Cervical sprouting of corticospinal fibers after thoracic spinal cord injury accompanies shifts in evoked motor responses

The adult central nervous system (CNS) of higher vertebrates displays a lim ited ability for self repair after traumatic injuries, leading to lasting f unctional deficits [1]. Small injuries can result in transient impairments, but the mechanisms of recovery are poorly understood [2]. At the cortical level, rearrangements of the sensory and motor representation maps often pa rallel recovery [3, 4]. In the sensory system, studies have shown that cort ical and subcortical mechanisms contribute to map rearrangements [5, 6], bu t for the motor system the situation is less clear. Here we show that large -scale structural changes in the spared rostral part of the spinal cord occ ur simultaneously with shifts of a hind-limb motor cortex representation af ter traumatic spinal-cord injury. By intracortical microstimulation, we def ined a cortical area that consistently and exclusively yielded hind-limb mu scle responses in normal adult rats. Four weeks after a bilateral transsect ion of the corticospinal tract (CST) in the lower thoracic spinal cord, we again stimulated this cortical field and found forelimb, whisker, and trunk responses, thus demonstrating reorganization of the cortical motor represe ntation. Anterograde tracing of corticospinal fibers originating from this former hind-limb area revealed that sprouting greatly increased the normall y small number of collaterals that lead into the cervical spinal cord rostr al to the lesion. We conclude that the corticospinal motor system has great er potential to adapt structurally to lesions than was previously believed and hypothesize that this spontaneous growth response is the basis for the observed motor representation rearrangements and contributes to functional recovery after incomplete lesions.