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. 2019 May 22;597(12):2975–2991. doi: 10.1113/JP277849

Figure 1. Conditioning of the FCR H‐reflex with TMS.

Figure 1

A, schematic representation of the experimental set‐up. B, principle of conditioning an H‐reflex evoked by PNS with TMS. TMS and PNS were applied together, so that TMS‐triggered volleys and the afferent volleys from PNS coincided at the spinal motoneurons. This leads to an increased recruitment of spinal motoneurons (middle part) and a corresponding increase in the size of the electromyographic response in the flexor carpi radialis (FCR) H‐reflex (lower part). We tested three different delays between TMS and PNS. At EFD −0.5 ms, the fastest conducting volley from TMS will not yet have arrived at spinal motoneurons at the time when the fastest conducting afferent volley from PNS arrives (blue and orange arrow). At EFD 0 ms, the fastest conducting corticospinal volley arrive at the same time at the spinal motoneurons as the fastest conducting afferent volley from PNS. At EFD +0.6 ms, subsequent volleys (orange arrow) arrive at the same time as the fastest afferent volley. C, mean traces of the unconditioned H‐reflex and of the conditioned H‐reflex at intervals EFD −0.5 ms, EFD 0 ms and EFD +0.6 ms. The yellow rectangle illustrates the 0.5 ms time window from H‐reflex onset used to quantify the H‐reflex facilitation. D, expanded view showing this time window in more detail. E, EFD 0 ms was determined by a two‐step procedure in each individual: we first tested delays between the application of TMS and the application of PNS from −5 ms to −2 ms, in steps of 0.5 ms (negative delays indicate that TMS was triggered after PNS). EFD 0 ms in this example was at a delay of −3.5 ms; conditioned H‐reflexes at this delay and at the subsequent delays were higher than unconditioned test H‐reflexes. The grey rectangle illustrates the time window tested in the second step of the procedure, shown in F. F, delays from −4.5 ms to −3.5 ms were again tested to denote EFD 0 ms in 0.1 ms steps (* P < 0.05). G, the histogram illustrates the distribution of delays across all subjects between TMS and PNS indicating EFD 0 ms. [Color figure can be viewed at wileyonlinelibrary.com]