The effect of stimulus strength on the jaw-jerk response in man.

The jaw-jerk response was elicited in seven adult male subjects by tapping on the chin in a downward direction. The magnitude of the applied taps was varied subjectively by the operator, and the downward acceleration of the mandible measured by means of a calibrated piezo-electric accelerometer fixed to the lower anterior teeth. The taps were applied during voluntary clenching by the subject, controlled by means of a force transducer placed between upper and lower teeth. The electrical response of masseter and temporal muscles was found to increase both with increasing stimulus strength and with voluntary clenching force. However, only by averaging a number of responses were clear relationships demonstrated, there being other variables, affecting the muscles independently, which it has not been possible to identify and control. It is concluded therefore, that quantitative analysis of the electromyograms of the jaw closing muscles following chin-tap stimuli is not a satisfactory method for testing for abnormality in excitability of the neuromuscular system involved in the jaw-jerk response.