Extended Data Fig. 2: Importance of modeling rootlet bundles.

Step 1, Models of the same spinal cord wherein the dorsal roots are modelled as single tubular structures (left) versus multiple tubular structures mimicking the topology of rootlet bundles observed in humans (right), as shown in Step 2, side by side comparison of the rootlet bundles in the model and in a real spinal cord. To create the model of the rootlets, we determined the entry point of the uppermost rootlet for each spinal segment, and then populated the space from the uppermost rootlet of a given dorsal root to the uppermost rootlet of the next dorsal root by distributing rootlets homogeneously across this space. Step 3, A pulse of EES was delivered with increasing intensities through the electrode depicted in step 1, over the L3 dorsal root. The plots show the resulting recruitment curve of each dorsal root. The explicit models of rootlets led to pronounced differences in the recruitment curves of each dorsal root. Step 4, Performance of the new paddle lead evaluated in 15 computational models of the atlas. The top left electrode of the paddle lead was positioned over the dorsal root innervating the L1 spinal segment, as depicted in the model on the left. The plot on the left reports the selectivity of this electrode for each model, organized laterally based on the length of the spinal cord (as reported in Fig. 1). The plot on the right reports the selectively of the bottom left electrode to recruit the dorsal root projecting to the S1 spinal segment. Lower Panel, Horizontal bar plots on the left report the variability of the width of the dorsal root entry zone (n = 15 healthy volunteers). Horizontal bar plots on the right report the variability of length of each spinal segment (n = 27 spinal cords). The bar plot between these two plots reports the variability of the width of the dorsal root entry zones and of the length of spinal segments. p = 0.000035, ***, P < 0.0001, two-tailed t-test.