Modulation of human rhythmical jaw motor function by perioral argon laser stimuli

The influence of peripheral afferent activity on human jaw motor function was examined in this study. Painful argon laser stimuli were applied to the upper lip in 10 subjects during rhythmical jaw movements and the effects on the jaw-closer electromyogram (EMG) and jaw movements were studied. The duration of the open-close cycle (1 sec) and the voluntary level of the jaw-closer EMG burst were standardized by auditory and visual feedback, respectively. Laser stimuli (0.2 sec) were delivered at predetermined percentages of the open-close cycle. Laser stimulation consistently had an excitatory effect on the jaw-closer EMG burst. The root-mean-square (RMS) value of the stimulus-related EMG burst was significantly larger than the preceding and the following EMG bursts. The excitatory effects on the jaw-closer EMG burst were modulated with respect to where in the open-close cycle the laser stimulus was applied, the voluntary force of the jaw-closer burst, and the laser stimulus intensity. Thus, the largest excitatory effects were seen during the start of jaw closure, with the most forceful voluntary contractions, and with the most painful laser stimuli. The stimulus-related changes in the jaw-closer EMG bursts were always associated with a significant shortening (30-90 msec) of the duration of the open-close cycle. The present results suggest that argon laser-induced activity in sensory afferents may generally have an excitatory effect on rhythmical jaw-closer motor performance.