Pterygoid muscle activity in speech: A preliminary investigation.

BACKGROUND: Speaking depends on refined control of jaw opening and closing movements. The medial pterygoid muscle (MPT), involved in jaw closing, and the lateral pterygoid muscle (LPT), involved in jaw opening, are two key mandibular muscles in mastication and are likely to be recruited for controlled movements in speech. OBJECTIVES: Three hypotheses were investigated, that during speech the MPT and LPT: (1) were both active, (2) but exhibited different patterns of activity, (3) which fluctuated with the vowels and consonants in speech. METHODS: Intramuscular EMG recordings were made from the right inferior head of the LPT and/or the right MPT in five participants during production of 40 target nonsense words (NWs) consisting of three syllables in the form /V1 C1 V2 C2 ə/ (V = vowel; C = consonant; ə = unstressed, reduced vowel), spoken by each participant 10 times per NW; analysis focussed on the target syllable, C1 V2 . RESULTS: Both MPT and LPT exhibited robust increases in EMG activity during utterance of most NWs, relative to rest. Peak LPT activation was time-locked to the final part of the target consonant (C1 ) interval when the jaw begins opening for the target vowel (V2 ), whereas peak MPT activation occurred around the temporal midpoint of V2 , when the jaw begins closing for C2 . EMG amplitude peaks differed in magnitude between "high" vowels, i.e., for which the tongue/jaw are high (e.g., in SEEK), and "low" vowels, i.e., for which the tongue/jaw are low (e.g., in SOCK). CONCLUSIONS: These novel findings suggest a key role for the LPT and MPT in the fine control of speech production. They imply that speech may impose major synergistic demands on the activities of the MPT and the LPT, and thereby provide insights into the possible interactions between speech activities and orofacial activities (e.g. mastication) and conditions (e.g. Temporomandibular Disorders) that involve the masticatory muscles.

Functional properties of single motor units in the human medial pterygoid muscle: Thresholds.

BACKGROUND: Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements. OBJECTIVES: The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle. METHODS: Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P < .05. RESULTS: A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening-closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties. CONCLUSIONS: The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.

Threshold variations of medial pterygoid single motor units during vertical or horizontal force tasks.

OBJECTIVES: To test the hypotheses that (a) the force thresholds at onset of medial pterygoid muscle single motor unit (SMU) activity do not decrease with an increase in the rate of force generation in standardised vertical or horizontal jaw-force tasks, and (b) there is evidence for functional heterogeneity within the medial pterygoid muscle. METHODS: In 14 healthy participants, electromyographic recordings of the right medial pterygoid muscle were performed with intramuscular fine-wire electrodes during four isometric force tasks: vertical, horizontal contralateral, horizontal protrusion and horizontal ipsilateral, performed at two rates of force development (slow ramp, fast ramp). Computer tomography scans confirmed electrode location within the muscle, which was divided into medial and lateral parts. Force thresholds of onset of discriminated SMUs were compared between rates in each task; significance accepted at p < 0.05. RESULTS: Of 45 SMU force thresholds studied in one or more tasks, there was no significant difference between slow and fast ramp within each force task, except slow ramp thresholds from the lateral part during the vertical force task were significantly higher than fast ramp thresholds. Reversals of recruitment order between tasks provided evidence for functional heterogeneity within the muscle. Force thresholds of the vertical tasks (range: 1-292.6 N) were mostly higher than for the horizontal tasks (range: 0.1-12.5 N). CONCLUSION: The data are consistent with the proposal that the medial pterygoid muscle stabilises the jaw in the vertical plane during isometric force generation in the jaw closing, as well as horizontal directions.

The role of progressive oral implant rehabilitation in mastication, cognition and oral health-related quality of life outcomes-A pilot to define the protocol.

BACKGROUND: The implications of oral rehabilitation after tooth loss require further investigation. OBJECTIVES: To conduct a pilot study to investigate: (a) changes in masticatory performance with progressive oral implant rehabilitation (POR); (b) association between POR and neurocognitive function using functional magnetic resonance imaging (fMRI); and (c) oral health-related quality of life (OHQoL) outcomes. METHODS: Four completely edentulous patients (mean age: 73 ± 1.4 years) participated. Each received new complete removable dental prostheses (RDPs) transitioned to mandibular two implant-retained RDPs (IR-RDP). Assessments were performed at 4 time points for neurocognitive skills, fMRI with functional tasks (jaw clenching, working memory and sustained attention, inhibition), masticatory performance with colour-changing gum and OHQoL. Assessments were performed with new complete RDPs (T0 as baseline data) and IR-RDPs at 1 week (T1), 6 weeks (T2) and 12 months (T3) post-insertion. Data analyses were based on intra-patient and inter-patient results. RESULTS: Masticatory performance and QoL improved with an IR-RDP at each time point. FMRI jaw clenching sensory and motor cortical activity decreased at T1, with motor cortical activity increasing to T0 levels at T2. For cognitive fMRI activation tasks, cortical activity decreased from T0 to T1 across all regions of interests (ROI) and increased at T2 throughout the cognitive brain regions. Neurocognitive skills declined at T1, followed by improvement to or beyond T0 levels at T2. CONCLUSION: Improvements in masticatory performance and OHQoL occurred from complete RDPs to IR-RDP. Prosthetic adaptation was associated with neurocognitive changes to pre-insertion activity levels or greater after 6 weeks. These pilot data suggest both behavioural and neural associations between POR and cognition; however, larger study numbers are required.

Single motor units from the medial pterygoid muscle can be active during isometric horizontal and vertical forces.

OBJECTIVES: To determine (a) whether the medial pterygoid muscle is active in an isometric vertical force task and in isometric horizontal force tasks in the contralateral, protrusion and ipsilateral directions; (b) whether the same single motor units (SMUs) could be active across different directions of isometric force generation; and (c) whether different regions of the medial pterygoid muscle exhibit different patterns of SMU activation during the generation of any one direction of isometric force. METHODS: Intramuscular electromyographic (EMG) recordings were made from the right medial pterygoid muscle in 15 healthy participants during isometric force tasks: vertical and horizontal contralateral, protrusion and ipsilateral. A computed tomography scan divided the EMG recording site into a medial or lateral part in each participant. Single motor units were discriminated in each task. RESULTS: Medial pterygoid SMU activity was recorded in 100% of participants for the vertical biting tasks, 86% of participants for the horizontal contralateral and horizontal protrusion tasks and 57% of the horizontal ipsilateral tasks. Of the 72 SMUs that were discriminated, 36% were active in all tasks; 18% were active only in the vertical tasks and 17% were active in the vertical, horizontal contralateral and horizontal protrusion tasks. The proportion of SMUs that was active in at least 1 horizontal task in the lateral part (33/39) was significantly higher than the proportion (21/33) in the medial part (Chi-Square, P < 0.05). CONCLUSION: The data are consistent with a stabilisation role for the medial pterygoid muscle in isometric jaw forces in the vertical and horizontal planes.

Reorganization of motor unit activity at different sites within the human masseter muscle during experimental masseter pain.

The aims were to test the hypotheses that experimental masseter muscle pain leads to recruitment and/or derecruitment of motor units at different sites within the masseter and that the patterns of change in motor unit activity differ between sites. Single motor unit (SMU) activity was recorded at two sites within the right masseter [superior/anterior, inferior/posterior (IP)] during isometric biting tasks (ramp, step level) on an intraoral force transducer in 17 participants during three experimental blocks comprising no infusion (baseline), 5% hypertonic saline infusion (pain), or isotonic saline infusion (control). A visual analog scale (VAS) was used to score pain intensity. The VAS scores were statistically significantly greater during infusion of hypertonic saline than during infusion of isotonic saline. No significant differences in force levels and rates of force change were found between experimental blocks. In comparison with isotonic saline infusion, SMUs could be recruited and derecruited at both sites during hypertonic saline infusion. The frequency of recruitment or derecruitment, in comparison with no change, was statistically significantly greater at the IP site than at the superior/anterior site. Experimental noxious masseter stimulation results in a reorganization of motor unit activity throughout the muscle, and the pattern of reorganization may be different in different regions of the muscle.

Experimental noxious stimulation of the right masseter muscle does not affect bilateral masseter and temporalis muscle activity and force parameters during standardised isometric biting tasks.

AIM: To determine if the electromyographic (EMG) activity of the left and right masseter and anterior temporalis muscles is altered by experimental right masseter muscle noxious stimulation during goal-directed isometric biting tasks in asymptomatic humans. METHODS: Isometric biting tasks (slow and fast ramp biting tasks, 2-step biting task) were performed on an intraoral force transducer in 18 participants during the following blocks: baseline block, hypertonic saline infusion into the right masseter muscle (painful block) and isotonic saline infusion into the right masseter (control block). Bipolar surface electrodes recorded EMG activity from the bilateral masseter and anterior temporalis muscles. A 100-mm visual analogue scale (VAS) quantified pain intensity, and the McGill Pain Questionnaire (MPQ), the Depression, Anxiety and Stress Scales-21 (DASS-21) and the Pain Catastrophizing Scale (PCS) were completed. Repeated measures ANOVA assessed the effects of pain on the force rates (N/s), force amplitudes (N) and the root mean square (RMS) jaw muscle EMG activity across blocks. Statistical significance accepted at P < 0.05. RESULTS: VAS scores were significantly (P < 0.001) higher during hypertonic than isotonic saline infusion blocks. There was no significant effect of pain on the force rates, or force levels or the RMS EMG activity of each masseter and anterior temporalis muscle. CONCLUSION: The findings suggest that experimentally induced right masseter muscle pain does not modify force or surface jaw muscle EMG activity during isometric biting tasks.

Effect of a brief episode of experimental muscle pain on jaw movement and jaw-muscle activity during chewing.

The aims of this study were to determine whether: (i) the jaw motor system develops a new pattern of jaw movement and/or jaw-muscle activity after resolution of an acute episode of jaw-muscle pain; and (ii) if jaw-muscle activity and jaw-movement features change progressively with repetition of a chewing sequence. Jaw movement and jaw muscle (masseter, anterior temporalis, and digastric) activity were recorded during free and rate-standardized chewing in eight asymptomatic participants (pain infusion group), before and at three time blocks up to 45 min after a single 0.2-ml bolus infusion of 5% hypertonic saline into the right masseter muscle. The same procedure, without infusion, was performed in another eight participants (control group). There were no significant main effects of group on jaw movement and muscle activity, suggesting that there were no persistent post-pain effects on chewing. Across groups, repetitions of free and unstandardized chewing movements were associated with progressive increases in velocity and amplitude of jaw movement and masseter and temporalis electromyographic (EMG) activity. These findings suggest that factors unrelated to pain, such as practice effects, may be playing a role in the changes in jaw movement and jaw-muscle activity observed after resolution of an acute episode of jaw-muscle pain.

Oral health and dental care in aged care facilities in New South Wales, Australia. Part 3 concordance between residents' perceptions and a professional dental examination.

OBJECTIVES: To determine the perceptions of dental care held by the residents in aged care facilities (ACFs) in New South Wales (NSW) and to compare these perceptions with clinical observations. BACKGROUND: No specific data exist relating to NSW residents' perceptions of dental care compared with a clinical examination. Planning for appropriate oral health programs in ACFs necessitate such data. MATERIALS AND METHODS: Four Area Health Services of Sydney and 25 low care ACFs were selected from which representative residents were sampled who completed a survey and underwent a basic dental examination. RESULTS: Of the subjects (25 males, 96 females), 76.9% had never received a dental visit as entering the ACF; 14.1% suffered from dental pain; 69.4% wore dentures and of these 18.3% required assistance in cleaning. Dentures were cleaned twice/day in 54.9% of cases. Natural teeth were reported present in 71.9% of residents, and 85.1% did not require assistance in cleaning. Appropriate dental care facilities and dry mouth were most frequent problems highlighted. Clinical examinations showed that 69% were denture wearers; oral hygiene and denture hygiene were considered good in 15.7% of cases. A high level of concordance existed between self-reports and examination. CONCLUSIONS: Increased awareness about oral health across leadership, caregivers and residents with appropriate dental health education and dedicated space within facilities would provide a much needed improvement for addressing oral health issues of the ACF residents. This might be the right time to plan for the future challenges that will need to be met by the NSW care system.

Provision of dental care in aged care facilities NSW Australia- Part 2 as perceived by the carers (care providers).

OBJECTIVES: To investigate carers' perception of the provision of dental care in aged care facilities (ACFs) New South Wales (NSW), Australia. BACKGROUND: Carers are responsible for 'hands-on, day-to-day' care of residents, including dental care, yet there were no specific figures available concerning their role in NSW ACFs. MATERIALS AND METHODS: Questionnaires were mailed to 406 NSW directors of nursing (DONs) requesting completion by a carer who was proficient in English and without the influence of the DON. The 23-item questionnaire was presented in 4 sections, and the data qualitatively analysed. RESULTS: 211 questionnaires were completed and returned, giving a response rate of 52%. Carers were mostly female (91.9%) in the 40-50 and >50 age groups. Oral health training had been received by 66.7% of carers, and although 73.2% thought that their training was adequate, carers in general requested further training. Long waiting periods for government dental services (69.4%) and resident unable to communicate oral health problems (69.2%) were seen as the most frequent barriers to dental care. Almost all carers reported the availability of electric tooth brushes, fluoride gel, disclosing tablets/gel, interdental brushes and the use of a foam mouth prop, while few reported the use of other dental care products. CONCLUSION: As carers provided almost all of oral health care for residents, emphasis should be placed on training in geriatric dental care techniques and use of dental products.

Neuroplasticity in the adaptation to prosthodontic treatment.

AIMS: To investigate cerebral cortical changes by using functional magnetic resonance imaging (fMRI) after denture renewal and to test how these relate to prosthodontic treatment adaptability as measured by chewing efficiency and maximum bite force. METHODS: Ten complete denture wearers (five women and five men, mean age ± standard deviation: 70.3 ± 9.1 years) participated in the study. Each had their complete dentures renewed and underwent an fMRI examination with three functional tasks (lip pursing; jaw tapping; jaw clenching) as well as a color-mixing test for chewing efficiency and unilateral maximum bite force measurements. Recordings were performed with the old dentures (T0) and with the new dentures on insertion (T1) and at 1 week (T2) and 3 months postinsertion (T3). At T1, denture stability and retention (S/T) were assessed by two independent operators. Wilcoxon signed rank tests and Spearman's rho correlation were carried out for data analysis. RESULTS: The right and the left precentral gyrus (PRCG) and postcentral gyrus (POCG) were identified with significant activation across all three functional tasks. A statistically significant increase in the level of activity between T0 and T2 (POCG: P = .022; PRCG: P = .017) was found during jaw clenching tasks. Both regions of interest (PRCG, POCG) appeared to correlate with S/T of the new dentures while the subject performed a lip-pursing task (PRCG: r = 0.689, P = .027; POCG: r = 0.665, P = .036). The chewing efficiency and maximum bite force increased significantly during the adaptation to replacement dentures (chewing efficiency: T1-T2 P = .032, T2-T3 P = .012; maximum bite force right side: T2-T3 P = .047). CONCLUSION: Changes in brain activity occurred in the adaptation to replacement dentures and appeared to regain preinsertion activity levels during motor tasks involving the dental occlusion after 3 months postinsertion.

Provision of dental care in aged care facilities, NSW, Australia - part 1 as perceived by the directors of nursing (care providers).

OBJECTIVE: To ascertain the Directors of Nursing (DONs) perception of the provision of dental care and the difficulties in oral health maintenance for residents in New South Wales (NSW) aged care facilities (ACFs). BACKGROUND: There were no specific figures relating to DONs' perceptions of oral health needs and dental care nor obstructions to dental care of residents. MATERIALS AND METHODS: A total of 414 questionnaires, encoded for confidentiality, were posted to all DONs of ACFs in NSW and the data analysed. RESULTS: Questionnaires were returned from 255 ACFs (response rate 61.6%) representing 16 861 residents with a male-to-female ratio of 1:3.45. Of these residents, 48% had a dental assessment on admission by qualified dental staff. In 74.2% of facilities, no regular visits by dental staff took place, and 58.6% considered the perceived unwillingness of dental professionals to visit residents a barrier to good oral health. DONs reported that 53.6% of residents had dentures only, while 18.3% had natural teeth only. The most frequent difficulties reported by staff were residents' use of abusive language (78.2%) and residents refusing to open mouth (60.9%). CONCLUSION: Lack of involvement of dental professionals in admission assessment and in ongoing programmes was of concern to DONs in ACFs.

Association between resting jaw muscle electromyographic activity and mandibular advancement splint outcome in patients with obstructive sleep apnea.

INTRODUCTION: The muscles of mastication are important in positioning the mandible and can therefore affect the patency of the upper airway. The aim of this study was to determine whether resting masticatory muscle activity influences the response to mandibular advancement splint treatment in patients with obstructive sleep apnea. METHODS: Thirty-eight adult patients with obstructive sleep apnea were recruited for the study. Baseline electromyographic activities of the right anterior and posterior temporalis, masseter, and submandibular muscles were recorded with surface electrodes while the patients were awake, in the upright and supine positions, with the jaw in the postural position, and with and without a mandibular advancement splint. Muscle activity of the patients with obstructive sleep apnea was compared between responders (apnea-hypopnea index change ≥50%, and <10 events per hour) and nonresponders (apnea-hypopnea index change <50%) to mandibular advancement splint treatment. RESULTS: There were 18 responders and 20 nonresponders to mandibular advancement splint treatment. The responders had a trend for increased muscle activity in all muscle groups and scenarios. The resting muscle activity of the submandibular and masseter muscles while lying at rest and of the submandibular and posterior temporalis muscles while lying with the mandibular advancement splint in place were significantly greater (P <0.05) in the responders than in the nonresponders. CONCLUSIONS: Inherent baseline differences in muscle activity between responders and nonresponders to mandibular advancement splint treatment in adults with obstructive sleep apnea were observed. This preliminary study suggests that there might be a correlation between responsiveness with mandibular advancement splint treatment and baseline muscle activity.

The effects of capsaicin-induced intraoral mucosal pain on jaw movements in humans.

AIMS: To determine whether mucosal pain, evoked through a novel topical capsaicin model, has an effect on jaw movement and whether psychologic factors have an association with any pain-induced movement effects. METHODS: Mandibular movement was recorded from 26 asymptomatic subjects during free opening and closing, resistant opening jaw movements, and free and standardized chewing, at baseline and in test sessions while the subjects were wearing a custom maxillary mouthguard coated with either capsaicin cream (pain group, 13 subjects) or placebo cream (control group, an additional 13 subjects). All subjects completed the Depression Anxiety Stress Scales (DASS) and the Pain Catastrophizing Scale (PCS). Statistical analyses were made with independent t tests and bivariate correlation analyses. RESULTS: Capsaicin induced moderate pain in the pain group, but there were no significant differences between the two groups in the change of kinematic variables from baseline except for a significantly greater increase from baseline in the number of chewing cycles per second (chewing rate) for free (t = 2.74, P = .011) and standardized chewing (t = 2.10, P = .047) in the pain group compared with the control group. In the pain group, the DASS anxiety score was negatively correlated (r = -.70, P = .007), with the change of mean opening velocity from the baseline to the test session in the free opening task, and the DASS depression score was negatively correlated to the increase of chewing rate in the free chewing task from the baseline to the test session (r = -.56, P = .046). CONCLUSION: Capsaicin-induced mucosal pain resulted in a significant increase in chewing rate but had no effect on amplitude or velocity in opening/closing jaw movements and chewing. Anxiety and depression scores correlated negatively with velocity in free opening jaw movement and chewing rate, respectively.

Thresholds, Firing Rates, and Order of Recruitment of Anterior Temporalis Muscle Single-Motor Units During Experimental Masseter Muscle Pain.

AIMS: To test the hypothesis that, in comparison with control, experimental noxious stimulation of the right masseter muscle would result in significant changes in the firing rates, thresholds, and recruitment orders of single-motor units (SMUs) of the nonpainful, synergistic right anterior temporalis muscle during goal-directed isometric biting task performance. METHODS: Twenty healthy volunteers received an infusion of hypertonic saline (HS; 5% sodium chloride) into the right masseter to produce pain intensity of 40 to 60 on a 100-mm visual analog scale (VAS). Isotonic saline (IS) infusion was a control. Standardized biting tasks were performed with an intraoral force transducer, and intramuscular electromyographic activity was recorded from the right anterior temporalis muscle. Tasks (slow and fast ramp biting tasks, two-step biting task) were performed in 3 blocks: baseline, HS infusion, and IS infusion. Across blocks, SMU thresholds and firing rates were statistically compared, and SMU recruitment sequences were qualitatively compared. Statistical significance was set at P < .05. RESULTS: No significant differences (P > .05) were noted between HS and IS infusion blocks in thresholds or firing rates of anterior temporalis SMUs. Individual SMUs showed increases or decreases in thresholds or firing rates or changes in recruitment sequences mostly during HS compared to IS infusion. CONCLUSION: The reorganization of SMU activity that has been suggested to occur in both painful and nonpainful agonist jaw muscles may involve not only recruitments and de-recruitments of SMUs, but may also extend to more subtle increases and/or decreases in firing rates, thresholds, and recruitment sequences of individual SMUs in the nonpainful synergistic muscles.

Reorganization of Masseter and Temporalis Muscle Single Motor Unit Activity During Experimental Masseter Muscle Pain.

AIMS: To test the hypothesis that experimental noxious stimulation of the right masseter muscle results in a reorganization of motor unit activity within the right temporalis and right masseter muscles during jaw closing tasks. METHODS: A total of 20 healthy participants received hypertonic saline (5% sodium chloride) infusion into the right masseter muscle, and pain intensity was maintained at 40-60/100 mm on a visual analog scale. Standardized isometric biting tasks were performed with an intraoral force transducer while single motor units (SMUs) were recorded from the right masseter and temporalis muscles. Tasks were repeated in four blocks: block 1 (baseline 1), block 2 (hypertonic saline [HS] infusion or isotonic saline [IS] infusion), block 3 (infusion of the other solution), and block 4 (baseline 2). The occurrences of SMUs were tabulated across blocks. Statistical significance was considered to be P < .05. RESULTS: There were no significant effects of block on the tasks. A total of 83 SMUs were discriminated in the temporalis and 58 in the masseter. For the comparison between HS and IS across tasks, the occurrences of 74.6% to 82.8% of SMUs were unchanged (70.2% to 94.3% for masseter), while during HS, 10.3% to 17.1% of SMUs were recruited (0% to 12.8%, masseter) and 6.9% to 12.7% were de-recruited (5.7% to 17%, masseter). CONCLUSION: The present findings suggest that most biting-task-related jaw muscle SMUs remain active during experimental muscle noxious stimulation. There was some evidence in both the anterior temporalis and masseter muscles for motor unit recruitment and de-recruitment consistent with a motor unit reorganization during experimental pain.

Simultaneous Noxious Stimulation of the Human Anterior Temporalis and Masseter Muscles. Part II: Effects on Jaw Muscle Electromyographic Activity.

AIMS: To test the hypotheses that, in comparison to control, the effects of simultaneous noxious stimulation of the right masseter and anterior temporalis muscles on jaw muscle activity (1) vary with the task; (2) are different between different agonist or antagonist muscles involved in a task; and (3) are correlated with mood or pain-related cognition scores. METHODS: In 15 asymptomatic participants, recordings were made of jaw movement and electromyographic (EMG) activity of the right digastric and bilateral masseter and anterior temporalis muscles during standardized open/close and free and standardized chewing tasks. The tasks were repeated in three blocks: block 1 (baseline), block 2 (during simultaneous infusion of 5% hypertonic or 0.9% isotonic saline infusion into the right masseter and anterior temporalis muscles), and block 3 (infusion sequence reversed). The Depression, Anxiety and Stress Scales questionnaire was completed prior to the experiment, and the Pain Catastrophizing Scale was completed before and after the experiment. Linear mixed-effects model analysis compared root mean square (RMS) EMG activity under baseline, hypertonic saline, and isotonic saline (control), and Spearman correlations between RMS and psychologic scores were calculated. P < .05 was considered significant. RESULTS: The significant effects of pain on the activity of a jaw muscle varied with the task, were different between different agonist and antagonist muscles in a task, and were significantly correlated with some of the psychologic scores. Qualitatively, the effects noted in a particular muscle could be different between different participants. CONCLUSION: Simultaneous noxious masseter and anterior temporalis stimulation results in changes in jaw muscle activity that can vary with the task, the muscle, the participant, and some psychologic variables.

Simultaneous Noxious Stimulation of the Human Anterior Temporalis and Masseter Muscles. Part I: Effects on Jaw Movements.

AIMS: To test the hypotheses that, in comparison to control (isotonic saline), simultaneous noxious stimulation (hypertonic saline) of the masseter and anterior temporalis muscles would result in (1) reductions in amplitude and velocity of jaw movements during standardized open/close jaw movements and during free and standardized chewing and (2) changes in amplitude and velocity of jaw movements that relate to higher levels of negative mood or pain-related thoughts. METHODS: Standardized open/close and free and standardized chewing were recorded in 15 asymptomatic participants in three blocks: block 1 (baseline), block 2 (during 5% hypertonic or 0.9% isotonic saline infusion into the right masseter and anterior temporalis muscles simultaneously), and block 3 (infusion sequence reversed). The Depression, Anxiety, and Stress Scale (DASS-21) and the Pain Catastrophizing Scale (PCS) were completed by the participants before the experiment, and the PCS was completed after the experiment. The amplitude and velocity of opening and closing movements for each task were compared between blocks (repeated-measures analysis of variance). Spearman rank correlation coefficient was used to explore correlations. Statistical significance was considered to be P < .05. RESULTS: In comparison to isotonic saline control, hypertonic saline resulted in significantly smaller opening and closing amplitudes and lower velocity during closing in free chewing, but no significant effects in the open/close task or standardized chewing. There were significant correlations between PCS scores and amplitude or velocity during isotonic saline and baseline, but not hypertonic saline. CONCLUSION: The pain-related reduction in amplitude and/or velocity of free chewing is consistent with the Pain Adaptation Model, but the absence of effects on the open/close task and standardized chewing is not. The few significant correlations between psychologic variables and jaw movement may reflect the low scores.

Effects of experimental pain on jaw muscle activity during goal-directed jaw movements in humans.

To study the effects of masseter muscle pain on jaw muscle electromyographic (EMG) activity during goal-directed tasks. Mandibular movement was tracked and EMG activity was recorded from bilateral masseter, and right posterior temporalis, anterior digastric, and inferior head of lateral pterygoid muscles in 22 asymptomatic subjects at postural jaw position, and during three tasks: (a) protrusion, (b) contralateral (left), (c) open jaw movement. Tasks were performed during three conditions: control (no infusion), test 1 [continuous infusion into right masseter of 4.5% hypertonic saline to achieve 30-60 mm pain intensity on 100-mm visual analog scale (VAS)], and test 2 (isotonic saline infusion; in 16 subjects only); the sequence of hypertonic and isotonic saline was randomized. The average EMG root-mean-square values at 0.5 mm increments of mid-incisor-point displacement were analysed using linear mixed effects model statistics (significance: P < 0.05). Right masseter hypertonic saline infusion resulted in significantly (P < 0.0005) more pain (mean +/- SD VAS 47.3 +/- 14.3 mm) than isotonic infusion (12.2 +/- 17.3 mm). Although there was evidence of inter-subject variation, the principal EMG findings were that the significant effects of hypertonic saline-induced pain on EMG activity varied with the task in which the muscle participated irrespective of whether the muscle was an agonist or an antagonist in the tasks. The direction of the hypertonic saline-induced pain effect on EMG activity (i.e., whether the hypertonic saline-induced EMG activity was less than or greater than control EMG activity) could change with the magnitude of jaw displacement. Hypertonic saline infusion had no significant effect on postural EMG activity in any of the recorded jaw muscles. The data suggest that under constrained goal-directed tasks, the pattern of pain-induced changes in jaw muscle EMG activity is not clear cut, but can vary with the task performed, jaw displacement magnitude, and the subject being studied.

Regional properties of the superior head of human lateral pterygoid muscle.

The aim of this study was to investigate functional heterogeneity within the superior head of the human lateral pterygoid (SHLP) muscle by comparing the functional properties (e.g. firing rates) of single motor units (SMUs) between different arbitrarily defined regions of the SHLP, namely, medial, middle or lateral; origin or insertion; and superior or inferior regions. Jaw movement and electromyographic (EMG) activity was recorded from computed tomography-verified locations within the SHLP of 27 asymptomatic human subjects during goal-directed contralateral, ipsilateral, and protrusive jaw movements. The SMU firing rates for protrusion in the medial, origin, and inferior regions were significantly lower than, respectively, the firing rates in the middle, insertion, and superior regions. For contralateral movement, the firing rates were significantly greater in the medial and middle regions than those in the lateral region. The data provide additional evidence that the SHLP is functionally heterogeneous and, together with previous evidence for functional heterogeneity within the inferior head of the lateral pterygoid, support the proposition that both heads should be regarded as functionally heterogeneous.