Intense Chewing Causes Transient Suppression of an Inhibitory Jaw Reflex

Objectives: Electrical stimulation of the upper lip evokes an inhibitory jaw reflex. Previous studies have shown that chemically induced experimental muscle pain reduces the late inhibitory wave of this reflex. Intense chewing, which provokes muscle fatigue and/or pain non-invasively, also caused a significantly smaller inhibition. So far, the reproducibility of this finding has not been tested. The aim of the present study therefore was to test the reproducibility of this reduced inhibitory reflex wave between two experimental sessions. Further, the recovery to pre-provocation reflex sizes was tested in both sessions. Methods: Five healthy participants (3 men, 2 women; mean age ± SD = 25.0 ± 8.5 yrs) volunteered for this study. Two sessions per participant were performed, separated by at least one week. Both sessions included a baseline recording (16 sweeps; stimuli at 1.25 times the individually established reflex threshold intensity), followed by masticatory muscle fatigue and/or pain provocation (about 3 min chewing on 3 pieces of hard chewing gum at 1.5 times the habitual chewing pace and with an intensity of about 50% of the maximum voluntary EMG contraction level per chewing stroke), immediately followed by an experimental recording, and finally, separated by 5-min breaks, followed by two recovery recordings. Results: No differences in reflex size in any of the four recordings were found between both sessions (F = 0.510; P = 0.515). Within-session testing yielded a significant effect for ‘recording' (F = 7.897; P = 0.004). Bonferroni contrasts indicated, that intense chewing caused a significantly reduced size of the inhibitory reflex wave (P = 0.000), while pre-provocation size was already regained after the first 5-min break (P = 0.899). Conclusion: These preliminary data suggest that fatigue and/or pain, provoked by intense chewing, causes a transient yet reproducible reduction of the late inhibitory reflex wave. (Supported by the IOT.)