Model for the study of plasticity of the human nervous system: features of residual spinal cord motor activity resulting from established post-traumatic injury.

Established post-traumatic spinal cord injuries can serve as an 'experimental model' in which trauma has partially separated the 'spinal neuronal pool' from supraspinal influence. Our findings show that: (1) when the muscle is deprived of upper motor neuron activity, fatigue resistance is diminished and external, electrically induced daily contractions will restore the level of fatigue resistance close to that of muscles in healthy, active subjects; (2) the spinal interneuron network, when completely deprived of brain influence, is a 'spinal reflex centre' with a relatively restricted and low excitability level; and (3) the 'discomplete spinal cord injury' model illustrates that spasticity is of supra-segmental origin and that there are two basic features of brain motor control of the spinal interneuron system: the command to restrict interneuronal pool activity and the command to activate the interneuronal network. Moreover, I have described the modifiability of fatigue resistance, locomotor patterns and different alternatives in the neurocontrol of motor activity, depending on the kind and degree of residual brain influence. Such significant modifiability can be thought of as plasticity of the neuromuscular system and impaired control of the nervous system.