Fig. 2: Computational analysis of the direct targets of EES of the monkey cervical spinal cord.

a Electric currents and potential distribution (ϕ) generated by a lateral electrode contact at the C6 spinal level for a stimulation current of 1 μA estimated with the finite element method. Left: transversal cross-section cutting the electrode contact in two halves. Red arrows: current density vectors. Dark gray surface: ϕ ≥ 2 mV. Mild gray: ϕ ≥ 1 mV. Light gray: ϕ ≥ 0.8 mV. Right: ϕ along trajectories of virtual nerve fibers and motoneurons. DC: dorsal columns. ST: spinocerebellar tract. DR: dorsal root. CST: corticospinal tract. MN: motoneuron. b Direct recruitment of nerve fibers and motoneurons estimated from neurophysical simulations using the potential distribution of (a). Left: recruitment curves (dark blue and red curves are almost superimposed). Stimulus amplitudes are expressed as multiples of the threshold amplitude (10% recruitment) for DR-Aα-fibers. Right: threshold amplitudes and saturation amplitudes (90% recruitment) for the different neural entities, expressed as multiples of the threshold for DR-Aα-fibers. Inset: threshold for DR-Aα-fibers and DR-Aβ-fibers. c Simulated recruitment of Ia-fibers of individual dorsal roots using the potential distribution of (a). Amplitudes are expressed as multiples of the threshold for the Ia-fibers of the C6 root. d Recruitment for the C5, C6, and C7 roots split by recruitment localization. Amplitudes are expressed as multiples of the threshold for the Ia-fibers of the C6 root. e Same as (a) for a medially-positioned electrode contact. f Same as (b) with the potential distribution of (e). Inset: threshold and saturation amplitudes for the DC-fibers. Amplitudes are expressed as in (b). g Same as (c) with the potential distribution of (e). h Maximal selectivity indexes for each root using lateral or medial electrodes (see “Methods”). Recruitment curves (panels b [left], c, d, f [left], g): curves are made of 80 data points (except for d, 60 data points) consisting in the mean and standard deviation of the recruitment computed across 10,000 bootstrapped populations (see “Methods”). Lines and filled areas represent the moving average over three consecutive data points. Threshold, saturation, and selectivity bars and whiskers (panels b [right], f [right], h): mean ± standard deviation of the represented quantity computed across 10,000 bootstrapped populations (see “Methods”).