Quantitative analysis of reflex inhibition in single motor units in human masseter muscle: effects of stimulus...

Inhibitory reflex responses to electrical stimulation of the mental nerve were studied in recordings from single motor units (SMU) in the human masseter muscle. A new analytical technique for spike train data was used. Electrical 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 preceding SMU action potential (AP). Sham, non-painful, and painful stimulus intensities were applied. The post-stimulus firing probability of the SMU was progressively 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 effect of the increasing stimulus intensity on the duration and amplitude of the inhibitory post-synaptic potential. The methods used in the present study provide a useful means of quantifying the effects of motoneuron excitability in detail.