F157. Presynaptic inhibition of the soleus H reflex and gating of event-related potentials by ankle taps

Abstract

Introduction A single tendon tap may cause at the same time inhibition of reflex responses through post-activation depression and reduction of the afferent volley reaching the cortex through sensory gating. We aimed at characterizing the temporal profile of the two effects and establish the differences in the timing and degree of inhibition between them. Methods In 10 healthy subjects aged 22–65 years we recorded both the H reflex from the soleus muscle and the long-latency evoked potentials (LLEPs) from Cz to electrical stimuli to the posterior tibial nerve at the popliteal fossa. Stimulus intensity was appropriate to induce an H reflex with a small or absent M wave. In test trials, the responses were conditioned by an Achilles tendon tap at interstimulus intervals 60, 80, 100, 200, 400, 800, 1200, 1600 and 2000 ms. Results The H reflex was inhibited test trials at short intervals with progressive recovery at long inte, reaching 72% of control values at the longest interval tested (2000 ms). The LLEPs were inhibited at short intervals, reaching 90% of control values at 200 ms. The M wave remained unmodified in all control and test trials. Conclusion Homosynaptic depression of the H reflex is far more powerful than gating of the same sensory volley by mechanical taps. The long lasting inhibition of the H reflex may imply the activation of inhibitory circuits that do not participate in afferent gating.