Mental Stress-induced Physiological Changes in the Human Masseter Muscle

Differences in the regulatory mechanism of blood flow in the orofacial area mediated by neural and humoral systems

Marked blood flow (BF) changes mediated by the autonomic neural and humoral systems may be important for orofacial hemodynamics and functions. However, it remains questionable whether differences in the autonomic vasomotor responses mediated by neural and humoral systems exist in the orofacial area. This study examined whether there are differences in changes in the BF and vascular conductance (VC) between the masseter muscle and lower lip mediated by autonomic neural and humoral systems in urethane-anesthetized rats. Electrical stimulation of the central cut end of the lingual nerve elicited BF increases in the masseter (mainly cholinergic) and lower lip (mainly non-cholinergic), accompanied by an increase in arterial blood pressure (ABP), while cervical sympathetic trunk stimulation consistently decreased BF at both sites. The lingual nerve stimulation induced a biphasic change in the VC in the masseter, consisting of an initial decrease and a successive increase. This decrease in VC was positively correlated with changes in ABP and diminished by guanethidine. Cervical vagus nerve stimulation also induced BF increases at both sites; the increases were greater in the masseter than in the lower lip. Adrenal nerve stimulation and isoproterenol administration induced BF increases in the masseter but not in the lower lip. These results indicate that cholinergic parasympathetic-mediated hemodynamics evoked by trigeminal somatosensory inputs are closely related to ABP changes. The sympathetic nervous system, including the sympathoadrenal system and visceral inputs, may be more involved in hemodynamics in the muscles than in epithelial tissues in the orofacial area.

Hemodynamic Changes in the Masseter and Superior Orbicularis Oris Muscles before and after Exercise Load: A Comparison between Young Adult Women and Middle-Aged to Old Adult Women

Background

The vascularity index (VI) is useful for measuring the hemodynamics on ultrasound imaging. However, there are no reports concerning the application of the VI to facial muscles.

Objective

The aim of this study was (1) to establish a method of measuring the hemodynamics in facial muscles in a constant way and (2) to evaluate the hemodynamic changes in the masseter and superior orbicularis oris muscles (SOOMs) before and after exercise load in two subject groups of females of different ages.

Methods

(1) The VI in the SOOM was calculated, and the test-retest reliability was assessed in seven healthy adults. (2) The VIs in the left-side masseter and SOOM were calculated in 3 sessions: before exercise loading (T0), immediately after loading (T1), and 5 minutes after T1 (T2) for the young adult group (YAG, n = 20; age range, 20–35 years) and the middle-aged to old group (MOG, n = 20; age range, 50–70 years). Tasks were gum chewing for the masseter muscle and lip sealing for the SOOM. The differences in the mean peak flows between two sessions were examined.

Results

(1) Significant differences were not noted for the repeatedly measured average volumes of blood flow with good test-retest agreement (intraclass correlation coefficient = 0.81). (2) In both muscles of the YAG, there were a significant increase in T1 compared with T0 and a significant decrease in T2 compared with T1 (all p < 0.05). In both muscles of the MOG, no significant differences were noted in either comparison.

Conclusions

A method of measuring the hemodynamics in facial muscles was developed and showed good reliability. Changes in the blood flow after exercise load in these muscles may vary with age in women.

Differential control of blood flow in masseter and biceps brachii muscles during stress

OBJECTIVE

The present study aimed to compare sympathetic hemodynamic effects in masticatory and limb muscles in response to different stressors.

DESIGN

Twelve healthy participants were subjected to a randomized series of stressors, including cold pressor test (CPT), mental arithmetic test, apnea, isometric handgrip (IHG) and post-handgrip muscle ischemia (PHGMI), while in the supine position. Spatially-resolved near-infrared spectroscopy was used to measure relative changes in blood volume and oxygenation (TOI) of the resting masseter and biceps muscles. Cardiac output, heart rate, and arterial blood pressure (ABP) were also monitored.

RESULTS

Except apnea, all tests increased ABP. Different response patterns were observed in the 2 muscles: TOI significantly increased during contralateral IHG (1.24 ± 1.17%) but markedly decreased during CPT (-4.84 ± 4.09%) and PHGMI (-6.65 ± 5.31%) in the biceps muscle, while exhibiting consistent increases in the masseter (1.88 ± 1.85%; 1.60 ± 1.75%; 1.06 ± 3.29%, respectively) (p < 0.05).

CONCLUSIONS

The results allow us to infer differential control of blood flow in head and limb muscles. In general, the masseter appears more prone to dilatation than the biceps, exhibiting opposite changes in response to painful stimuli (CPT and PHGMI). Several mechanisms may mediate this effect, including reduced sympathetic outflow to the extracranial vasculature of the head, generally exposed to lower hydrostatic loads than the rest of the body.

Evaluation of the effect of a newly developed steering unit with enhanced self-alignment and deadband on mental workload during driving of agricultural tractors

The aim of present study was to investigate the effect of a newly developed steering unit with enhanced self-alignment and deadband on mental workload (MW) during heavy vehicle operation. Fourteen participants performed two tasks consisting of a lane keeping and a double lane shift with two tractors equipped with 1) a conventional and 2) an enhanced steering system. Physiological measurements, i.e., electromyography, electrodermal activity and heart rate were recorded during the tasks. Furthermore, performance measurements and subjective perception of MW were collected. Present study demonstrated that participants perceived the enhanced steering system requiring less mental demands to operate. Participants improved their performance during the lane keeping task and tended to improve in the double lane shift task with the enhanced system. Physiological measurements did not reveal differences between the steering systems. This study highlighted the dissociation of subjective indices of mental workload from physiological indices in driving of heavy vehicles.

The intensity of awake bruxism episodes is increased in individuals with high trait anxiety

OBJECTIVES

Trait anxiety is associated with an increased occurrence of awake bruxism episodes, a behavior characterized by clenching of the teeth contributing to temporomandibular disorders in some individuals. Here we measured the activity of the masseter and the intensity and duration of spontaneous wake-time tooth clenching episodes in healthy individuals with different levels of trait anxiety (TA).

MATERIALS AND METHODS

Two hundred fifty-five individuals completed a web survey. Using their TA scores, we allocated them in low (< 20^th percentile of the TA score distribution), intermediate (between 20^th and 80^th ), and high (> 80^th) TA groups. We analyzed the electromyographic (EMG) activity of the right masseter during a 15-min silent reading task in forty-three individuals with low (n = 12), intermediate (n = 17), and high TA (n = 14). We tested between-group differences in EMG activity of the masseter, as well as postural activity-the muscular activity that maintains mandibular posture, and amplitude and duration of spontaneous tooth clenching episodes.

RESULTS

The activity of the masseter (mean ± SEM %maximum voluntary contraction/MVC) was greater in the high TA (10.23 ± 0.16%MVC) than the intermediate (8.49 ± 0.16%MVC) and low (7.97 ± 0.22%MVC) TA groups (all p < 0.001). Postural activity did not differ between groups (all p > 0.05). The EMG amplitude of tooth clenching episodes was greater in the high TA (19.97 ± 0.21 %MVC) than the intermediate (16.40 ± 0.24%MVC) and low (15.48 ± 0.38 %MVC) TA groups (all p < 0.05). The cumulative duration of clenching episodes was not different between groups (p = 0.390).

CONCLUSIONS

Increased TA is associated with both increased masseter muscle activity and intensity of wake-time tooth clenching episodes.

CLINICAL RELEVANCE

TA may contribute significantly to masticatory muscle overload.

Introduction of Application of Gini Coefficient to Heart Rate Variability Spectrum for Mental Stress Evaluation

Background

The Gini coefficient is a statistical tool generally used by economists to quantify income inequality. However, it can be applied to any kind of data with unequal distribution, including heart rate variability (HRV).

Objectives

To assess the application of the Gini coefficient to measure inequality in power spectral density of RR intervals, and to use this application as a psychophysiological indicator of mental stress.

Methods

Thirteen healthy subjects (19 ± 1.5 years) participated in this study, and their RR intervals were obtained by electrocardiogram during rest (five minutes) and during mental stress (arithmetic challenge; five minutes). These RR intervals were used to obtain the estimates of power spectral densities (PSD). The limits for the PSD bands were defined from 0.15 to 0.40 Hz for high frequency band (HF), from 0.04 to 0.15 Hz for low frequency band (LF), from 0.04 to 0.085 Hz for first low frequency sub-band (LF1) and from 0.085 to 0.15 Hz for second low frequency sub-band (LF2). The spectral Gini coefficient (SpG) was proposed to measure the inequality in the power distribution of the RR intervals in each of above-mentioned HRV bands. SpG from each band was compared with its respective traditional index of HRV during the conditions of rest and mental stress. All the differences were considered statistically significant for p < 0.05.

Results

There was a significant decrease in HF power (p = 0.046), as well as significant increases in heart rate (p = 0.004), LF power (p = 0.033), LF2 power (p = 0.019) and LF/HF (p = 0.002) during mental stress. There was also a significant increase in SpG(LF) (p = 0.009) and SpG(LF2) (p = 0.033) during mental stress. Coefficient of variation showed SpG has more homogeneity compared to the traditional index of HRV during mental stress.

Conclusions

This pilot study suggested that spectral inequality of Heart Rate Variability analyzed using the Gini coefficient seems to be an independent and homogeneous psychophysiological indicator of mental stress. Also, HR, LF/HF, SpG(LF) of HRV are possibly important, reliable and valid indicators of mental stress.

Visual and psychological stress during computer work in healthy, young females-physiological responses

Purpose

Among computer workers, visual complaints, and neck pain are highly prevalent. This study explores how occupational simulated stressors during computer work, like glare and psychosocial stress, affect physiological responses in young females with normal vision.

Methods

The study was a within-subject laboratory experiment with a counterbalanced, repeated design. Forty-three females performed four 10-min computer-work sessions with different stress exposures: (1) minimal stress; (2) visual stress (direct glare); (3) psychological stress; and (4) combined visual and psychological stress. Muscle activity and muscle blood flow in trapezius, muscle blood flow in orbicularis oculi, heart rate, blood pressure, blink rate and postural angles were continuously recorded. Immediately after each computer-work session, fixation disparity was measured and a questionnaire regarding perceived workstation lighting and stress was completed.

Results

Exposure to direct glare resulted in increased trapezius muscle blood flow, increased blink rate, and forward bending of the head. Psychological stress induced a transient increase in trapezius muscle activity and a more forward-bent posture. Bending forward towards the computer screen was correlated with higher productivity (reading speed), indicating a concentration or stress response. Forward bent posture was also associated with changes in fixation disparity. Furthermore, during computer work per se, trapezius muscle activity and blood flow, orbicularis oculi muscle blood flow, and heart rate were increased compared to rest.

Conclusions

Exposure to glare and psychological stress during computer work were shown to influence the trapezius muscle, posture, and blink rate in young, healthy females with normal binocular vision, but in different ways. Accordingly, both visual and psychological factors must be taken into account when optimizing computer workstations to reduce physiological responses that may cause excessive eyestrain and musculoskeletal load.

Blood oxygenation of masseter muscle during sustained elevated muscle activity in healthy participants

Myofascial pain associated with temporomandibular disorders has often been linked to pathological muscle hyperactivity. As a result, localised disturbances of intramuscular blood flow could lead to a lower level of oxygen distribution, hypoxia and microcirculatory changes. To assess haemodynamic changes in the masseter muscle during sustained elevated muscle activity (SEMA). Sixteen healthy participants performed thirty 1-min bouts of SEMA with intervals of 1-min 'rest' periods between the bouts on a bite force transducer device. The participants completed three sessions with different percentage of their maximal voluntary occlusal bite force (MVOBF): 0% (no task), 10% or 40% MVOBF tasks. The order of the sessions was randomised with 1- to 2-week intervals. Haemodynamic characteristics of the masseter muscle were estimated with use of a laser blood oxygenation monitor. Tissue blood oxygen saturation (StO₂ ) during SEMA was lower than during rest (P < 0·001). The relative changes in total haemoglobin (Total-Hb) and StO₂ were influenced by condition (SEMA and rest) and with interactions between condition and session (0%, 10% and 40% MVOBF tasks). These results suggest that SEMA may lead to hypoxia in the masseter muscle and that the haemodynamic characteristics and muscle symptoms depend on the magnitude of muscle contractions. Overall, the present findings may help to provide better insights into relationships between jaw muscle activity, haemodynamic changes and symptom developments with implications for clinical conditions such as bruxism characterised by different levels of tooth-grinding and tooth-clenching muscle activity.

Symptoms and physiological responses to prolonged, repeated, low-level tooth clenching in humans

BACKGROUND

The traditional view contends bruxism, such as tooth grinding/clenching, is part of the etiology of temporomandibular disorders (TMD) including some subtypes of headaches. The purpose of this study is to investigate if a low-level but long-lasting tooth-clenching task initiates TMD symptoms/signs.

METHODS

Eighteen healthy participants (mean age ± SD, 24.0 ± 4.3 years) performed and repeated an experimental 2-hour tooth-clenching task at 10% maximal voluntary occlusal bite force at incisors (11.1 ± 4.6 N) for three consecutive days (Days 1-3). Pain and cardiovascular parameters were estimated during the experiment.

RESULTS

The task evoked pain in the masseter/temporalis muscles and temporomandibular joint after 40.0 ± 18.0 minutes with a peak intensity of 1.6 ± 0.4 on 0-10 numerical rating scale (NRS) after 105.0 ± 5.0 minutes (Day 1). On Day 2 and Day 3, pain had disappeared but the tasks, again, evoked pain with similar intensities. The onset and peak levels of pain were not different between the experimental days (P = .977). However, the area under the curve of pain NRS in the masseter on Day 2 and Day 3 were smaller than that on Day 1 (P = .006). Cardiovascular parameters changed during the task but not during the days.

CONCLUSIONS

Prolonged, low-level tooth clenching evoked short-lived pain like TMD. This intervention study proposes that tooth clenching alone is insufficient to initiate longer lasting and self-perpetuating symptoms of TMD, which may require other risk factors.

Biosignal analysis to assess mental stress in automatic driving of trucks: palmar perspiration and masseter electromyography

Nowadays insight into human-machine interaction is a critical topic with the large-scale development of intelligent vehicles. Biosignal analysis can provide a deeper understanding of driver behaviors that may indicate rationally practical use of the automatic technology. Therefore, this study concentrates on biosignal analysis to quantitatively evaluate mental stress of drivers during automatic driving of trucks, with vehicles set at a closed gap distance apart to reduce air resistance to save energy consumption. By application of two wearable sensor systems, a continuous measurement was realized for palmar perspiration and masseter electromyography, and a biosignal processing method was proposed to assess mental stress levels. In a driving simulator experiment, ten participants completed automatic driving with 4, 8, and 12 m gap distances from the preceding vehicle, and manual driving with about 25 m gap distance as a reference. It was found that mental stress significantly increased when the gap distances decreased, and an abrupt increase in mental stress of drivers was also observed accompanying a sudden change of the gap distance during automatic driving, which corresponded to significantly higher ride discomfort according to subjective reports.

Lamotrigine reverses masseter overactivity caused by stress maybe via Glu suppression

Experimental and non-experimental stress significantly increase masseter muscle tone, which has been linked to the symptoms and pathogenesis of several stomatognathic system diseases. Until now, the mechanism underlying this phenomenon has remained unclear. The current study was performed to determine the mechanism of the stress-induced increase in masseter muscle tone and to investigate the effect of lamotrigine on this change. Animals challenged by repeated restraint stress received either saline as a vehicle or lamotrigine in doses of 20, 30 or 40 mg/kg body weight, whereas control animals received saline without stress treatment. Masseter muscle tone was assessed using electromyography. The activity of glutamate-related metabolic enzymes (glutaminase and glutamine synthetase) in the trigeminal motor nucleus was also investigated. Our results showed an interesting phenomenon: masseter muscle activity increased concurrently with the upregulation of the glutamate concentration after stress treatment. The activities of glutaminase and glutamine synthetase in the trigeminal motor nucleus were also upregulated and downregulated, respectively, when the rats were challenged by prolonged stress. The animals treated with lamotrigine at moderate and high doses had significantly decreased masseter muscle tone compared with stressed animals treated with vehicle. These results suggested that increased glutaminase activity and decreased glutamine synthetase activity increased glutamate production and decreased glutamate decomposition, causing an increase in glutamate levels in the trigeminal motor nucleus and eventually increasing masseter muscle tone. The administration of lamotrigine at doses of 30 or 40 mg/kg body weight effectively mitigated the adverse effects of stress on masseter muscle tone via inhibition of glutamate release.

A Paced Auditory Serial Addition Task evokes stress and differential effects on masseter-muscle activity and haemodynamics

This study aimed to determine autonomic and jaw-muscle activities, and haemodynamic responses, to acute experimental mental stress in humans. Eleven healthy men (25.2 ± 3.0 years of age) and five healthy women (23.0 ± 3.7 years of age) performed a standardized mental stress task, the Paced Auditory Serial Addition Task (PASAT). Autonomic function, such as heart rate variability (HRV), and haemodynamic changes were recorded simultaneously. The success rate of the PASAT decreased with increased pace and duration. Low-frequency (5.8 ± 1.1 ms(2)) and high-frequency (5.3 ± 0.6 ms(2)) components of HRV decreased during the PASAT (to 5.0 ± 0.9 ms(2) and 4.6 ± 1.1 ms(2), respectively) as an indication of acute stress. Oxygenated haemoglobin in the masseter muscle (14.6 ± 2.2 10(4) units mm(-3)) remained at an elevated level during the PASAT (15.5 ± 2.5 10(4) units mm(-3)), whereas deoxygenated haemoglobin (7.8 ± 2.3 10(4) units mm(-3)) showed a consistent decrease (to 6.8 ± 2.1 10(4) units mm(-3)). Total haemoglobin and jaw-muscle electromyographic (EMG) activity did not change during the PASAT. Thus, PASAT-induced mental stress changed the parasympathetic/sympathetic balance of the heart and had an acute influence on jaw-muscle haemodynamics, but not on jaw-muscle EMG activity. This non-invasive experimental set-up can be applied to study the effects of repeated or longer-lasting mental stress in order to further the understanding of pathophysiological mechanisms in craniofacial pain conditions.

The influence of psychological stress on the rat temporomandibular joint with the application of countermeasures

OBJECTIVE

This study aimed to provide an experimental theoretical basis for the treatment of temporomandibular disorders by observing the effects of psychological stress and countermeasures on the rat temporomandibular joint (TMJ).

METHODS

Rats were exposed to psychological stress via a communication box and the lateral pterygoid muscle and TMJ were observed with transmission electron microscopy and scanning electron microscopy. Furthermore, the expression of interleukin-1 and tumor necrosis factor-α was assessed in control animals and psychological stress (PS) and stress with diazepam (PS+DI) groups.

RESULTS

Transmission electron microscopy of the lateral pterygoid muscle fibers in the PS showed vacuolar changes in the mitochondria, loss of cristae, and reduced matrix density to variable degrees after 1, 3, and 5 wk of stress. After 5 wk stress+recovery, the cristae and matrix were normal in the PS and PS+DI groups. Scanning electron microscopy of PS rats showed some synovial membranes were detached from the surface of the articular disc after 1 wk. After 3 wk, collagen fibers appeared to have wider waves and worn strips changing in size on the articular disc; after 5 wk, the distribution of collagen fibers was distorted. In PS+recovery and PS+DI rats, no obvious changes were observed on the surface of the articular disc after 1 to 5 wk stress. In PS rats, interleukin-1 and tumor necrosis factor-α expression increased significantly but was at control levels in the PS+DI and PS+recovery groups.

CONCLUSION

Counteracting psychological stress can antagonize its effects on the TMJ and provide a reference for the treatment of stress-related temporomandibular disorders.

Interactions of visual and cognitive stress

BACKGROUND

The objective of this research is to assess the ocular and muscular response to long-duration reading under different visual and cognitive difficulty levels.

METHODS

Thirty-five subjects, with 20/20 vision and without history of ocular pathology or cognitive deficits, participated in the study. Subjects read under different visual and cognitive difficulty levels for 6 (30-minute) conditions. Upper and lower orbicularis oculi, frontalis, and trapezius muscle activities were recorded using surface electromyography (EMG). Aperture size, pupil diameter, and pulse rate of the subjects were recorded with a video camera, pulse meter, and ISCAN eye tracker (ISCAN Inc.), respectively.

RESULTS

The results show that the texts read with a refractive error caused increased orbicularis oculi EMG power and reduced aperture size (P < 0.001). There was no statistically significant difference between the conditions for pulse rate, pupil diameter, or EMG activity of the frontalis and trapezius muscles with either visual or cognitive stress presented in this experiment.

CONCLUSION

Visual stress experienced due to reading under an induced refractive error is potentially mediated by a local mechanism, different from the mechanism underlying reading under low contrast or high cognitive demand.

Oxidative damage and HSP70 expression in masseter muscle induced by psychological stress in rats

Psychological stressors are generally associated with masseter muscle dysfunction and disorders in emotional response. In addition, oxidative states and HSP70 expression, which are involved in the physical and pathological changes of the masseter muscle, could be altered in the stressed tissues and organs. However, the link between psychological stress and the redox homeostasis or the expression of HSP70 in masseter muscles in rats has not been examined. Therefore, we used a communication box paradigm to induce psychological stress in rats. The successful establishment of the animal model was evidenced by an increase in plasma corticosterone and adrenocorticotropic hormone (ACTH). Meanwhile, the stressed rats showed a decrease in the number of entries on open arms, percentage of time spent in open arms, and distance moved in the elevated plus-maze test. The stressed rats also displayed a decrease in the time spent in the center zone, active velocity, and the distance moved in the open-field test. These results demonstrate affective-like behavioral changes in the stressed rats. Moreover, compared with the control rats, a decrease in SOD, GSH-Px and catalase activities and an increase in MDA content were observed in the masseter muscles in stressed rats after 3 weeks and 5 weeks, and the HSP70 expression was elevated in muscles in the rats exposed to stress for 5 weeks. These results indicate that psychological stress induces oxidative damage and up-regulates the expression of HSP70 in masseter muscles in rats, which are associated with behavior resembling anxiety.

Psychological stress alters ultrastructure and energy metabolism of masticatory muscle in rats

To investigate the effects of psychological stress on the masticatory muscles of rats, a communication box was applied to induce the psychological stress (PS) in rats. The successful establishment of psychological stimulation was confirmed by elevated serum levels of adrenocorticotropic hormone (ACTH) and changed behaviors in the elevated plusmaze apparatus. The energy metabolism of the bilateral masseter muscles was tested via chemocolorimetric analysis, whereas muscle ultrastructure was assessed by electron microscopy. In comparison to the control group, the PS group showed evidence of swollen mitochondria with cristae loss and reduced matrix density in the masticatory muscles after three weeks of stimulation; after five weeks of stimulation, severe vacuolar changes to the mitochondria were observed. Increased vascular permeability of the masticatory muscle capillaries was found in the five-week PS rats. In addition, there was decreased activity of Na⁺-K⁺ATPase and Ca²⁺-ATPase and a simultaneous increase in the activity of lactate dehydrogenase and lactic acid in the masticatory muscles of PS rats. Together, these results indicate that psychological stress induces alterations in the ultrastructure and energy metabolism of masticatory muscles in rats.

Circulating adrenaline released by sympathoadrenal activation elicits acute vasodilatation in the rat masseter muscle

The present study was designed to examine the effects of circulating catecholamines released by sympathoadrenal system on the haemodynamics of the masseter muscle in deeply urethane-anaesthetized, artificially ventilated, cervically vagotomized and sympathectomized rats. Intravenous administration of adrenaline induced a biphasic change of blood flow in the masseter muscle (MBF). The change of blood flow showed an initial marked increase and successive slight decrease in a dose-dependent manner (0.01-1 microg/kg). The administration of noradrenaline had no significant effect on the MBF. The increase in the MBF evoked by exogenously applied adrenaline was markedly reduced by the intravenous administration of propranolol (100 microg/kg), whereas pretreatment with either hexamethonium (10 mg/kg), atropine (100 microg/kg), or phentolamine (1 mg/kg) failed to affect the MBF increase. Electrical stimulation of splanchnic nerve (SPLN) preganglionic neurones projecting to the adrenal medulla elicited frequency-dependent (1-20 Hz) increases in the MBF. The intravenous administration of the beta(2)-adrenergic receptor selective antagonist, ICI 118551 (0.5 mg/kg), almost abolished the MBF increase induced by SPLN stimulation, but pretreatment with the beta(1)-adrenergic receptor selective antagonist, atenolol (1 mg/kg), had no effect on this response. The results of the present study indicate that circulating adrenaline elicits acute vasodilatation through a beta-adrenergic mechanism in the rat masseter muscle. Vascular beta(2)-adrenergic receptors in the masseter muscle may be activated preferentially by adrenaline released from the adrenal medulla, suggesting that the sympathoadrenal system is involved in the marked MBF increase during sympathoexcitation.

Quality of reporting masticatory muscle electromyography in 2004: a systematic review

This study evaluated the quality of reporting electromyography in studies evaluating the masticatory muscles published during 2004. Several electronic databases were searched. Abstracts and later articles were selected by a consensus from the five reviewers. An adaptation of the methodological checklist published by the International Society of Electrophysiology and Kinesiology (ISEK) was used. The following information regarding electrodes was reported on the 35 finally selected articles: location (94.3%), interelectrode distance (48.6%), and material (42.9%); detection and amplification: amplification type (54.3%), gain (25.7%), low high pass filters (60%) and cut-off frequencies (60%); electromyography (EMG) processing: sampling rate (74.2%), rectification (46.6%), root-mean square (RMS) (39.2%); number of bits and model of A/D card (17.1%); and normalization procedure (40%). Reasons for the poor reporting are discussed. Because of the general poor quality of reporting of the analysed studies, findings of studies using surface electromyography of masticatory muscles should be interpreted with caution.

Inhibitory effects of excess sympathetic activity on parasympathetic vasodilation in the rat masseter muscle

The present study was designed to examine the effect of sympathetic tonic activity on parasympathetic vasodilation evoked by the trigeminal-mediated reflex in the masseter muscle in urethane-anesthetized rats. Sectioning of the superior cervical sympathetic trunk (CST) ipsilaterally increased the basal level of blood flow in the masseter muscle (MBF). Electrical stimulation of the peripheral cut end of the CST for 2 min using 2-ms pulses ipsilaterally decreased in a dependent manner the intensity (0.5-10 V) and frequency (0.1-5 Hz) of the MBF. The CST stimulation for 2 min at <0.5 Hz with 5 V using 2-ms pulses seems to be comparable with the spontaneous activity in the CST fibers innervating the masseter vasculature, because this stimulation restored the basal level of the MBF to the presectioned values. Parasympathetic vasodilation evoked by electrical stimulation of the central cut end of the lingual nerve in the masseter muscle was markedly reduced by CST stimulation for 2 min with 5 V using 2-ms pulses in a frequency-dependent manner (0.5-5 Hz). Intravenous administration of phentolamine significantly reduced the vasoconstriction induced by CST stimulation in a dose-dependent manner (0.1-1 mg/kg), but pretreatment with either phentolamine or propranolol failed to affect the sympathetic inhibition of the parasympathetic vasodilation. Our results suggest that 1) excess sympathetic activity inhibits parasympathetic vasodilation in the masseter muscle, and 2) alpha- and beta-adrenoceptors do not contribute to sympathetic inhibition of parasympathetic vasodilation, and thus some other types of receptors must be involved in this response.

Influence of sympathetic nervous system on sensorimotor function: whiplash associated disorders (WAD) as a model

There is increasing interest about the possible involvement of the sympathetic nervous system (SNS) in initiation and maintenance of chronic muscle pain syndromes of different aetiology. Epidemiological data show that stresses of different nature, e.g. work-related, psychosocial, etc., typically characterised by SNS activation, may be a co-factor in the development of the pain syndrome and/or negatively affect its time course. In spite of their clear traumatic origin, whiplash associated disorders (WAD) appear to share many common features with other chronic pain syndromes affecting the musculo-skeletal system. These features do not only include symptoms, like type of pain or sensory and motor dysfunctions, but possibly also some of the pathophysiological mechanisms that may concur to establish the chronic pain syndrome. This review focuses on WAD, particular emphasis being devoted to sensorimotor symptoms, and on the actions exerted by the sympathetic system at muscle level. Besides its well-known action on muscle blood flow, the SNS is able to affect the contractility of muscle fibres, to modulate the proprioceptive information arising from the muscle spindle receptors and, under certain conditions, to modulate nociceptive information. Furthermore, the activity of the SNS itself is in turn affected by muscle conditions, such as its current state of activity, fatigue and pain signals originating in the muscle. The possible involvement of the SNS in the development of WAD is discussed in light of the several positive feedback loops in which it is implicated.

Utilization of pharmaceuticals among patients with temporomandibular disorders: a controlled study

OBJECTIVE

Pharmaceuticals are among factors that might be associated with temporomandibular disorders (TMDs), but knowledge about their utilization is limited. The purpose was to systematically register the regular use of medication in general among TMD patients and matched controls to enable comparisons to be made.

MATERIAL AND METHODS

Three hundred consecutive patients referred for diagnosis and treatment of TMDs and fulfilling the Research Diagnostic Criteria were examined prospectively and any medication recorded. Matched controls were registered parallel in time. The pharmaceuticals used were categorized according to the Anatomical Therapeutic Chemical Classification System (ATC).

RESULTS

Forty-four percent of the patients received a main diagnosis of "muscle disorder", 39% "disk disorder", and 17% "joint disorder". Fifty-one percent of all patients used some medication on a regular basis compared to 36% of the controls (p<0.001). The average number of ATC categories used among all patients was 0.9 and among controls 0.5 (p<0.001). Of the female patients with the diagnosis "muscle disorder", 23% used antidepressants (N06A), 6% tranquilizers (N05B), and 7% sleep medication or sedatives (N05C) significantly more frequently than controls. Of the female patients diagnosed with a "joint disorder", 26% used antidepressants (N06A) significantly more frequently than controls. All other ATC categories differed non-significantly.

CONCLUSIONS

The results suggest that the use of pharmaceuticals differs between patients and controls. TMD patients, particularly women diagnosed with "muscle" or "joint" disorders, appear to use drugs for depression more frequently than ordinary dental patients.

Evidence for parasympathetic vasodilator fibres in the rat masseter muscle

The present study was designed to examine (1) whether there are vasodilator fibres in the masseter muscle, and (2) if there are, to establish the neural pathways mediating these responses in urethane-anaesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited intensity- and frequency-dependent increases of the blood flow in the masseter muscle (MBF) and lower lip (LBF). Increases in both the MBF and LBF evoked by the LN stimulation were reduced by hexamethonium in a dose-dependent manner (1-10 mg kg(-1)). Pretreatment with phentolamine or propranolol at a dose of 100 microg kg(-1) had no effect on the increases in either MBF or LBF evoked by LN stimulation. Pretreatment with atropine (100 microg kg(-1)) significantly reduced the MBF increase induced by LN stimulation, but not that in the LBF. The sectioning of the superior cervical sympathetic trunk did not affect the responses. MBF increases occurred with electrical stimulation of the trigeminal ganglion, and these increases were significantly reduced by the administration of hexamethonium and atropine. Lidocaine microinjection into the trigeminal spinal nucleus or salivatory nuclei caused a significant attenuation of the LN-induced MBF increases. When wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the masseter muscle, labelled neurones were abundantly observed in the otic ganglion. The present study indicates that there are parasympathetic cholinergic and noncholinergic vasodilator fibres originating from cell bodies in the otic ganglion in the rat masseter muscle. The MBF increase evoked by activation of the parasympathetic fibres occurred via the trigeminal mediated reflex, suggesting that the novel parasympathetic vasodilator response may play an important role in the regulation of the haemodynamics of jaw muscles.