Modeling LLPG Activity in Incomplete SCI Subjects

In Austria, approx. 150 persons per year suffer a spinal cord injury (SCI); 55 - 85 % of them remain wheelchair- bound. Now physicians, biomedical engineers and computer scientists join their forces to find out what electrical stimulation of the spinal cord could do for these patients to help them recover the ability to walk. The clinical outcome of traumatic SCI mainly depends on the severity of the lesion, and on the program of neurorehabilitation which may include daily exercise on a running treadmill. This is a very promising method of treatment - but only for paraplegics with an incomplete transection of the spinal cord. Recently, it has been shown that stepping movements of the legs may be induced by the electrical stimulation of some kind of "locomotor lumbar pattern generator" (LLPG) within the lower spinal cord by means of an electrode. SCI patients who are able to cover short distances on a treadmill may improve their endurance essentially with the help of "spinal cord stimulation" (SCS) - even though the nervous connections of the lower limbs to the brain have been cut off almost completely by the trauma. The question arises in how far stimulation of the LLPG can replace missing brain control. Neuroscientists, medical specialists and technicians will cooperate to explore the processes within the human spinal cord that are triggered by SCS. In order to learn how the spinal cord processes the information converging from different brain areas and the legs, incomplete SCI subjects (who give their consent) will be recruited to a comprehensive examination: The electrical activity of the leg muscles is recorded by means of surface electrodes (attached to the skin) while the subjects are requested to perform certain movements - both with and without SCS. Subsequently, hypotheses about the emergence of different responses in the LLPG of incomplete paraplegics shall be deducted, and examined by computer simulation: The electrical activity of single nerve cells and simple networks will be simulated with the help of specific software. This way our knowledge about the influence of spinal cord stimulation on the control of locomotion shall be improved. Depending on the extent of brain control preserved after the injury different results may be expected.

Scientists from Vienna and Houston cooperated in order to analyze the neuronal coding principles for human standing and walking. They found in which way a rather simple neuro-prosthetic implant enables people with spinal cord injury to stretch the leg and to elicit walking like movements of the lower limbs. They proved the existence of a neural pattern generator within the spinal cord that can be driven by a single stimulating electrode. Continuation of such research may lead to a therapeutic walking program for paralyzed people. A serious lesion of the spinal cord disconnects important neural information lines and this way the brain cannot activate neural regions below the level of injury and for many people conscious motion control of the lower limbs is therefore not possible. In such cases a rather simple spinal cord implant is currently in clinical use for spasm suppression. The electrodes with to active contacts are located in the vertebral canal but even outside of the cerebral fluid region in order not to damage the highly sensitive elements of spinal cord. Analyses of biophysical modeling and recorded muscle activities have shown where the implant generates neuronal signals artificially. This happens in the sensory fibers from the lower limbs, where a train of electrical impulses generates a phase locked train of nerve impulses that propagate into the spinal cord. Mono- or polysynaptic pathways activate pattern generators of the spinal cord, and this way, the artificially evoked signal trains in the sensory fibers partially substitute for the signaling from the brain of a healthy subject. The research demonstrates the existence of a much simpler coding mechanism as generally assumed because the changing of a single parameter, namely a change of the stimulation frequency, cause either leg stretching or rhythmical walking-like lower limb movements. An improvement for rhythmic muscle activation as necessary for walking was observed when assisted stepping on a treadmill is combined with electrical stimulation of the spinal cord.