Effects of transcranial electrical and magnetic stimulation on reciprocal inhibition in the human arm

We studied the effects of transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS), delivered at intensities below the threshold for evoking an electromyographic response, on the disynaptic and presynaptic phases of reciprocal inhibition in 8 healthy subjects. After TES, the H-reflex evoked in the flexor carpi radialis (FCR) muscle was strongly facilitated when the cortical stimulus was given 4.0–4.5 ms after the test stimulus (median nerve stimulus). TES reduced the disynaptic phase of reciprocal inhibition most strongly when the cortical stimulus followed the test stimulus by 3.0–3.5 ms. TES also reduced presynaptic inhibition, but with a time course that was identical to that of the facilitation of the uninhibited H-reflex. After subthreshold TMS, the facilitation of the H-reflex showed at least 2 peaks, one occurring when the cortical stimulus was given 2 ms after the test stimulus and the other when the cortical stimulus followed the test stimulus by 0.5 to −1.5 ms. The effects of TMS on the 2 phases of reciprocal inhibition were similar, and in both cases the disinhibitory effects had essentially the same time course as the facilitatory effect of TMS on the uninhibited H-reflex. The different effects of TES on the 2 phases of reciprocal inhibition provide evidence of the presynaptic natuer of the second phase. The absence of a difference in the effect of TMS on the 2 phases could be due to the more temporally dispersed descending volley after TMS.