Sensory-motor interactions of human experimental unilateral jaw muscle pain: a quantitative analysis

Experimental muscle pain was elicited by bolus injection of 0.15 ml of 5% hypertonic saline into the human masseter muscle. The sensory experience was described using 10-cm visual analogue scales (VAS) and the McGill Pain Questionnaires (MPQ) on 10 subjects. Effects of pain on deliberately unilateral mastication were quantitatively assessed in 13 other male subjects using kinematic recordings of the mandible and jaw muscle electromyography (EMG). Jaw movement and EMG data were transformed into single masticatory cycles which were averaged within subjects to produce mean masticatory cycles. Injection of 5% saline through normal and anesthetized skin produced similar VAS profiles and MPQ features. Displacement of the mandible during painful mastication was significantly smaller in the vertical axis (10.0 +/- 11.5%, P < 0.05) and in the lateral axis (22.6 +/- 20.9%, P < 0.05) as compared to pre-pain values. The mean opening and closing velocities of the mandible were significantly reduced (10.5 +/- 16.3% and 15.3 +/- 21.2%, P < 0.05) and the cumulated distance of the jaw movement was also significantly smaller during pain (10.5 +/- 11.8%, P < 0.05). Moreover, agonist EMG activity during pain was significantly lower in the ipsilateral masseter muscle (20.3 +/- 25.4%, P < 0.05) as compared to pre-pain root-mean-square (RMS) values. The observed sensory-motor interactions can be explained by a facilitatory effect of activity in nociceptive muscle afferents on inhibitory brain-stem interneurons during agonist action. Thus, generated movements have smaller amplitudes and they are slower which most likely represents a functional adaptation to experimental jaw muscle pain.