Decomposition of the human electromyogramme in an inhibitory reflex

Abstract

The activity of up to 4 motor units was recorded simultaneously with electrodes placed in the masseter muscle in human subjects. Mildly noxious electrical shocks were applied to the ipsilateral lip while one of the units was kept firing at a steady frequency by voluntary control. The pattern of reflex responses of each of the units was determined, and spike-triggered averaging was used to measure the potential that each action potential contributed to the rectified surface electromyogramme (EMG). Finally, the average contribution made to the surface EMG by each unit throughout the whole course of the reflex was determined. The contribution of each unit to the reflex response in the surface EMG was found to depend on its firing frequency throughout the course of the response, and on its amplitude measured at the surface. The timing of the various phases of inhibition and activation of different units depended on their pre-stimulus firing frequency. In a given bite, the lowest-threshold units were more likely to be firing most rapidly, and these were least susceptible to the inhibitory stimulus. Higher-threshold units tended to produce larger potentials at the muscle surface but, because they fired more slowly in a given bite, they were more powerfully inhibited by the stimulus. Most units showed the same general pattern of inhibitory and excitatory activity that can be seen in the surface EMG. However, the timing of these various phases in the surface EMG did not necessarily correspond with the timing of inhibition and activation in the records of individual units. Rather, the surface signal is the sum of many similar, but out-of-phase frequency changes in the motor units of which it is constituted.