Macaque study examines role of interhemispheric pathways in spinal cord injury healing

Stroke and spinal cord injuries can severely impair motor function, so understanding how to promote recovery is critical. Although damaged neurons in the brain and spinal cord have limited capacity to regenerate, the brain can form or strengthen alternative neural pathways involving uninjured parts of the brain, allowing for functional recovery. This reorganization of pathways in the brain is called neural plasticity. Identifying the pathways involved and understanding their functions can help researchers develop more effective rehabilitation strategies.

Previous research has shown that when one side of the corticospinal tract, an important pathway that transmits movement signals from the brain to the spinal cord, is damaged, activity in the motor cortex on the opposite side of the brain increases. This has led to questions about whether this increased activity promotes or hinders healing, and the precise role of these pathways is unclear.

In a study published in Nature CommunicationsResearchers from WPI-ASHBi, Kyoto University, and the National Institute of Physiological Sciences conducted experiments on macaque monkeys to study how this change in neuronal activity affects recovery after spinal cord injury.

To manipulate the neural activity of both motor cortexes, viral vectors were injected into targeted brain areas to block communication between the left and right motor cortexes by administering drugs, the effect of which was reversible. This allowed the researchers to observe how disrupting these pathways affected the monkeys’ ability to perform precise reaching and grasping tasks before and after injury. In addition, they measured neural activity in the motor cortex on both sides during the task. The results showed that blocking the pathways had no effect on the monkeys’ ability in the absence of injury.

However, a notable decrease in the monkeys’ abilities was observed due to the blockade during the initial phase of recovery. This suggests that the interhemispheric pathway, which is not usually involved in motor functions, becomes relevant for motor recovery at the early stage. The study also highlighted changes in the activity patterns of the motor cortex when the pathways were blocked. With the signal blocked from the opposite side, the activity on the affected side decreased during the initial phase of recovery after injury. On the other hand, the same blockade increased the activity in the intact state.

These results suggest that the interhemispheric pathway between the left and right motor cortices, which plays an inhibitory role in the intact state, plays a facilitative role in the early phase of recovery after injury. This pathway activates the motor cortex on the side that is not involved in motor functions in general, thus contributing to the recovery of motor function.

“These results show that while the interhemispheric pathway is inhibitory in the intact state, it becomes facilitative early in recovery, promoting recovery of motor function by activating the motor cortex on the uninvolved side,” said Dr. Masahiro Mitsuhashi, lead researcher of the study. This finding underscores the adaptability of spared neural pathways to promote recovery after central nervous system injuries.

The results of this study provide valuable information about how brain pathways contribute to motor control and how the brain responds to perturbations. This information is essential for developing better rehabilitation strategies and treating motor impairments resulting from brain injuries.

The researchers plan to study how similar pathways work in other types of central nervous system injuries, such as those caused by stroke. By integrating these findings with traditional rehabilitation methods, they hope to improve treatments for severe central nervous system injuries and reduce long-term impairments.

“Further research is needed, but we hope that revealing the neural pathways crucial for recovery and developing methods to activate them will lead to advances in future neurorehabilitation therapies,” Mitsuhashi said.