Putative feed-forward control of jaw-closing muscle activity during rhythmic jaw movements in the anesthetized rabbit

When a thin plastic test strip of various hardness is placed between the upper and lower teeth during rhythmical jaw movements induced by electrical stimulation of the cortical masticatory area (CMA) in anesthetized rabbits, electromyographic (EMG) activity of the masseter muscle is facilitated in a hardness-dependent manner. This facilitatory masseteric response (FMR) often occurred prior to contact of the teeth to the strip, and thus preceded the onset of the masticatory force. Since this finding suggests involvement of a feed-forward mechanism in the induction of the FMR, the temporal relationship between the onset of the FMR and that of the masticatory force was analyzed in five sequential masticatory cycles after application of the strip. The FMR was found to precede the onset of masticatory force from the second masticatory cycle after application of the strip, but never did in the first cycle. This finding supports the concept of a feed-forward control mechanism that modulates FMR timing. Furthermore, the FMR preceding the force onset disappeared after making a lesion of the mesencephalic trigeminal nucleus (MesV) where the ganglion cells of the muscle spindle afferents from the jaw-closing muscles are located. In contrast, no such change occurred after blocking periodontal afferents by transection of both the maxillary and the inferior alveolar nerves. The putative feed-forward control of the FMR is therefore dependent mainly on sensory inputs from the muscle spindles, but little on those from the periodontal receptors, if any. We further examined the involvement of the CMA with the putative feed-forward control of the FMR via the transcortical loop. For this purpose, rhythmical jaw movements were induced by stimulation of the pyramidal tract. No significant change in the timing of the FMR occurred after the CMA ablation, which strongly suggests that the CMA is not involved in the putative feed-forward control of the FMR. The FMR was also noted to increase significantly in a hardness-dependent manner even after the MesV lesion, although the rate of increment decreased significantly. Contribution of muscle spindles and periodontal receptors to the hardness-dependent change of the FMR is discussed.