Influence of methodological parameters on human jaw-stretch reflexes

The influence of methodological parameters and experimental conditions on the human jaw-stretch reflex was studied in healthy subjects in order to develop a reliable tool for investigation of the excitability of motoneuron pool. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. The displacement and ramp time of the stretches were accurately controlled and automatically triggered by a computer. The reflex responses were measured in the surface electromyogram (EMG) of the masseter and anterior temporalis muscles with online monitoring of the clenching level. The peak-to-peak amplitude of the jaw-stretch reflex was shown to be proportional to the level of EMG activity during isometric contractions, to increase proportionally with increasing stretch displacement at a given ramp time, and to decrease proportionally with increasing ramp time at a given stretch displacement. There were no significant differences in the reflex amplitude between repeated recordings within one session or between different sessions. Local anesthetic around the lower incisors as well as the upper incisors had no significant influence on the reflex amplitude. However, different biting positions on the bars of the stretch device significantly influenced the amplitude of the stretch reflex.