Distribution of periodontal afferent input to motoneurons of human masseter

The distribution of the synaptic input from the periodontal mechanoreceptors onto the motoneurons of the human masseter is studied. Periodontal mechanoreceptors were activated using slowly rising force profiles of 2.5 N, which are known to induce predominantly excitatory reflex responses in the surface electromyogram (EMG) of the masseter. The reflex responses of single motor units (SMUs) were recorded to quantify the distribution of the periodontal input onto the masseter motoneurons. The relative sizes of motoneurons were estimated by comparing the peak-to-peak amplitude of the MacroRep (i.e. the representation of the SMU in the Macro EMG record). It was found that the larger SMUs had more excitatory and less inhibitory reflex responses than those of smaller size. This study demonstrates that the inputs from the periodontal mechanoreceptors, activated by slowly rising force profiles, are not distributed equally to the masseteric motoneurons. This may cause recruitment of motoneurons contrary to the size principle under some circumstances.