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. Furthermore, in the masseter EMG, two periods of apparent excitation can be distinguished from the background activity: inter-suppression period (ISP) and post-suppression period (PSP). In the present study, the relationship between nociception and ES1, ISP and PSP was investigated. Abstract Methods: The reflex responses were electrically elicited in 15 volunteers. A novel computer algorithm was used to detect and quantitate ES1, ES2, ISP and PSP. Abstract Results: (a) ES1 and ES2 were more susceptible to changes in magnitude of suppression and excitation, respectively, than ISP and PSP; (b) the on-set latencies were almost unaffected; (c) both ES1 and ES2 occurred at stimulus intensities well below pain threshold; (d) magnitude of ES1 and ES2 suppression increased with increasing stimulus intensity, but supra pain threshold stimulation did not result in further suppression; (e) no correlation was found between perceived stimulus intensity and ES1, ISP or PSP. Abstract Conclusions: The increase in ES1 and ES2 suppression seen in our study at increasing stimulus intensity is suggested to be mediated by activity in both nociceptive and non-nociceptive fibres converging onto central interneurones in the reflex circuits. We suggest that neither ISP nor PSP are reflex responses. It is unlikely that ES1, ES2, ISP and PSP evoked by electrical stimuli are directly linked to noxious activity.