The overall goal of this work is to investigate the changes in the spinal cord resulting from limb amputation. Limb amputation results in an extreme form of peripheral nerve injury. Damage to peripheral nerves, such as with neuropathy, crush injuries, nerve transection, or limb amputation often results in chronic pain, which may be associated with altered excitability of spinal sensorimotor pathways. These spinal pathways become hyperexcitable due to a lack of sensory input, which causes tonic disinhibition of descending circuits and spontaneous activity in the dorsal root ganglia (DRG). Spinal excitability can be measured using the H-reflex, in which electrical stimulation of muscle spindle Ia afferents activates spinal motoneurons via the myotatic reflex, as well as the posterior root-muscle (PRM) reflex, which is elicited by transcutaneous stimulation over the dorsal roots and is considered to be half of the H-reflex, excluding the peripheral primary afferents, but with multiple root activation. Spinal excitability has not been measured in amputees but may offer a potential biomarker for PLP. Neuromodulation may restore normal spinal excitability and reduce PLP, thus offering the potential to improve the quality of life in individuals with a lower limb amputation. The results of this study will provide the foundation for future development of a neuroprosthesis to restore spinal excitability and reduce PLP in individuals with a lower limb amputation. Subjects will undergo 5 testing and stimulation sessions in 1 week. An additional 3 days of recording sessions may be necessary if a phantom limb pain episode does not occur during normal testing days.

Specific Aim 1: Quantify spinal excitability. A lack of sensory input results in spinal hyperexcitability through several pathways including tonic disinhibition of descending circuits and spontaneous activity in the DRG. Spinal cord excitability is directly related to reflex modulation; impaired or enhanced reflex modulation indicates abnormal spinal cord excitability. Spinal cord excitability will be determined in people with a lower limb amputation using the H-reflex and posterior root-muscle (PRM) reflex. The H-reflex is elicited with electrical stimulation of peripheral nerves, exciting muscle spindle Ia afferents projecting to spinal motoneurons via the myotatic reflex. Stimulation of the peripheral nerves also elicits a direct motor (M) wave. The PRM reflex is elicited by electrical stimulation of the posterior roots on the back. It is considered to be half of the H-reflex, excluding the peripheral motor efferents, but activates multiple dorsal roots. Reflex amplitude and latency, threshold, recruitment curves, and rate-dependent depression will be measured and compared to intact controls. The investigators hypothesize that H and PRM reflex hyperexcitability will be present in the residual limb of amputees with PLP. These results will provide insight into the role of limb amputation on spinal cord health and excitability.

Specific Aim 2: Characterize the effects of transcutaneous spinal cord stimulation on spinal cord excitability and phantom limb pain.

Neuromodulation of sensorimotor pathways using transcutaneous electrical nerve stimulation (TENS), dorsal root ganglia stimulation (DRGS), and epidural spinal cord stimulation (eSCS) to reduce phantom limb pain have been explored with mixed results. The most promising methods for pain reduction were DRGS or laterally-placed eSCS, indicating that the DRG and dorsal roots are optimal targets for reducing PLP. However, these methods require surgical implantation of electrodes. Transcutaneous spinal cord stimulation (tSCS) is a non-invasive method for stimulating the dorsal roots in a similar way as eSCS. Through activation of the primary afferents, tSCS may inhibit pain pathways and reduce the hyperexcitability that leads to chronic pain. tSCS in people with spinal cord injury has been shown to restore spinal inhibition and reduce H-reflex hyperexcitability. The investigators hypothesize that tSCS can reduce PLP through modulation of sensorimotor pathways. By comparing the H- and PRM reflex excitability recorded from the residual limb before and after each session of tSCS, a potential mechanism of PLP could be elucidated. H- and PRM reflex modulation, and any differences in the extent of modulation for each, can further inform on the mechanisms of tSCS and how it modulates sensorimotor pathways. The investigators will also quantify the subjects' experience of PLP before and after the 5 days of tSCS and correlate their pain experiences with spinal excitability measures. The investigators will use a visual analog scale and the McGill Pain Questionnaire to assess changes in pain perception. The investigators will also use an algometer to determine changes in local pain threshold.