Unsuspected Plasticity of Single Neurons after Connection of the Corticospinal Tract with Peripheral Nerves in Spinal Cord Lesions

ABSTRACT

We sought to understand an unsuspected plasticity of single neurons found after connection of the cord with peripheral nerves in paraplegics. Our research aimed at making paraplegics walk again, after 20 years of experimental surgery in animals that, among other things, demonstrated the alteration of the motor end plate receptors from cholinergic to glutamatergic; the same connection was done in humans. The grafts were put in the corticospinal tract of the cord randomly, without possibility of choosing the axons coming from different areas of the brain cortex. As a result, the patient was able to selectively activate the muscles she wanted without cocontractions of the other muscles connected with the same cortical areas. We believe that unlike in nerve or tendon transfers, where the whole cortical area corresponding to the transfer changes its function (a phenomenon that we call “brain plasticity by areas”), in the connection of the lateral bundle of the thoracic cord (the CST) with different peripheral nerves and muscles, the brain plasticity occurs by single neurons; in fact, there are no cocontractions. We propose to call it “brain plasticity by single neurons.” We speculate that this phenomenon is due to the simultaneous activation of neurons spread in different cortical areas for a given specific movement while the other neurons of the same areas connected with peripheral nerves of different muscles are not activated. Why different neurons of the same area fire at different times according to different voluntary demands remains to be discovered, and we are committed to solve this enigma.

KEYWORDS

Brain plasticity - neuron activity - paraplegia - corticospinal tract - central nervous system–peripheral nervous system connection - cocontractions

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