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

Site-to-site variation of muscle activity and sensitivity in the human anterior temporalis muscle: Implications for contingent stimulation

Objective: To evaluate variation of electromyographic (EMG) activity and sensitivity between different sites of anterior temporalis (AT) muscle. Materials and methods: Sixteen healthy subjects (eight men: 28.8 ± 5.2 year old and eight women: 29.1 ± 3.9) participated in one experimental session. EMG activity during masticatory muscle contraction was recorded from nine sites at the AT muscle in a 3 x 3 grid with 1 cm between. The subjects maintained steady 30% of maximal voluntary contraction (MVC) using visual feedback. The surface EMG electrode was moved sequentially between these nine test sites and the contractions were repeated. One site was tested four times to assess test-retest variability. The sensory threshold to electrical stimulation and impedance was also measured at the same sites as the placement of EMG electrodes. Results: The 30% MVC force values did not differ between sites (p = 0.863) or within the same site (p = 0.995) due to the feedback. The EMG activity during 30% MVC was highest at the anterior-superior site (p < 0.05) with a marginal difference within the same site (p = 0.044). Impedance was higher at the posterior-superior, posterior-middle, and posterior-inferior sites (p < 0.05). The sensory threshold was highest at the posterior-superior site (p < 0.05). Conclusions: These findings showed that electrodes close to the hairline have higher impedance and sensory thresholds and should be avoided. The anterior-superior site produces the highest EMG activity and lower sensory thresholds and can be recommended as the optimal site to place the electrode for contingent stimulation

Haemodynamic reactions in human masseter muscle during different types of contractions

Objectives : To investigate to what extent different types of jaw-muscle contractions cause haemodynamic reactions in human masseter muscle. Materials and Methods : Eleven healthy volunteers (seven males : 25.0±2.9 years and four females: 23.3±4.3 years) performed three standardized oral-motor tasks : maximal voluntary contractions (MVC ; duration 5 sec, 3 times repetition), tooth grinding (repetitive left and right side grinding from intercuspal position to canine-to-canine position at 0.5 Hz keeping 50% MVC for a total of 10 times), and 1-min left-side gum chewing at 1 Hz. Haemodynamic characteristics were measured in the left masseter muscle with the use of a laser blood oxygenation monitor (BOML1TRW, OMEGAWAVE INC., Tokyo, Japan). Electromyographic (EMG) activity from right and left masseter muscle was simultaneously monitored (500 Hz sample frequency) during the tasks. 1-ANOVA followed by Dunnett’s test was used. Results : Oxygenated haemoglobin (OXYHb : 13.5±0.2 104 units/mm3) and deoxygenated haemoglobin (deOXYHb : 7.6 ± 0.3 104 units/mm3) did not change significantly during the MVC task (13.9±0.2 and 7.8±0.3 104 units/mm3, respectively, P>0.065), however, the total haemoglobin (TOTALHb : 22.1±0.3 104 units/mm3) showed a significant increase (22.7±0.3 104 units/mm3, P=0.003) during the MVC. Tissue blood oxygen saturation was not changed during the MVC (P=0.164). During the tooth grinding task, OXYHb, deOXYHb, TOTALHb, and tissue blood oxygen saturation (StO2) remained constant (P>0.127). Finally, the chewing task was associated with significant decreases in StO2 (67.9±0.7%, P=0.006) related to a decrease in OXYHb (14.0±0.2 104 units/mm3, P=0.040) compared to baseline (68.8±0.7% and 14.2 ±0.3 104 units/mm3, respectively). Conclusion : These results showed that high-intensity experimental tooth clenching caused constriction-like reactions in the masseter muscle whereas tooth grinding did not cause detectable changes in haemodynamic characteristics of masseter muscle. Finally, the findings indicated that rhythmic dynamic contractions might lead to oxygen deficit in the masseter muscle. The present data may have implications for understanding the potential pathophysiological consequences of different types of oral-motor tasks, e.g., bruxism and prolonged mastication

Effects of the Paced Auditory Serial Addition Task (PASAT) with different rates on autonomic nervous system responses and self-reported levels of stress

To characterise self-reported levels of stress and autonomic responses in healthy humans evoked by different rates of the Paced Auditory Serial Addition Task (PASAT). Fifteen participants performed PASATs with different rates (3·6-, 2·4-, 1·6- or 1·2-s intervals) and a control task, in random order. Correct responses, self-reported levels of stress and autonomic responses to the PASATs were estimated. Increased PASAT rates were associated with decreases in correct responses (P < 0·001) and increases in self-reported levels of stress (P < 0·001). For autonomic responses, significant changes were seen in 10 variables during 2·4-s PASAT compared with the respective baseline; however, significant differences in relative changes from baseline were found between the 2·4-s PASAT and control task only for mean RR-intervals (P < 0·001), systolic and diastolic blood pressure (P = 0·002 and P = 0·006) and cardiac output (P < 0·001). Regarding comparison between the four PASATs, significant differences in the relative changes from baseline were seen between the 3·6-s PASAT and faster PASATs, for example mean RR-intervals, high-frequency power and respiration rate; however, there were no differences between the faster PASATs. The autonomic responses during the PASATs with different rates were quite similar for the faster PASATs (intervals < 2·4 s); however, the slowest 3·6-s PASAT evoked significantly less self-reported stress and autonomic arousal compared with the faster PASATs. Standardization of the PASAT rate may be important for studies on autonomic nervous system function and self-reported measures of stress. Future studies may test more complex interactions between stress, autonomic responses and pain responses