FIG. 6.

Mechanisms underlying the specificity of L2-S1 inter-stimulation intervals in enabling stepping. (A) Schematic of the progression of a spatiotemporally-independent monopolar stimulation to electrically-enabled motor control (SIM-eEmc) trial. Inset demonstrates that the L2 pulse (blue dot and arrows) appears within one of the four time windows (W1-W4) relative to the S1 pulse (red dot and arrows). (B) Bilateral raw medial gastrocnemius (MG) and tibialis anterior (TA) electromyographic (EMG) bursts in a representative animal along with averaged evoked responses from individual bursts (corresponding to black and tan rectangles) secondary to L2 only, S1 only, and four SIM-eEmc trials each with four distinct time windows (W1 to W4) and related step kinematics. W1 shows a partial inhibition of the interaction evoked response (gray arrows) and W2 is accompanied by the presence of a polysynaptic response (green arrows). L2 stimulation from 10.1–18.0 msec (W3) is either partially inhibited or does not show any obvious alteration in the response. L2 preceding the S1 pulse (W4) results in a shift of the latency of the response in one muscle (purple dotted line for the MG) relative to both the L2 and S1 monopolar evoked responses and is frequently accompanied by a polysynaptic response. (C) Mean (± standard deviation [SD]) latency periods of the monosynaptic and polysynaptic evoked responses for conditions similar to that described in (B). (D) Average (± SD) percentage differences in the amplitude of interaction-evoked responses in each window in comparison to the L2 or S1 only stimulation trials in all animals for MG and TA muscles. *Significant latency shift differences of first response in W4 from both S1 only and L2 only. †Significantly different from W1 and W3. ‡Significantly different from W3. *Significantly different from L2 monopolar stimulation. *Significantly different from S1 monopolar stimulation. Color image is available online at www.liebertpub.com/neu