Figure 8. Distribution of spinal networks generating FW and BW stepping. A, A hypothesis about the control of step direction (modified from Musienko et al., 2012). The locomotor system includes two principal mechanisms, one generating a vertical component of the step (limb elevation and lowering) and the other generating a horizontal component (limb transfer from one extreme point to the other). The latter includes networks generating the horizontal component of step in different directions (for simplicity, only the networks generating steps in four directions are shown: F, forward; B, backward; R, rightward; and L, leftward). These networks receive sensory input signaling limb motion in stance; reaching an extreme position triggers a limb transfer. ES of the spinal cord activates a network generating a vertical component of the step. It also causes subthreshold activation of all networks generating a horizontal component. Due to the treadmill motion (e.g., forward), the limb will reach an extreme anterior position, and sensory input will activate network B (marked in blue), which will evoke the BW step. Thus, ES stimulation can evoke stepping opposite to the direction of treadmill motion. B, Areas of the gray matter in L6 and L7 where the neurons of the network generating the horizontal component of BW steps are located (marked in blue). Abbreviations are as in Figure 5C. C, A scheme for the rostrocaudal distribution of a network generating the vertical component of the step (green thick line), a network generating the horizontal component for FW steps (red thick line) and a network generating the horizontal component for BW steps (blue thick line) in the lumbosacral enlargement. Insets 1–4, Hindlimb configurations in the middle of swing (pink and light blue) and at extreme limb positions during one step cycle (red and blue). Thick and thin black arrows show the direction of the treadmill motion and the directions of the limb swing movement, respectively. ES of rostral (L4–L5) and caudal (S1–S2) segments containing only two of three networks (generating the vertical component of the step and the horizontal component for the FW step) evokes FW locomotion (insets 1 and 4). ES of L6–L7 segments containing all three networks (generating the vertical component of the step, the horizontal component for the FW step and the horizontal component for the BW step), depending on the direction of the treadmill belt motion, evokes FW (inset 2) or BW (inset 3) stepping, respectively. Note that FW-stepping movements evoked by ES of rostral segments are performed at a more rostral position in relation to the trunk and with more flexed limb compared with those evoked by ES of caudal segments (compare insets 1 and 4).