Reflex responses of the human jaw-closing system depend on the locus of intraoral mechanical stimulation.

Innocuous mechanical stimuli were applied to eight sites on the tongue dorsum and palate while subjects used feedback to maintain a constant isometric biting force. Reflex responses of the jaw-closing system were measured as changes in force and in EMGs recorded from right and left masseter muscles. Stimulation at each of the eight sites produced reflex force and EMG responses in most subjects tested. The nature of the reflex responses strongly depended on the site of stimulation. Stimulation of the palate tended to produce suppression of ongoing EMG activity and decreases in background biting force. In contrast, stimulation of sites on the tongue posterior to the tip, most often resulted in excitatory EMG responses and increases in jaw-closing force. Unilateral, early excitatory responses were observed in the right masseter with stimulation of the right side of the tongue. The existence of spatially organized responses of the human jaw-closing system to innocuous intraoral stimulation is not consistent with the view that these cutaneous reflexes are primarily of protective significance.