Progressive plastic changes in the hand representation of the primary motor cortex parallel incomplete recovery from a unilateral section of the corticospinal tract at cervical level in monkeys.

After a sub-total hemisection of the cervical cord at level C7/C8 in monkeys, a paralysis of the homolateral hand is rapidly followed by an incomplete recovery of manual dexterity, reaching a plateau after about 40-50 days, whose extent appears related to the size of the lesion. During a few days after the lesion, the hand representation in the contralateral motor cortex disappeared, replaced by representations of either face or more proximal body parts. Later, however, following a time course (about 40 days) consistent with the functional recovery, progressive plastic changes in the contralateral motor cortex took place, as demonstrated by a progressive reappearance of digit movements elicited by intracortical microstimulation. These progressive plastic changes, which parallel the functional recovery, correspond to a reinstallation of a hand representation, though substantially diminished in size as compared to pre-lesion. Regarding the functional recovery, the motor cortex (including the reestablished hand area) contralateral to the unilateral cervical cord lesion played a crucial role in reestablishing control on finger movements, as assessed by reversible inactivation experiments. In contrast, the motor cortex ipsilateral to the cervical cord lesion, with largely intact projections to the spinal cord, did not contribute significantly to the recovered movements by the affected hand. These observations indicate that the CS fibers spared by the lesion are not sufficient, at least in their pre-lesion condition, to control the motoneurones innervating the digit muscles and that the pathways conveying signals from the contralateral M1 underwent reorganization.