Hindlimb motor responses evoked by microstimulation of the lumbar dorsal root ganglia during quiet standing

Somatosensory afferent pathways have been a target for neural prostheses that seek to restore sensory feedback from amputated limbs and to recruit muscles paralyzed by neurological injury. These pathways supply inputs into spinal reflex circuits that are necessary for coordinating muscle activity in the lower limb. The dorsal root ganglia (DRG) is a potential site for accessing sensory neurons and DRG microstimulation selectively recruits major nerve branches of the cat hindlimb. Previous studies of DRG microstimulation have been performed in anesthetized animals, but the effects on muscle recruitment and behavior in awake animals have not been examined. In this study, 32-channel penetrating microelectrode arrays were implanted chronically in the left L6 and L7 DRG of four cats. During each week of testing, one DRG electrode was selected to deliver microstimulation pulse-trains during quiet standing. Electromyographic (EMG) signals were recorded from intramuscular electrodes in 10 hindlimb muscles, and ground-reaction forces (GRF) were measured under the foot of the implanted limb. Microstimulation in the DRG evoked a mix of excitatory and inhibitory responses across muscles. Response rates were highest when microstimulation was applied on the L7 array, producing more excitatory than inhibitory responses; hamstrings excitation occurred most frequently. Response rates for the L6 array were lower, and the composition of responses was more evenly balanced between excitation and inhibition. On approximately one third of testing weeks, microstimulation induced a transient unloading of the hindlimb as indicated by a decrease in GRF during stimulation. Reciprocal inhibition at the knee was a prevalent response pattern across testing days and also contributed to the unloading force on this subset of testing weeks. Results show that single-channel microstimulation in the lumbar DRG evokes stereotyped patterns of muscle recruitment in awake animals, demonstrating that even limited sensory input can elicit hindlimb behavior. These findings imply that DRG microstimulation may have utility in neural prosthetic applications aimed at restoring somatosensory feedback or promoting motor function after neurological injury.