Sherrington1 demonstrated that the neuronal circuitry of the spinal cord is capable of autonomously generating coordinated muscle activity. Since then, an abundance of evidence has shown that the spinal cord can mediate a number of motor behaviors including standing,1 locomotion,1-4 scratching,5 micturition,6-8 defecation,9 and reach-ing-like limb movements.10 Additionally, recent evidence indicates that locomotor control circuits are present in the human spinal cord.11-15 These results suggest that electrical activation of spinal neural networks and artificial control of the behaviors they subserve may simplify the generation of complex motor acts using neural prosthetic devices.
Neural motor prostheses use electrical activation or recording of the nervous system to restore lost functions to individuals with neurological impairment.16-19 All motor system neural prostheses to date electrically excite the "lower" or segmental motor neuron. They thus access the nervous system at the lowest possible level and act by stimulating the motor axons directly. An unfortunate effect of stimulating motor axons directly is that the normal size-ordered recruitment of motor units is reversed, and large fast-fatiguing twitch units are recruited first, followed by smaller axons.20,21 However, if recruitment order problems can be overcome, interfacing with the nervous system at the motor neuron level provides the most flexible access to the motor apparatus. It allows arbitrary activation of muscles in arbitrary combinations. However, the cost of this enormous flexibility is that each and every detail of the control must be solved by the FES designer and then implemented by the FES controller. These include the solution of numerous ill-posed problems using the motor apparatus. These problems represent fundamental problems for control of complex behaviors involving the interaction of multiple muscles. However, these same sets of control problems have been solved by evolution and tuned during ontogeny by the individual organism. Viable solutions to the control and selection problems posed by the limbs and trunk are at least in part embedded in the spinal motor apparatus itself. To restore complex motor functions it may be advantageous to access the nervous system at a higher level and use the intact neural circuitry to control the individual elements of the motor system.22 However, it is also important to acknowledge that the spinal embedded motor functions are most often coarse representations of behaviors such as locomotion or grooming and reaching. Thus while it is conceivable that reaching behaviors might be restored to the upper limb by using FES to access intraspinal "reaching circuits," it is also not unlikely that those finely adjusted details of reach and grasp behaviors which we associate with skilled execution require fractionated control of muscles in the cervical enlargement. Ideally each of these capacities should be restored, together with broader controls of the spinal motor apparatus, such as neuromodulatory state, that normally occur in parallel with voluntary actions in intact individuals.
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