Biphasic inhibitory responses and their IPSPs evoked by tibial nerve stimulation in human soleus motor neurones

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Abstract
Summary A novel stimulating protocol was used to demonstrate the existence of a biphasic inhibitory reflex in single soleus motor neurones in response to weak stimulation of the tibial nerve in human subjects. The stimulus intensity was sufficient to evoke a small M-response but was subthreshold for the H-reflex. Stronger stimulation caused the 2 phases of inhibition to fuse. The experimental protocol allowed excitatory and inhibitory responses to be recognized unambiguously. The criterion for excitation was stimulus-induced advance in the timing of the next action potential in certain trials: for inhibition, the criterion was stimulus-induced delay in the timing of the next action potential. The shorter-latency inhibitory response began at about 45–50 ms and lasted for about 20 ms. With the stimulus intensity used, only inhibitory responses were evoked at short latency in tibialis motor neurones, which suggests that the soleus inhibition resulted from recurrent inhibition via the Renshaw pathway rather than by activation of Ib afferents in the tibial nerve. A novel approach to the determination of the waveform of the IPSP underlying the shorter-latency (presumably Renshaw-mediated) inhibition revealed only part of the waveform owing to the sharp fall in the membrane potential at the onset of (presumably-Renshaw) IPSP. This approach was more successful when applied to the longer-latency, slower IPSP. The longer-latency inhibitory response in soleus motor neurones began at about 80–120 ms. Overall, the latency of this inhibition was less when it followed an M-response than when it followed an H-reflex. It was inferred from this and other circumstantial evidence that the longer-latency response was secondary to twitch-induced activation of soleus tendon organs (and/or disfacilitation of homonymous spindles). In contradiction to the conclusion reached in a recent study (Kudina and Pantseva 1988), no evidence was found for a reflex excitatory response after the first phase of inhibition.

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