Quantitative analysis of reflex inhibition in single motor units in human masseter muscle: Effects of stimulus intensity

Inhibitory reflex responses to electrical stimulation of the mental nerve w ere studied in recordings from single motor units (SMU) in the human masset er muscle. A new analytical technique for spike train data was used. Electr ical stimuli were delivered to the mental nerve when the SMU fired with two consecutive inter-spike intervals (ISIs) within the range of 90 ms to 110 ms. Stimuli were delivered with increasing stimulus delays after the preced ing SMU action potential (AP), Sham, non-painful, and painful stimulus inte nsities were applied. The post-stimulus firing probability of the SMU was p rogressively decreased among the three conditions. Analysis of the relation between stimulus delays and ISI for the first post-stimulus APs revealed a linear relation which was shifted upward, and the slope was increased with increasing stimulus intensity. This may be explained by a differential eff ect of the increasing stimulus intensity on the duration and amplitude of t he inhibitory post-synaptic potential. The methods used in the present stud y provide a useful means of quantifying the effects of motoneuron excitabil ity in detail. (C) 2000 John Wiley & Sons, Inc.