Convergence of ipsi- and contralateral muscle afferents on common interneurons mediating reciprocal inhibition of ankle plantarflexors in humans.

Recent studies have shown that afferents arising from muscle receptors located on one side can affect the activity of muscles on the contralateral side. In animal preparations, evidence supports that afferent pathways originating from one limb converge onto interneurons mediating disynaptic reciprocal Ia inhibition of the opposite limb. This study was designed to investigate whether this pathway is similar in humans to that described in animals. Thirteen healthy volunteers participated in one of two experiments. In experiment 1, the effects of ipsilateral posterior tibial nerve (iPTN) stimulation were assessed on the reciprocal Ia inhibition of the contralateral soleus (cSOL) motoneuronal pool (n = 8). Across all participants, iPTN stimulation intensity was 1.69 ± 0.3 × Motor Threshold (MT) and contralateral common peroneal (cCPN) stimulation intensity was 0.86 ± 0.16 × MT. iPTN and cCPN stimulation were delivered separately or in combination and changes in the ongoing electromyography (EMG) quantified. In experiment 2, the amplitude of a test SOL H-reflex elicited by contralateral PTN (cPTN) stimulation was quantified following iPTN, cCPN or iPTN + cCPN nerve stimulation (n = 5). Intensities used during the H-reflex conditioning experiment were 1.79 ± 0.4 × MT for the iPTN stimulation and 0.88 ± 0.16 × MT for cCPN stimulation. Across all participants, the onset of the cSOL EMG suppression was 42 ± 4, 44 ± 3 and 44 ± 3 ms for iPTN, cCPN and iPTN + cCPN conditions, respectively. The inhibition from the combined iPTN and cCPN stimulation was significantly greater compared to the algebraic sum of their separate effects. When conditioning the cSOL H-reflex, the ISI between the test cPTN and the iPTN or cCPN stimulus was 5.4 ± 0.5 and 2.6 ± 0.5, respectively. The combined stimulation induced a significantly greater inhibition compared to their separate effects. These data provide evidence of convergence on common inhibitory interneurons by muscle afferents activated by iPTN and cCPN stimulation during sitting. Since the inhibition elicited by cCPN stimulation is known to be mediated by the disynaptic Ia inhibitory pathway, this suggests that the crossed inhibition of cSOL motoneurones elicited by muscle afferents from the ipsilateral plantarflexor muscles is at least partly mediated by Ia inhibitory interneurons in the contralateral human spinal cord. This is similar to what has been observed in the cat.