A Lack of modulation of motor evoked potential in sensory-impaired individuals with spinal cord injuries

Corticospinal excitability can be facilitated by peripheral electrical stimulation (ES) in healthy individuals. This facilitation could possibly be via the muscle afferent pathway, however, no direct evidence has been documented. This study was to examine the corticospinal excitability following ES in spinal cord-injured (SCI) individuals with sensory impairments to justify the mechanism. Eight individuals with sensory impairments following SCI and nine healthy subjects were recruited. Motor evoked potentials (MEPs), silent period (SP) elicited by transcranial magnetic stimulation (TMS), and H-reflex elicited by median nerve stimulation were recorded on flexor carpi radialis (FCR) muscle before, during, immediately and 30 minutes after low-intensity 20-Hz median nerve electrical stimulation (ES). The result showed that the MEP increased to 154 ± 29% (p < 0.01) of initial in healthy controls but not in SCI individuals following ES. The H-reflex did not change in both groups. The SP increased (from 41.8 ms to 53.8 ms, p < 0.05) in healthy controls following ES. The SP was not present in individuals with SCI. These data confirm that the lasting facilitation of corticospinal excitability elicited by peripheral nerve stimulation is due to supra-segmental modulation and the muscle afferent is an essential pathway which contributes to the facilitation.