Changes in the H-reflex amplitude with different characteristics of the preceding conditioned stimulus.

The post-stimulus inhibition of the H-reflex was studied in human subjects with the following experimental sets: Presentation of a pair of stimuli, with intensity of the conditioned stimulus, S1, of 30% Hmax, and with varying intensities of the test stimulus, S2, from threshold to maximal values. Varying intensity of S1 from threshold values, 30% Hmax, 50% Hmax, and varying intensity of S2 from threshold to maximal values, for the H2-reflex-curve to be obtained. Varying intensity of S1 from 0.9 to 1.15-1.20 threshold and constant intensity of S2 of 30-40% Hmax. In the first two experiments the curve "intensity of S1/H2-reflex amplitude" at different intensities of S1 and different delays of S2 has been investigated. In the third experiment the H2-reflex recovery with diminishing the S1 to subthreshold values has been investigated. The results have shown: H2-reflex was strongly suppressed even with threshold values of S1. H2-reflex-curve after a conditioned stimulus was significantly lower to the H-reflex-curve after a single stimulus. The recovery of the H-reflex amplitude after S1 was observed with subthreshold values of S1. The possible role of the presynaptic inhibition for the suppression of the H1-reflex with the presentation of a pair of stimuli has been considered.