Reorganization of corticospinal projections after prominent recovery of finger dexterity from partial spinal cord injury in macaque monkeys.

We investigated morphological changes in the corticospinal tract (CST) to understand the mechanism underlying recovery of hand function after lesion of the CST at the C4/C5 border in 7 macaque monkeys. All monkeys exhibited prominent recovery of precision grip success ratio within a few months. The trajectories and terminals of CST from the contralesional (n = 4) and ipsilesional (n = 3) hand area of primary motor cortex (M1) were investigated at 5-29 months after the injury using an anterograde neural tracer, biotinylated dextran amine (BDA). Reorganization of the CST was assessed by counting the number of BDA-labeled axons and bouton-like swellings in the gray and white matters. Rostral to the lesion (at C3), the number of axon collaterals of the descending axons from both contralesional and ipsilesional M1 entering the ipsilesional and contralesional gray matter, respectively, were increased. Caudal to the lesion (at C8), axons originating from the contralesional M1, descending in the preserved gray matter around the lesion, and terminating in ipsilesional laminae VI/VII and IX were observed. In addition, axons and terminals from the ipsilesional M1 increased in the ipsilesional laminae IX after recrossing the midline, which were not observed in intact monkeys. Conversely, axons originating from the ipsilesional M1 and directed toward the contralesional laminae VII decreased. These results suggest that multiple reorganizations of the corticospinal projections to spinal segments both rostral and caudal to the lesion originating from bilateral M1 underlie a prominent recovery in long-term after spinal cord injury. SIGNIFICANCE STATEMENT Previous studies have shown that dexterous finger movements recover prominently after lesion of the corticospinal tract (CST) at the mid-cervical segments through rehabilitative training in macaque monkeys. Here, we show reorganization of the CST including sprouting of axons originating from the contralesional and ipsilesional motor cortex in the gray matter both caudal and rostral to the lesion, including a re-direction of the CST to hand motoneurons in the monkeys 5-29 months after the lesion. Thus, multiple mechanisms of reorganization of CST axons underlie the recovery of impaired cortico-motoneuronal pathways for the long-term recovery of finger dexterity.