Spatial Facilitation of Reciprocal Inhibition and Crossed Inhibitory Responses to Soleus Motoneurons During Walking

In humans, short-latency crossed spinal inhibitory reflexes are elicited in the contralateral soleus (cSOL) muscle following stimulation of the ipsilateral posterior tibial nerve (iPTN). To date, the spinal interneurons mediating the cSOL inhibition are unknown. This study investigated whether the Ia inhibitory interneurons in the disynaptic reciprocal inhibition pathway mediate the short-latency cSOL inhibition. Following combined stimulation of the iPTN and the contralateral common peroneal nerve (cCPN), we quantified the spatial facilitation of the ongoing electromyography (EMG; Experiment 1) or the test H-reflex (Experiment 2) in the cSOL during walking. There was a significant increase in the cSOL inhibition when the two stimuli were elicited in combination compared to that expected from the algebraic sum of the two if elicited separately. It is therefore likely that the Ia inhibitory interneurons in the disynaptic reciprocal inhibitory pathway contribute to the short-latency cSOL inhibitory reflex.