Relation between perceived stimulus intensity and exteroceptive reflex responses in the human masseter muscles

Objectives: Two periods of exteroceptive suppression are elicited in human masseter muscle following perioral electrical stimulation: ES1 and ES2. Fur thermore, in the masseter EMG, two periods of apparent excitation can be di stinguished from the background activity: intersuppression period (ISP) and post-suppression period (PSP). In the present study, the relationship betw een nociception and ES1, ISP and PS was investigated. Methods: The reflex responses were electrically elicited in 15 volunteers. A novel computer algorithm was used to detect and quantitate ES1, ES2, LSP and PSP. Results: (a) ES1 and ES2 were more susceptible to changes in magnitude of s uppression and excitation, respectively, than ISP and PSP; (b) the on-set l atencies were almost unaffected; (c) both ES1 and ES2 occurred at stimulus intensities well below pain threshold; (d) magnitude of ES1 and ES2 suppres sion increased with increasing stimulus intensity, but supra pain threshold stimulation did not result in further suppression; and (e) no correlation was found between perceived stimulus intensity and ES1, ISP or PSP. Conclusions:The increase in ES1 and ES2 suppression seen in our study at in creasing stimulus intensity is suggested to be mediated by activity in both nociceptive and non-nociceptive fibres converging onto central interneuron es in the reflex circuits. We suggest that neither ISP nor PSP are reflex r esponses. It is unlikely that ES1, ES2, ISP and PSP evoked by electrical st imuli are directly linked to noxious activity. (C) 1999 Elsevier Science Ir eland Ltd. All rights reserved.