Periodontal mechanoreceptor input reduces synchronous discharge of voluntarily activated masseter motor units in man

The control exerted by inputs from periodontal mechanoreceptors (PMRs) on the tonic activity of 35 pairs of single motor units in the left masseter muscle was investigated with and without the presence of continuous pressure on the upper left central incisor tooth. Crosscorrelograms were computed to assess the temporal coupling between the discharges of the motor unit pairs. In the absence of continuous pressure, central peaks in the cross-correlograms revealed the presence of significant synchronous discharge in 16 out of the 35 pairs tested. In contrast, during PMR stimulation only nine pairs were found to discharge with a significant amount of synchronization. It is concluded that short-term synchronization due to common, partially common and synchronized inputs shared by the motoneurons was reduced whenever extraneous periodontal inputs were superimposed on the voluntary command. This indicates that the interneurons which mediate the periodontal inputs arising from one single tooth are not distributed widely throughout the masseter motoneuron pool. In contrast, it appears that periodontal inputs are liable to reduce the efficiency of common inputs distributed to the masseter motoneurons during voluntary contraction ("desynchronization").