Short latency contralateral inhibition identified in single motor units in human

In static and dynamic tasks a short latency (37-41 ms) inhibition of the contralateral Soleus (cSOL) muscle occurs following ipsilateral tibial nerve (iTN) stimulation [3, 4]. To gain further insights into the pathways that induce this response we have recently investigated the convergence with the disynaptic inhibitory pathway evoked by common peroneal stimulation on the contralateral side (cCPN) [1]. For such studies it is necessary to determine the exact timing of the responses in relation to the stimuli. Surface electromyography (sEMG) has limitations in this regard as it relies upon the occurrence of action potentials at a particular time in relation to the stimulus and a strong response can synchronize their timing thus distorting their onset [2]. The aim of the current study was to use the peristimulus frequencygram (PSF), a technique which illustrates the time course of the net input current to a motor neuron more accurately compared with sEMG, to study the time-course of the postsynaptic potential in cSOL motor neurons induced by the two stimuli (iTN and cCPN stimulation). Three subjects (age: 26-40 years) were seated in a chair with their cSOL contracted to 5-10% maximum. Bipolar Teflon® insulated silver wires (75 µm in core diameter) were inserted into the cSOL muscle. sEMG was recorded with bipolar electrodes from the same muscle. Three conditions were investigated and the stimuli were applied randomly: 1) iTN at 2×motor threshold, 2) cCPN at motor threshold, and 3) iTN+cCPN stimulation. Signals were recorded at 10 kHz for offline PSF and trigger averaged sEMG analysing. The onset of inhibition measured from trigger averaged sEMG was 44 ± 1.7 ms in the iTN stimulation, 48 ± 2.0 ms for cCPN stimulation, and 41 ± 1.7 ms for the iTN+cCPN condition. Six motor units were decomposed from the intramuscular EMG signals. The PSF showed that the iTN stimulation induced short latency contralateral inhibition on all cSOL motor units with an average latency of 42 ± 7 ms. The amount of inhibition (27.7 ± 10.5%) in the iTN+cCPN condition was above that found for the summation of the inhibition response in iTN (12.9 ± 7.6%) and cCPN (13.8± 2.8%) conditions for all motor units. This is the first study to show a significant change in the firing rate of single motor units of the cSOL following iTN stimulation. It further supports our previous finding that it is at least in part mediated via Ia inhibitory interneurons in the disynaptic reciprocal inhibitory pathway.