Effect of functional electrical stimulation on the central state of excitability of the spinal cord

The recent years brought a growing awareness for the existence of complex neuronal structures within the spinal cord that act as movement controllers. The presented pilot study addresses an extension of pure non-invasive spinal cord stimulation by adding an afferent input from electrically stimulated peripheral nerve. The goal was to assess electrophysiologically the influence of peripheral nerve stimulation (peroneal nerve) on the function of the lumbar spinal cord neural circuitries. In a subject in supine position 3 series of conditioning stimulus trains (10 s, 30 Hz) at sub-threshold, sensory threshold and motor threshold intensity were applied to the peroneal nerve, immediately followed by a stimulus via a paravertebral electrode pair, positioned between the T11 and T12 spinal processes. The stimulus intensity at the lumbar spinal cord was tuned to just eliciting measurable posterior root-muscle (PRM) reflexes in four major muscle groups of both lower extremities. The PRM reflex of the ipsilateral quadriceps was suppressed and with increasing peroneal nerve stimulation strength this effect spread to other muscles. Reduced amplitudes were observed in EMG-recordings from ipsilateral proximal and distal flexor and extensor muscle groups. Further studies on this effect are ongoing. Present preliminary results suggest that measurement of modification of excitability with conditioning peripheral nerve stimulation or even brain generated residual input to the spinal cord can contribute to the therapeutical programs based on FES. Keywords— Spinal cord, afferent input, pattern generator, electrical stimulation, FES.