Activity of Spinal Interneurons during Forward and Backward Locomotion

Abstract

Higher vertebrates are capable not only of forward but also backward and sideways locomotion. Also, single steps in different directions are generated for postural corrections. While the networks responsible for the control of forward walking (FW) have been studied in considerable detail, the networks controlling steps in other directions are mostly unknown. Here, to characterize the operation of the spinal locomotor network during FW and backward walking (BW), we recorded the activity of individual spinal interneurons from L4 to L6 during both FW and BW evoked by epidural stimulation (ES) of the spinal cord at L5-L6 in decerebrate cats of either sex. Three groups of neurons were revealed. Group 1 (45%) had a similar phase of modulation during both FW and BW. Group 2 (27%) changed the phase of modulation in the locomotor cycle depending on the direction of locomotion. Group 3 neurons were modulated during FW only (Group 3a, 21%) or during BW only (Group 3b, 7%). We suggest that Group 1 neurons belong to the network generating the vertical component of steps (the limb elevation and lowering) because it should operate similarly during locomotion in any direction, while Groups 2 and 3 neurons belong to the networks controlling the direction of stepping. Results of this study provide new insights into the organization of the spinal locomotor circuits, advance our understanding of ES therapeutic effects, and can potentially be used for the development of novel strategies for recuperation of impaired balance control, which requires the generation of corrective steps in different directions.

Simultaneous control of forward and backward locomotion by spinal sensorimotor circuits

“…In decerebrate cats, the spinal networks generating forward locomotion are distributed over several segments from L3 to S2, as determined by epidural electrical stimulation and c‐Fos immunostaining, while those for backward locomotion were found from L5 to L7 (Merkulyeva et al., 2018). Different and shared populations of spinal interneurons are also recruited during forward and backward locomotion in decerebrate cats (Musienko et al., 2022). However, some muscle synergies are conserved for forward and backward locomotion in cats, suggesting a similar modular organization at the pattern formation level (Harnie et al.…”