Sustained non-patterned electrical stimulation of the lumbar spinal cord can induce automatic stepping-like lower limb movements in complete spinal cord injured individuals. The underlying neuronal mechanisms were studied in the present thesis.
Stimulation (1-10 V and 2.2-50 Hz) was applied to the posterior lumbar spinal cord by epidural electrodes in seventeen spinal cord injured subjects. The induced lower limb muscle activity was assessed by surface electromyography. These data on the input-output behavior of the spinal cord were analyzed. Computer modeling of the applied electric field together with the activating function concept was carried out to identify the directly stimulated neural structures.
Epidural 2.2 Hz-stimulation induced muscle twitches in the lower limbs.
The neurophysiological features of these responses and computer simulations identified posterior roots as the directly stimulated neural targets. Stimulation at 25-50 Hz induced alternating burst-style electromyographic activity in the paralyzed lower limbs leading to stepping-like movements. Stimulus-evoked compound muscle action potentials were subject to well-defined amplitude modulations and demonstrated a 10 ms-increase of latency during burst-style phases.
Muscle twitches induced by 2.2 Hz-stimulation are monosynaptic responses to stimulation of afferent fibers within the posterior roots. Same afferent input is transmitted to spinal interneurons. Continuous posterior root stimulation at 25-50 Hz activated spinal neuronal circuits that dynamically modulated the afferent flow through monosynaptic pathways by inhibitory mechanism and controlled motoneuronal discharge by excitatory longer pathways. These capabilities of the lumbar cord isolated from brain control are an evidence for a human spinal pattern generator for locomotion.