Simultaneous modulation of the exteroceptive suppression periods in the trapezius and temporalis muscles by experimental muscle pain

Objective To investigate whether the exteroceptive suppression (ES) periods in the trapezius and temporalis muscles could be simultaneously modulated by either experimental trapezius muscle pain or temporalis muscle pain. Methods Fourteen healthy volunteers participated in two sessions with randomised injections of either hypertonic or isotonic saline into the right trapezius muscle or right temporalis muscle. The pain intensity was continuously scored on a 10 cm electronic visual analogue scale. During muscle contraction, the early (ES1) and late (ES2) reflex in the temporalis and late (ES2) reflex in the trapezius elicited by electrical stimulation of the right infraorbital nerve were recorded pre-, during-, and post-injection. Results Hypertonic saline injection induced local muscle pain and referred pain with the maximal pain intensity of 6.3±0.5 cm in the right trapezius and 6.7±0.6 cm in the right temporalis muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle was associated with significantly shortened ES2 duration and offset time in the injected right trapezius muscle. Only injection of hypertonic saline into the right trapezius muscle could significantly reduce the ES2 offset time and decrease the degree of suppression of ES2 in the right temporalis and left trapezius muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle evoked no changes in ES2 parameters in the left temporalis muscle. Injection of hypertonic saline, but not isotonic saline, into the right temporalis muscle caused a significantly shorter ES2 offset time and shorter ES2 duration bilaterally in the temporalis and trapezius muscles. Conclusions The findings of the present study provide evidence for a bi-directional effects reflecting convergence of muscle afferents from the trigeminal and upper cervical neural systems in humans, which may partly underlie the manifestations of pain referral between these two areas.