Human masseter inhibitory reflexes evoked by repetitive electrical stimulation

Objectives: The relationship between the masseter inhibitory reflex (MIR) and nociceptive processing in the trigeminal region was studied in 10 healthy subjects. Based on the known increase in perceived sensory intensity following repetitive stimulation of the nociceptive system, we examined the hypothesis that the MIR reflects noxious activity in the trigeminal system by determining the possible relation between changes in MIR and perceived sensory intensity. Methods: The MIR was quantitated and compared with psychophysical measures following repetitive peri-oral electrical stimulation (5 square wave pulses of 0.5 ms repeated at 2 Hz). In addition to the early (ES1) and late (ES2) periods of exteroceptive suppression, two periods with apparent excitation could be distinguished from the background electromyografical activity: (a) the inter-suppression period (ISP) between the ES1 and the ES2, and (b) the post-suppression period (PSP) after the ES2. A computer algorithm was used to detect and quantitate ES1, ES2, ISP, and PSP. The response variables were (a) onset latencies and (b) magnitudes of suppression (ES1 and ES2) and excitation (ISP and PSP). Results: Consistent reduction of the magnitude of ES2 suppression in response to repetitive stimuli was observed below as well as above the pain detection threshold. Conclusions: The observed reduction of the magnitude of ES2 suppression is not specifically related to nociceptive processing. Habituation or net inhibitory effects on inhibitory pre-motor neurones (i.e. disinhibition) are possible mechanisms for the observed reduction of the magnitude of ES2 suppression after repetitive stimulation.