Dongyuan Yao

Properties and plasticity of the primate somatosensory and motor cortex related to orofacial sensorimotor function.

1. The lateral pericentral region of the cerebral cortex has been well documented in primates to be important in sensorimotor integration and control and in the learning of new motor skills.

Effects of reversible bilateral inactivation of face primary motor cortex on mastication and swallowing.

The effects of reversible cold block-induced bilateral inactivation of the face primary motor cortex (face MI) on mastication and swallowing were studied in awake monkeys. A warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the intracortical microstimulation-defined face MI while the monkey chewed and swallowed food during pre-cool (thermode temperature 37 degrees C), cold block (4 degrees C), and post-cool (37 degrees C) sessions. Vertical and horizontal jaw movements and electromyographic (EMG) activity of several muscles were monitored. Each masticatory sequence was divided into three masticatory phases (i.e. food preparatory, rhythmic chewing, preswallow). The cold block markedly affected the ability of the monkey to carry out mastication although it did not prevent mastication from occurring. The masticatory deficit was characterized by a significant elongation of the total masticatory time, including in particular elongation of the food preparatory phase. The coordination of the jaw- and tongue-muscle activities was severely disrupted during the food preparatory phase. Face MI cold block also significantly affected the duration of some masticatory-related EMG activities and had some limited effects on the temporal relationships of the EMG activities during mastication. Although cold block significantly affected the duration and some EMG parameters of the preswallow phase, it had no significant effect on swallow duration or the EMG parameters during swallowing. These findings provide further evidence that the primate face MI plays a critical role in the regulation of mastication and that it plays a role in the preparation for swallowing.

Effects of functional disruption of lateral pericentral cerebral cortex on primate swallowing

Bilateral cold block of the intracortical microstimulation (ICMS)-defined swallow cortex markedly affected the ability of monkeys to carry out swallowing. Significant changes also occurred in swallow-related electromyographic (EMG) activity patterns. These findings provide further evidence that the lateral pericentral cortex plays a critical role in the initiation and regulation of swallowing in the primate.

Motor cortex neuroplasticity associated with lingual nerve injury in rats

The aim of this study was to determine if lingual nerve trauma affects the features of face primary motor cortex (MI) defined by intracortical microstimulation (ICMS). The left lingual nerve was transected in adult male rats by an oral surgical procedure; sham rats (oral surgery but no nerve transection) as well as naive intact rats served as control groups. ICMS was applied at post-operative days 0, 7, 14, 21, and 28 to map the jaw and tongue motor representations in face MI by analyzing ICMS-evoked movements and electromyographic activity recorded in the genioglossus (GG) and anterior digastric (AD) muscles. There were no statistically significant effects of acute (day 0) nerve transection or sham procedure (p > 0.05). The surgery in the sham animals was associated with limited post-operative change; this was reflected in a significant (p < 0.05) increase in the number of GG sites in left MI at post-operative day 14 compared to day 0. However, nerve transection was associated with significant increases in the total number of AD and GG sites in left or right MI or specifically the number of GG sites in rats at post-operative days 21 or 28 compared to earlier time periods. There were also significant differences between nerve-transected and sham groups at post-operative days 7, 14, or 21. These findings suggest that lingual nerve transection is associated with significant time-dependent neuroplastic changes in the tongue motor representations in face MI.

Effects of reversible cold block of face primary somatosensory cortex on orofacial movements and related face primary motor cortex neuronal activity

Our previous studies have revealed that face primary somatosensory cortex (SI) as well as face primary motor cortex (MI) play important roles in the control of orofacial movements in awake monkeys, and that both face MI and face SI neurons may have an orofacial mechanoreceptive field and show activity related to orofacial movements. Since it is possible that the movement-related activity of face MI neurons could reflect movement-generated orofacial afferent inputs projecting to face MI via face SI, the present study used reversible cold block-induced inactivation of the monkeys face SI to determine if face MI neuronal activity related to a trained tongue-protrusion task, chewing or swallowing was dependent on the functional integrity of the ipsilateral face SI and if inactivation of face SI affects orofacial movements. The effects of face SI cold block were tested on chewing, swallowing and/or task-related activity of 73 face MI neurons. Both task and chewing and/or swallowing-related activity of most face MI neurons was independent of the functional integrity of the ipsilateral face SI since SI cold block affected the movement-related activity in approximately 25% of the neurons. Similarly, unilateral cold block of SI had very limited effects on the performance of the task and chewing, and no effect on the performance of swallowing. These findings suggest that movement-induced reafferentation via face SI may not be a significant factor in accounting for the activity of the majority of ipsilateral face MI neurons related to trained movements, chewing and swallowing.

Effects on mastication of reversible bilateral inactivation of the lateral pericentral cortex in the monkey (Macaca fascicularis).

It is known that intracortical microstimulation (ICMS) of the lateral pericentral cortex can evoke masticatory movements and swallowing in awake monkeys. The aim was to determine if the ability of monkeys to carry out mastication is affected by reversible bilateral cold block of the ICMS-defined cortical masticatory area/swallow cortex. A cranial chamber was implanted bilaterally in two monkeys and a warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the ICMS-defined cortical masticatory area/swallow cortex while monkeys chewed standardised amounts of fruit during pre-cool (thermode temperature, 37 degrees C), cool (0-4 degrees C), and post-cool (37 degrees C) trials. Electromyographic (EMG) activity was recorded from masseter, genioglossus, anterior digastric, geniohyoid and thyrohyoid or perilaryngeal muscles. Vertical and horizontal jaw movements were recorded with a photodiode position transducer, which monitored movements of a light-emitting diode fixed to the mandible. Each masticatory period was divided into a food-preparatory phase, a rhythmic chewing phase and a preswallow phase. Both monkeys could readily accept and ingest the foodstuffs during pre-cool and post-cool trials. In contrast, cold block was associated with masticatory deficits, reflected in both monkeys as impaired food intake or manipulation and difficulty in carrying out a sequence of masticatory cycles, alterations in of the food-preparatory phase, and alterations in masticatory-related EMG patterns of the jaw and tongue muscles. The cold block-induced changes included significant (P<0.05) prolongations of the total masticatory time, the food-preparatory phase duration, and burst durations of the jaw and tongue muscle EMG activities; furthermore, the amplitudes and temporal correlations of the EMG activities of the jaw and tongue muscles were significantly (P<0.05) changed by cold block. These findings provide further evidence that the lateral pericentral cortex has a critical role in the initiation and regulation of masticatory movements in the primate, and that the programming of masticatory muscle activities may be dependent upon corticofugal influences for engaging masticatory motor activities appropriate to the masticatory conditions.

Nitroglycerin facilitates calcitonin gene-related peptide-induced behavior.

Nitric oxide and calcitonin gene-related peptide (CGRP) have been implicated in craniofacial pain including migraine headache, and CGRP induces face-grooming indicative of nocifensive behavior in animals. The aim of this study was to test whether systemic administration of nitroglycerin (NTG) influences the CGRP-induced behavior in awake rats and whether sumatriptan, a 5-HT1B/1D receptor agonist, can block the effects of NTG. CGRP was not significantly different from normal saline in inducing face-grooming behavior but NTG facilitated the effect of CGRP. Furthermore, sumatriptan was found to block the effect of NTG. These data suggest that facilitatory processes involving nitric oxide may be necessary for CGRP to play a role in some craniofacial pain conditions including possibly migraine headache.

Adaptability to pain is associated with potency of local pain inhibition, but not conditioned pain modulation: A healthy human study

Summary Participants demonstrated reactions adaptive or nonadaptive to pain during cold pressor. The 2 groups differed in local pain inhibition, but not in conditioned pain modulation. ABSTRACT This study investigated the relationship between pain sensitivity, adaptability, and potency of endogenous pain inhibition, including conditioned pain modulation (CPM) and local pain inhibition. Forty‐one healthy volunteers (20 male, 21 female) received conditioning stimulation (CS) over 2 sessions in a random order: tonic heat pain (46°C) on the right leg for 7 minutes and cold pressor pain (1°C to 4°C) on the left hand for 5 minutes. Participants rated the intensity of pain continuously using a 0 to 10 electronic visual analogue scale. The primary outcome measures were pressure pain thresholds (PPT) measured at the heterotopic and homotopic location to the CS sites before, during, and 20 minutes after CS. Two groups of participants, pain adaptive and pain nonadaptive, were identified based on their response to pain in the cold pressor test. Pain‐adaptive participants showed a pain reduction between peak pain and pain at end of the test by at least 2 of 10 (n = 16); whereas the pain‐nonadaptive participants reported unchanged peak pain during 5‐minute CS (n = 25). Heterotopic PPTs during the CS did not differ between the 2 groups. However, increased homotopic PPTs measured 20 minutes after CS correlated with the amount of pain reduction during CS. These results suggest that individual sensitivity and adaptability to pain does not correlate with the potency of CPM. Adaptability to pain is associated with longer‐lasting local pain inhibition.

Jaw-Opening Reflex and Corticobulbar Motor Excitability Changes During Quiet Sleep in Non-Human Primates

STUDY OBJECTIVE To test the hypothesis that the reflex and corticobulbar motor excitability of jaw muscles is reduced during sleep. DESIGN Polysomnographic recordings in the electrophysiological study. SETTING University sleep research laboratories. PARTICIPANTS AND INTERVENTIONS The reflex and corticobulbar motor excitability of jaw muscles was determined during the quiet awake state (QW) and quiet sleep (QS) in monkeys (n = 4). MEASUREMENTS AND RESULTS During QS sleep, compared to QW periods, both tongue stimulation-evoked jaw-opening reflex peak and root mean square amplitudes were significantly decreased with stimulations at 2-3.5 × thresholds (P < 0.001). The jaw-opening reflex latency during sleep was also significantly longer than during QW. Intracortical microstimulation (ICMS) within the cortical masticatory area induced rhythmic jaw movements at a stable threshold (≤ 60 μA) during QW; but during QS, ICMS failed to induce any rhythmic jaw movements at the maximum ICMS intensity used, although sustained jaw-opening movements were evoked at significantly increased threshold (P < 0.001) in one of the monkeys. Similarly, during QW, ICMS within face primary motor cortex induced orofacial twitches at a stable threshold (≤ 35 μA), but the ICMS thresholds were elevated during QS. Soon after the animal awoke, rhythmic jaw movements and orofacial twitches could be evoked at thresholds similar to those before QS. CONCLUSIONS The results suggest that the excitability of reflex and corticobulbar-evoked activity in the jaw motor system is depressed during QS.

Chapter 55 – Sleep bruxism

Sleep bruxism is a sleep-related movement disorder with repetitive jaw muscle contraction. It is occasionally associated with tooth-grinding noises. Patients may experience jaw muscle and joint pain, headaches, and sensitivity to teeth due to forceful jaw muscle contractions. It is important to distinguish clenching while awake from sleep bruxism during sleep, because of different etiology and pathophysiology. In the absence of any medical or sleep disorders, use of medication or recreational drugs, sleep bruxism is classified as primary. Age seems to be an important variable, as the condition is reported in 14% in children, compared with 3% in older people. Furthermore, the prevalence of sleep bruxism–tooth grinding stands at 8% in adults. The pathophysiology of sleep bruxism remains unclear, but may be associated with: some neurological and psychiatric disorders; influence of sensory factors on motor control; interaction with neurotransmitters; and some mechanisms related to sleep arousal. A diagnosis of sleep bruxism is reached following patient interview, clinical examination, and sleep recordings. The most appropriate strategy to manage patients with sleep bruxism is to reassess the main motive for the consultation, such as tooth-grinding noises or morning pain. A dental device, adapted to either the maxillary or the mandibular arches, is the recognized procedure used to prevent the grinding sounds in absence of sleep apnea. If sleep-disordered breathing is suspected, the mandibular advancement appliance may be used. In mild cases, the first line of approach might be short-term use of a muscle relaxant.