Antoon De Laat

Experimental jaw-muscle pain does not change heteronymous H-reflexes in the human temporalis muscle

Abstract Muscle pain generally has an inhibitory effect on voluntary orofacial motor function. However, it is not known whether muscle pain causes direct or indirect changes in motoneuron excitability. In this study a monopolar needle stimulation technique was used to evoke the direct motor response (M-response) in the left masseter muscle and the heteronymous H-reflex in the left temporalis muscle as an indirect measure of motoneuron excitability. Series of 20 repeated electrical stimuli were delivered at 50% of maximal voluntary contraction (MVC) before, during, and after periods with experimental jaw-muscle pain in 11 healthy subjects. Pain was induced by standardized infusion of hypertonic (5%) saline into the mid-portion of the right masseter muscle. The mean pain intensity rating on a 100-mm visual analog scale was 42±5 mm. The short-latency responses (less than 6 ms) could be evoked in all subjects. Analysis of the latency and amplitude of the temporal H-reflex indicated no significant effect of jaw-muscle pain. The amplitude of the masseteric M-response was significantly smaller in the postpain condition than in the pain conditions (ANOVA, P=0.018), but no differences were found between the prepain and postpain conditions. In nine subjects, poststimulus periods (mean offset latency, 69.6±8.6 ms) with significantly (more than 50%) suppressed EMG activity were detected in the ipsilateral masseter muscle following the M-response (mean offset latency, 5.5±0.2 ms). These reflex responses did not show a systematic change during the pain conditions. In conclusion, acute contralateral jaw-muscle pain does not seem to modulate the motoneuron excitability as measured by the heteronymous H-reflex.

Long-term fluctuations of pressure pain thresholds in healthy men, normally menstruating women and oral contraceptive users.

The aim of this investigation was to evaluate whether the pressure pain threshold (PPT) in masticatory muscles of symptom‐free subjects was influenced by fluctuations of the sex hormones. The PPT was measured with an electronic algometer for at least 10 consecutive menstrual cycles in 10 women using oral contraceptives and 10 women not using oral contraceptives, with a regular menstrual cycle (26–31 days). In addition, 10 men were measured in a regular pattern over a period of 1 year. All subjects were symptom‐free with an age range between 18 and 39 years. Measurement sessions were held during three different cycle phases (follicular, luteal, perimenstrual) and each session consisted of four consecutive PPT measurements. By means of a linear mixed model (SAS), the PPTs of the masster, temporalis and thumb muscles were compared between: (1) groups, (2) sex‐hormonal phases, (3) the four consecutive measurements of each muscle per session and (4) time. The PPTs of the masseter (p  = 0.8419) and temporalis muscles (p  = 0.2786) did not change significantly over time. There was no significant difference in variance for the masseter (p  = 0.6250), temporalis (p  = 0.9705) and thumb (p  = 0.7446) between the three groups. The PPTs of all muscles were significantly lower during the perimenstrual phases in the two female groups. The present data showed similar patterns of PPTs for the three muscle groups. Moreover, the results have shown a very good consistency of the PPTs over a long time period, both in males and females.

Tactile and pain thresholds in the intra- and extra-oral regions of symptom-free subjects

&NA; The aim of the present study was to evaluate the tactile detection threshold (TDT), the filament‐prick pain detection threshold (FPT), the pressure pain threshold (PPT), and the pressure pain tolerance detection threshold (PTOL) at multiple measuring points in the orofacial region of normal subjects. Sixteen males and 16 females (age range from 20 to 41 years) participated. The TDT and the FPT were measured on the cheek skin overlying the central part of the masseter muscles (MM), on the maxillary gingiva, and at the tip of the tongue, using Semmes–Weinstein monofilaments. The PPT and PTOL were measured at the central part of the MM, using a pressure algometer. The pain intensity during the FPT, PPT and the PTOL measurements was assessed on a numeric rating scale (NRS). The tongue tip had the lowest value in TDT and FPT compared to the other sites. Females showed a significantly lower TDT and FPT at the cheek skin than males. Further, measurements of PPT and PTOL confirmed the previously reported higher thresholds in males. In contrast, while the intra‐oral threshold measurements revealed no gender differences, a significantly higher pain perception as evaluated using NRS, was seen in the males. A strong correlation was found between the pain responses at the same measuring site (FPT, PPT, and PTOL over the MM). In addition, the TDT and the pain responses were also correlated positively.

Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric poststimulus EMG complex following tapping of a tooth in man

Abstract Poststimulus EMG complexes (PSECs), consisting of a series of inhibitory and excitatory waves in full-wave rectified and averaged electromyogram (EMG), were elicited in the masseter muscles of 7 subjects following controlled tapping of a tooth, at a controlled clenching level. Applying local anaesthesia to this tooth decreased the total surface of the waves, on average by 89%. The excitatory and the inhibitory waves were similarly affected, indicating that mainly pressure receptors in the periodontium mediate the entire PSEC. In 4 subjects, who were exposed to acoustic noise to exclude a contribution of acoustic receptors, the recovery of the PSEC waves from local anaesthesia was tracked. In 3 subjects, one wave (the first inhibitory or the first excitatory one, respectively) recovered differently from the other waves, indicating that they are not necessarily mediated by one type of afferent axons. The evidence, nevertheless, suggests that the different PSEC waves in man reflect the projection of the periodontal afferents upon several brain structures, involved in the control of the activity of the masseteric motorneurones, as: (1) inhibitory and excitatory control requires different groups of interneurones; and (2) a mediation of the first inhibitory wave by slower conducting axons than the second inhibitory wave, or a mediation of both waves by axons of similar type, is not compatible with common interneurones.

Sleep bruxism as a motor disorder.

Keywords: sleep; bruxism; temporomandibular disorders; dopamine D2 receptors; polysomnography; arousal; parasomnias

Interactive periodontal and acoustic influences on the masseteric post-stimulus electromyographic complex in man.

Post-stimulus electromyogram (EMG) complexes (PSECs) were studied in the full-wave rectified and averaged EMGs of the masseter muscles in 15 subjects, who clenched at a controlled level. The PSECs, a series of downward- and upward-going waves reflecting inhibitory and excitatory influences upon the masseteric motoneurones, were elicited by mechanical stimulation of a tooth. The stimuli selectively activated mechanoreceptors in the periodontium and, by bone-conduction, acoustic receptors. Application of acoustic masking during the periods of stimulation revealed a series of inhibitory and excitatory acoustic influences in the PSEC, which were absent after local electrical stimulation of receptors in the periodontium or their afferents. By applying local anaesthesia to the periodontium of a mechanically stimulated tooth, the durations of the acoustic influences were on the average reduced by 76%. In subjects whose PSECs consistently included a second inhibitory period, the duration of the acoustic influences with respect to that of the PSEC (30%) was larger than otherwise (13%), suggesting a central gating of periodontal pathways which can block both periodontal and acoustic influences. The acoustic influences, of which the appearance in the PSEC largely depends upon activated periodontal pathways, represent a new finding of audio-motor reflexes.

Ethnic differences regarding sensory, pain, and reflex responses in the trigeminal region

OBJECTIVE The present study evaluated ethnic differences in sensory, pain responses, and the exteroceptive suppression reflex (ES) in the trigeminal region. METHODS Twenty-eight Caucasian (14 men and 14 women, age from 20 to 31) and 28 gender and age matched Japanese participated. Tactile detection thresholds and filament-prick pain detection thresholds were measured on the left cheek skin. Surface EMG was recorded from the left masseter muscle, and 13 different intensities of electrical stimuli were applied to the skin overlying the left mental nerve. The first stimulation intensity at which the late ES appeared was defined as the electrical reflex threshold. The duration and suppression degree of the ES were also determined. RESULTS Japanese subjects had a significantly lower sensory and pain threshold then Caucasian subjects. Caucasian subjects had a significantly lower electrical reflex threshold in comparison to Japanese subjects. Caucasian subjects had significantly longer duration of the ES and larger suppression degree of the ES compared to Japanese subjects. CONCLUSIONS Our results demonstrate that ethnic differences exist not only concerning sensory and pain responses but also regarding reflex (motor) responses. SIGNIFICANCE The findings of this study have direct implications for studies evaluating different types of orofacial pain conditions and trigeminal reflexes.

Gender difference in masseteric exteroceptive suppression period and pain perception

OBJECTIVE Use of brain stem reflexes in the assessment of orofacial function requires insight into the influence of demographic factors such as gender. The aim of this study was to characterize possible gender differences in the relation between quantitative measures of the masseteric exteroceptive suppression (ES) reflex response and pain perception evoked by incrementally increasing electrical stimulation. METHODS In 12 men and 12 women, the surface electromyogram was recorded from the left masseter muscle. Thirteen fixed stimulus intensities from 5 to 35 mA at 2.5 mA increments were applied to the skin above the left mental nerve. The stimulation intensity at which the late ES appeared first and the first intensity at which the subjects reported the stimulus intensity to be painful were defined as the reflex threshold (RT) and pain threshold (PT), respectively. Furthermore, data were analyzed using stimulus-response curves, and the reflex appearance levels (RAL), the saturation level, the slope from appearance to saturation of the reflex (SLP), and the pain appearance level (PAL) were determined. RESULTS The PT was equal to or higher than the RT in 9 of the 12 men, but only in 4 of the 12 women. Further, women had significantly lower PAL, RAL, and SLP (12.7 +/- 0.8, 12.9 +/- 1.4, and 3.0 +/- 0.9 mA, respectively) compared to men (20.3 +/- 1.6 mA, 16.7 +/- 1.1 T, and 4.1 +/- 0.4, respectively) (P < 0.05). CONCLUSIONS The present results document that women have a lower reflex threshold and pain threshold to cutaneous electrical stimulation than men. SIGNIFICANCE These findings suggest that gender differences may exist in the sensory-motor integration of primary afferent input from the orofacial region and that these differences should be considered in the design of future reflex studies.

Intraoral measurement of tactile and filament-prick pain threshold using shortened Semmes-Weinstein monofilaments.

Objectives Semmes-Weinstein monofilaments are too long for use in parts of the oral cavity. The present study used shortened Semmes-Weinstein monofilaments to evaluate reliability and spatial differences in the intraoral tactile detection threshold (TDT) and the filament-prick pain detection threshold (FPT) in healthy volunteers. Methods For practical purposes, classic Semmes-Weinstein monofilaments with 20 different diameters were cut to half their length (ie, 19 mm) and the bending forces were measured. Eighteen men and 18 women (age range, 20 to 33 y) were recruited to evaluate the reliability and reproducibility of measurements using half-cut monofilaments. The TDT and the FPT were measured on the labial maxillary gingiva, on the palatal maxillary gingiva, and at the anterior tip of the tongue, using a double random staircase method. Results According to the forces needed to bend the half-cut filaments, they were renumbered from 2.55 to 6.86. There were significant differences of bending force between the half-cut and original monofilaments (P<0.001), Using half-cut filaments, the following differences could be detected; the labial maxillary gingiva had a significantly higher TDT threshold compared with the other test sites (P<0.001). By contrast, the palatal posterior maxillary gingiva had a significantly higher FPT threshold compared with the other test sites (P<0.001). Discussions The present study illustrated that in healthy participants, half-cut Semmes-Weinstein monofilaments reliably and easily assess TDT and FPT intraorally. A combined examination of sensory and pain thresholds using these filaments contributes to the clinical examination for orofacial pain.

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.