Central modulation of compound motor action potential responses to epidural spinal cord stimulation

Stimulation at frequencies from 1 to 50 Hz was applied over the dorsal surface of the lumbar enlargement in spinal cord injury subjects. Low-frequency responses appeared that were similar to compound motor action potentials (CMAPs) evoked by peripheral nerve stimulation. Close examination of the EMG response to higher frequencies (30-50 Hz) revealed that the EMG was comprised of a series of CMAPs of varying shape and amplitude. Under appropriate conditions, a constant train of stimuli evoked cyclic bursting activity. Examination of the characteristics of that activity revealed that intra-burst and burst phases were comprised of CMAPs of very different morphologies with markedly different latencies. These findings suggest that stimulation-induced activity was processed through different pathways, depending on the system state.