Résumé
Electrical Stimulation (ES) induces action potentials by depolarization of the membrane of the targeted cell i.e. axons or muscle fibers. From the fifties, ES has been successfully used in a growing set of applications linked to motor and sensory impairments. The most popular devices are the pacemakers that originally allow triggering the cardiac muscle [Elmquist et al. 59]. In the sensory area, cochlear implants allow to recover sound perception for deep deaf persons. The principle is basically the same with a set of electrodes, around twenty, located in the cochlea in order to activate the remaining auditory neural circuits [Djourno et al. 57]. More recently deep brain stimulation, with a simple stimulation scheme applied to deep brain areas allows suppressing tremor in Parkinson disease [Benabid et al. 91]. Attempts to use ES have been made in movement rehabilitation, such as drop foot syndrome for hemiplegic patients [Liberson et al. 61] and more complex movements for patients with spinal cord injuries; The functional results could prove to be substantial, including, for instance, recovery of the grasp function for quadriplegic patients, who might then be able to grab hold of objects, eat, and even, in the best cases, write with a pen [Smith et al. 98]. The results for standing and walking restoration remain less functional in paraplegic patients. Although not optimal, functional ES systems remain the only way for movement restoration in a daily use context. The main drawbacks of the technique are well known and include insufficient reliability, the complexity of the surgery, limited stimulation selectivity and efficiency, the non-physiological recruitment of motor units and muscle control. However, recently, researches for movement restoration of the lower limb regained interest through new approaches [Harkema et al. 11]. In this study, the patient had a complete motor and incomplete sensitive spinal cord lesion at C7-T1 level. The patient experienced much exercising for months and empirical optimal stimulation patterns were found in order to provide lower limb muscle contractions through modulation of the afferent pathways at the lumbo sacral level. The patient could even produce rhythmic but non-functional gait-like movements. Even though the involved mechanisms are still not well described and the functional benefits quite low, the results are clearly important because they show that the spinal cord circuits can help to provide for functional movements. However no solutions are widely used. Why movements seem to be more complex to restore than hearing, cardiac pacing even tremor suppression? The following questions may explain partly the reason. Available implanted stimulators remained too limited to explore widely all the possibilities that these techniques could provide. Besides, functional movement modeling and advanced movement control in particular in a closed loop way, are still unused although it is known that the human nervous system is controlling movement through complex multilevel closed loops. Even though the last study tried to use the spinal cord network including its natural closed loops between afferent and efferent signals, they are neither controlled nor explicitly used by the stimulators so that the activation, from a global system point of view, remained open loop. On one hand, a more physiological stimulation that provides functional benefits implies that the activation of motor units is accurately controlled in order to focus the activation on the desired target with as less as possible side effects (fatigue, unwanted diffusion to other muscles, activation of reflexes...). On the other hand, an efficient functional movement would need for closed loop control (balance control, fatigue compensation...) or optimized synthesized patterns (sit-to stand movement, grasping...). The off-line synthesis may limit the duration and the number of clinical sessions needed to tune the neuroprosthesis, otherwise tuned empirically. To take up these challenges, the paper dealt with both a theoretical approach of movement restoration based on control theory and an innovative technological platform that foreshadows new designs of neuroprotheses. Finally, open questions were exposed among which the role of the afferent pathways both as an input signal for closed loop and as an input for the stimulation to indirectly modulate efferent responses or activate neural networks. Wide research areas are still opened and of high interest: neural engineering together with theoretical sciences may propose palliative solutions to deficiencies and tools to investigate the basics of the human sensory motor system functioning.