Corticospinal modulation of vibration-induced H-reflex depression

The purpose of this study was to examine corticospinal modulation of spinal reflex excitability, by determining the effect of transcranial magnetic stimulation (TMS) on soleus H-reflexes while they were almost completely suppressed by lower extremity vibration. In 15 healthy adults, a novel method of single-limb vibration (0.6 g, 30 Hz, 0.33 mm displacement) was applied to the non-dominant leg. Soleus muscle responses were examined in six stimulation conditions: (1) H-reflex elicited by tibial nerve stimulation, (2) tibial nerve stimulation during vibration, (3) subthreshold TMS, (4) subthreshold TMS during vibration, (5) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, and (6) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, during vibration. With or without vibration, subthreshold TMS produced no motor evoked potentials and had no effect on soleus electromyography (p > 0.05). In the absence of vibration, H-reflex amplitudes were not affected by subthreshold TMS conditioning (median (md) 35, interquartile range (IQ) 18-56 vs. md 46, IQ 22-59% of the maximal M wave (Mmax), p > 0.05). During vibration, however, unconditioned H-reflexes were nearly abolished, and a TMS conditioning pulse increased the H-reflex more than fourfold (md 0.3, IQ 0.1-0.7 vs. md 2, IQ 0.9-5.0% of Mmax, p < 0.008). Limb vibration alone had no significant effect on corticospinal excitability. In the absence of vibration, a subthreshold TMS pulse did not influence the soleus H-reflex. During limb vibration, however, while the H-reflex was almost completely suppressed, a subthreshold TMS pulse partially restored the H-reflex. This disinhibition of the H-reflex by a corticospinal signal may represent a mechanism involved in the control of voluntary movement. Corticospinal signals that carry the descending motor command may also reduce presynaptic inhibition, temporarily increasing the impact of sensory inputs on motoneuron activation.