# Neuronal mechanisms underlying activation of locomotor network by epidural electrical stimulation of the spinal cord

## TL;DR

This study explores how electrical stimulation of the spinal cord activates neurons involved in movement, shedding light on the mechanisms behind improved locomotion in patients.

## Contribution

The study identifies specific neuronal activity changes during electrical stimulation that are crucial for initiating locomotion.

## Key findings

- ES increases neuronal activity before locomotion onset but is insufficient alone to initiate movement.
- Neuronal activity patterns shift during locomotion, with highly active neurons decreasing and less active ones increasing.
- Changes in neuronal responses to stimuli help maintain locomotor network excitability without disrupting rhythm.

## Abstract

Epidural electrical stimulation of the spinal cord (ES) is used to restore/improve locomotor movements in patients. However, the neuronal mechanisms underlying activation of locomotor networks by ES are unknown. Here, we analyzed the effects of ES on the activity of individual spinal neurons of different functional locomotor groups. Neuronal activity was recorded in decerebrate cats before and during ES that evoked locomotion. During ES initial period (NW-period, before the locomotion onset), the activity was increased in most neurons as compared to that before ES. We found that ES-caused activation of neurons of locomotor network to the average level similar to that observed during locomotion is not sufficient for initiation of locomotion. We demonstrated that the onset of ES-evoked locomotion was associated with specific changes in the activity of individual neurons within functional locomotor groups and in their responses to epidural stimuli. With the locomotion onset, there was a trend for individual neurons with extremely high activity during NW-period to decrease their activity, while for neurons with extremely low activity to become more active. Also, in neurons contributing to generation of a specific mode of locomotion, peaks and troughs in responses to individual epidural stimuli were significantly less pronounced as compared with those observed during NW-period. We suggest that these changes allow ES to maintain the high level of excitability of the locomotor network necessary for its operation without distortion of the locomotor rhythm. The obtained results advance our understanding of the neuronal mechanisms underlying activation of locomotor network by ES.