Fig. 4 |. Residual supraspinal inputs can modulate motoneuron firing rate during SCS.

a, Motoneuron pool firing rate (5 seconds simulations) as a function of residual supraspinal inputs firing rate in the voluntary movement regime (SCS parameters: 45 Hz and 30% amplitude) and involuntary regime (SCS parameters: 45 Hz and 80% amplitude). b, Membrane potential of the same motoneuron with residual supraspinal inputs at 20 and 45 Hz. This motoneuron is recruited at 45 Hz but not at 20 Hz. c Membrane potential of another motoneuron that skips 3 SCS pulses at 20 Hz and 4 or 5 at 45 Hz. SCS parameters: 69 Hz and 30% amplitude. d, Percentage of motoneurons recruited (with a firing rate >8 Hz) as a function of the residual supraspinal fibers firing rate e,f, Two examples of ISI normalized probability distributions for residual supraspinal firing rates at 20 and 45 Hz during SCS (69 Hz, 30%). g, ISI normalized cumulative distributions show that residual supraspinal inputs can modulate motoneuron firing rate by controlling the number of skipped SCS pulses. h-i, Fine movement task in the voluntary (69 Hz, 30% amplitude) and involuntary (69 Hz, 80% amplitude) movement regimes. Firing rates are computed in 100 ms bin windows. From bottom to top: supraspinal inputs that follow a sinusoidal wave, motoneuron firing rate and normalized force. Blue area indicates the difference in firing rate from the prelesion case (i.e., error). j, Root-mean square (RMS) of the difference between the motoneuron firing rate pre- vs postlesion (dark blue regions in h and i) for all simulated SCS parameters. The gray lines in the heatmap indicate the voluntary movement regime defined in the isometric task (Fig. 3b).