Effect of tonic muscle pain on short-latency jaw-stretch reflexes in humans

The modulation of human jaw-stretch reflexes by experimental muscle pain was studied in three experiments. Short-latency reflex responses were evoked in the masseter and temporalis muscles by fast stretches (1 mm displacement, 10 ms ramp time) before, during and 15 min after a period with tonic pain. In Expt. I, a dose of 5.8% hypertonic or 0.9% isotonic (control) saline was infused in random order into the left masseter for up to 15 min (n=12). The level of excitation of the left masseter was kept constant at 15% of maximal effort by visual feedback and on-line calculation of the root-mean-square value of the surface electromyogram (sEMG). In Expt. II, a dose of 5.8% saline was infused into the left masseter but with feedback from the right masseter sEMG (n=12). In Expt. III, both sEMG and intramuscular (im) EMG was recorded from the left and right masseter muscles. The feedback was from either the sEMG or imEMG of the left masseter in which 5.8% saline was infused (n=12). In all experiments, subjects continuously rated their perceived pain intensity on a 10-cm visual analogue scale (VAS). Infusion of 5.8% saline caused moderate levels of pain (mean VAS 4.9-5.0 cm) whereas infusion of 0.9% saline was almost pain-free (mean VAS 0.3 cm). The pre-stimulus EMG activity in the masseter, which served as the feedback muscle during the recording, was constant across the different conditions. During painful infusion of 5.8% saline in Expts. I and III, the pre-stimulus sEMG activity in the non-painful masseter was significantly higher than baseline when the sEMG on the painful side was used as feedback signal, and in Expt. II significantly lower on the painful side when the non-painful side served as feedback signal (Student-Newman-Keuls: P<0.05). Isotonic saline did not affect the pre-stimulus sEMG activity or the jaw-stretch reflex parameters. The peak-to-peak amplitude of the stretch reflex in the painful masseter normalized to the pre-stimulus EMG activity was significantly higher during the pain conditions compared with the pre- and post-infusion conditions in all experiments. These results indicate that experimental jaw-muscle pain facilitates the short-latency (8-9 ms), probably monosynaptic, jaw-stretch reflex as revealed by both sEMG and imEMG. This effect could not be accounted for by variability in pre-stimulus EMG activity. An increased sensitivity of the fusimotor system at this level of static muscle excitation is suggested as a possible mechanism, which could contribute to an increased stiffness of the jaw-muscles during pain.