The Consistent Nature of the Nociceptive Modulation of Jaw Reflexes

Objective: Previous studies have established that inhibitory reflexes evoked in human jaw muscles are consistent in any individual over a period of time and can be modulated by the application of nociceptive stimuli to remote parts of the body. The purpose of the present study is to investigate whether these modulatory effects also show consistency over time.

Methods: Two identical experiments were carried out on each of 8 volunteer subjects, with an interval between experiments of at least 3 days. EMG recordings were made from a masseter muscle while the subjects maintained activity in the muscle at approximately 12.5% of maximum. Clear inhibitory reflexes were evoked by electrical test stimuli (mean: 2.75mA, 1ms duration; all producing a sharp or painful sensation) to the hair-bearing skin of the upper lip. Modulation of the reflex was produced by the application of a painful conditioning stimulus (15mA, 1ms) to the sural nerve 100ms before the test stimulus. Presentation of test stimuli or conditioning-test stimulus combinations were randomised. Following rectification and averaging of the EMGs, reflexes were quantified by integration of their waveforms. Data are presented as means±S.E.M..

Results: In the initial experiments, inhibitory reflexes were evoked in all 8 subjects, with a latency of 44±3ms for the test-stimulus-only presentations in the randomised sequences. The magnitude of this reflex was significantly reduced (by 45±11%) when the conditioning stimuli were applied. No significant differences were found in these parameters in the second experiments: latency, 39±3ms; reduction following conditioning, 39±12% (Paired t-tests, P=0.3 and P=0.4 respectively). More importantly, there was a strong positive correlation (Pearson's r=0.87, P=0.005) between the extents of the modulatory effects in the 2 experiments.

Conclusion: The nociceptive modulation of this inhibitory jaw reflex is consistent in any one subject, at least over a short period of time.