Organisation of common inputs to motoneuron pools of human masticatory muscles

Objective To determine the pattern of organization of common inputs to the motoneuron pools of individual muscles in the masticatory system. Methods Six subjects bit on a rubber-coated wooden splint placed between the upper and lower incisor teeth. We recorded the surface electromyogram (EMG) of co-contracting masseter, temporalis and digastric muscles bilaterally during isometric jaw closing at 5%, 10%, 20% and 40% of maximal voluntary masseter EMG. Results The cross-correlograms of the EMGs of homologous muscle pairs indicate that there are common synaptic inputs to the motoneuron pools of the left and right masseter, and left and right digastric muscles, but not to left and right temporalis. The amplitude of the central peak in masseter and digastric correlograms increased with bite force. When the activity of ipsilateral muscle pairs was cross-correlated, central peaks were prominent for masseter–digastric and masseter–temporalis muscle pairs, and the peak amplitudes increased significantly with bite force. In contrast, no significant central peak was observed for temporalis–digastric muscle pairs at any level of voluntary biting. Conclusions We conclude that there is synchronous modulation of input bilaterally to the masseter muscles and to the digastric muscles but not to the temporalis muscles. There is synchronous modulation of input to ipsilateral masseter–digastric and masseter–temporalis muscle pairs but not to temporalis and digastric muscles. Significance The extent of common input to motoneuron pools of muscles acting around a common joint varies for different muscle pairs, and is not simply a function of whether the muscles of the pair are synergists or antagonists