A study on synaptic coupling between single orofacial mechanoreceptors and human masseter muscle

The connection between individual orofacial mechanoreceptive afferents and the motoneurones that innervate jaw muscles is not well established. For example, although electrical and mechanical stimulation of orofacial afferents in bulk evokes responses in the jaw closers, whether similar responses can be evoked in the jaw muscles from the discharge of type identified single orofacial mechanoreceptive afferents is not known. Using tungsten microelectrodes, we have recorded from 28 afferents in the inferior alveolar nerve and 21 afferents in the lingual nerve of human volunteers. We have used discharges of single orofacial afferents as the triggers and the electromyogram (EMG) of the masseter as the source to generate spike-triggered averaged records to illustrate time-based EMG modulation by the nerve discharge. We have then used cross correlation analysis to quantify the coupling. Furthermore, we have also used coherence analysis to study frequency-based relationship between the nerve spike trains and the EMG. The discharge patterns of the skin and mucosa receptors around the lip and the gingiva generated significant modulation in EMGs with a success rate of 40% for both cross correlation and coherence analyses. The discharge patterns of the periodontal mechanoreceptors (PMRs) generated more coupling with a success rate of 70% for cross correlation and about 35% for coherence analyses. Finally, the discharges of the tongue receptors displayed significant coupling with the jaw muscle motoneurones with a success rate of about 40% for both analyses. Significant modulation of the jaw muscles by single orofacial receptors suggests that they play important roles in controlling the jaw muscle activity so that mastication and speech functions are executed successfully.