EMG, force and discharge rate analysis of human jaw reflexes in response to axial stimulation of the incisor

Reflex studies utilising controlled stimulation along the long axis of human incisors are relatively new, and the effects that various stimulus parameters have on the elicited reflexes are not fully understood. Twelve subjects were recruited to determine the effects that contraction level, stimulus force and amount of constant force applied between stimuli have on the reflex response of the masseter muscle. Multi-unit intramuscular electromyogram (EMG) was recorded alongside surface EMG to determine whether any differences existed between the two. Furthermore, cumulative peri-stimulus "dischargegrams" were constructed to determine whether events seen in the EMG corresponded to changes in the discharge rates of the underlying motor units. Axial stimulation of the incisor induced a response in the EMG comprising of peak-trough-peak, with the trough being the most dominant. The bite force record showed only a reduction (relaxation) in response to the stimulation. The most significant experimental factor affecting the reflex occurrence and strength was the stimulus force. Although the latency, duration and occurrence rates were not significantly different, the strength of the responses was greater in intramuscular recordings compared with the surface recordings. Discharge rate analysis showed that approximately two-thirds of the late peaks detected in the EMG did not correspond to an increase in the discharge rates of the underlying units; hence they were due to the clustering of action potentials following the trough and not to a change in the membrane potential of the motoneurone. It was also found that the duration of the trough, as seen by the reduced cumulative discharge rate of the underlying units, was longer than indicated by the EMG.