Fig. 4: Motor output potentiation is not through current spreading towards CST axons nor through spinal circuits.

a Example of antidromic responses in the spinal cord from VLL stimulation at 10 Hz (n = 30 traces) in MK-OP for three different channels (CH1, 16, and 31) along the multi-channel linear spinal probe. The spinal cord section in (a) was created with BioRender.com released under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International license. b Left: Heatmaps of peak-to-peak amplitude of the antidromic responses in the spinal cord at the C6-C7 spinal level with dorsal-ventral alignments for n = 3 animals. The dashed boxes are highlighting the putative intermediate-ventral zone where we see the greatest responses. Right: Boxplots of the antidromic response latency for each animal (n = 656 MK-OP, n = 600 MK-JC, n = 596 MK-HS). c MEPs of the hand (ECR: Extensor Carpi Radialis, 30 traces for each animal) elicited by VLL stim at 50 Hz. d EMG reflexes and boxplots of the AUC of the EMG reflexes of the ECR muscle elicited by radial nerve stimulation and radial nerve paired with continuous VLL stimulation at 50 Hz (30 example traces each). For all boxplots, the whiskers extend to the maximum spread not considering outliers. Central, top, and bottom lines represent median, 25th, and 75th percentile, respectively. For (d), statistical significance was assessed with one-tail bootstrapping with Bonferroni correction, however, in all cases the results were not significant. Source data for (b and d) are provided as a Source Data file.