Effects of deep pain of masticatory and neck/shoulder muscles to the tactile sensation from orofacial region and modification effect of autonomic nerve activation

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Aim : This study was investigated whether somatosensory cortex activity, evoked by air puff stimulation of the lower lip region, could be modulated by experimental masticatory and/or neck/shoulder muscles pain, and by hand immersion in cold water.Methods : Air puff stimulation, was applied to evoke tactile sensation from the lower lip surface. Somatosensory evoked potentials (SBPs) were recorded from C3, C4, and Cz positions. Masticatory muscle pain and neck/shoulder muscle pain was evoked by the injection of isotonic (0.9%, 0.5ml) and hypertonic saline (5%, 0.5ml) into the left masseter muscle and trapezius muscle, respectively. As cold pressor test, left hand was immersed into cold water (4℃).Results : SBPs were composed of two wave factors, N20 and P30. By injecting isotonic saline into the masseter and trapezius muscles, amplitudes of N20 and P30 and peak-to-peak amplitudes of N20/P30 were not significantly altered. On the other hand, peak amplitudes of N20 and P30 and peak-to-peak amplitude of N20/P30 were significantly decreased during the first phase of the post-injection period of hypertonic saline into masseter muscle. However, there were no significant alterations in SBPs during the injection of hypertonic saline into trapezius muscle. These results present further evidences for the pathofunctional interrelationship between masticatory muscle pain and modulation of homotopical tactile input on the bases of quantitative analyses of SEPs. We also analyzed the effect of the hand immersion into cold water (4℃) on the tactile sensation from the orofacial region and it was clarified that the tactile sensation from the lip surface was significantly enhanced by the cold pressor test.Conclusions : Masticatory muscle pain may diminish the tactile sensation from the homotopical orofacial region depending on the severity of pain, while the sympathetic nerve activation may enhance the tactile sensation of the orofacial region.