The motor cortices of both hemispheres are involved in control of trigeminal motoneurons, however the contralateral hemisphere has the greater excitatory influence. Corticomotoneuronal cells in each hemisphere project to jaw-closer and jaw-opener motoneurons. Less is known about cortically mediated inhibitory effects in the trigeminal motor system, but the available evidence suggests that drive to jaw muscles on each side is affected similarly by intracortical inhibitory processes activated in one hemisphere. Functional studies reveal that the two hemispheres play distinct roles in control of ipsilateral and contralateral muscles, particularly for jaw-closers. Masseter and digastric motor units recruited during low-force contractions do not receive uniform inputs from each hemisphere; the majority of masseter motor units are excited only from the contralateral hemisphere, and while digastric motor units are usually excited from both hemispheres the direct CM cell influence appears to be augmented on the contralateral side by corticobulbar activation of segmental excitatory interneurons. Differences in bilateral cortical control of jaw-closer and jaw-opener muscles may contribute to the more independent control of jaw-closers on each side during functional tasks. Corticobulbar control of the trigeminal muscles during natural tasks such as chewing and speech remains to be investigated with TMS.

Motor potentials evoked by transcranial magnetic stimulation during isometric and dynamic masseter muscle contraction in humans

The facilitation of muscle motor potentials evoked by transcranial magnetic stimulation (TMS) has been demonstrated convincingly during both isometric and dynamic activity in the limbs but not in the jaw muscles. An experimental design involving TMS, surface electromyography and controlled muscle-activity was employed to investigate the motor response of the human masseter during voluntary isometric and dynamic voluntary conditions. During the isometric condition, an increase in muscle facilitation resulted in a progressive increase in motor-evoked potential (MEP) amplitude that was consistently greater on the side contralateral to that subjected to TMS (P < 0.05). No difference in MEP amplitude or laterality of response was revealed for the two dynamic conditions. The sample size may have been too small to reveal any differences. The modulation of MEPs during isometric activity was probably due to cortical and brainstem mechanisms. Putative variation in masseter MEPs during dynamic conditions cannot be discounted.