Rewiring of Cortex to Brainstem After Brain Injury Contributes to Recovery of Dexterity
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Material below summarizes the article Hand Motor Recovery Following Extensive Frontoparietal Cortical Injury Is accompanied by Upregulated Corticoreticular Projections in the Monkey, published on July 11, 2018, in JNeurosci and authored by Warren G. Darling, Jizhi Ge, Kimberly S. Stilwell-Morecraft, Diane L. Rotella, Marc A. Pizzimenti, and Robert J. Morecraft.
Injuries to areas of the brain responsible for motor function can have devastating and permanent effects on motor abilities.
For example, after injury to motor areas on one side of the brain due to stroke or traumatic brain injury, use of the hand and digits on the opposite side of the body to grasp and manipulate small objects is often severely impaired.
With these impairments, individuals have difficulty performing activities of daily living, such as dressing, eating, and maintaining personal hygiene. Employment may also be affected when physical labor or fine motor control skills such as typing, writing, or manipulating a computer mouse are necessary.
However, animal studies have shown the brain can respond to injuries by making new connections from uninjured parts of the brain to neurons involved in controlling muscles that move the impaired hand and digits.
For example, in monkeys, we previously found after injury to brain motor areas located on the lateral surface in one hemisphere, a cortical motor area located on the medial surface in the same hemisphere can increase nerve connections to neurons in the spinal cord that control arm, hand, and digit muscles used for reaching and grasping objects.
Therefore, it is as if the medial cortical motor area takes over the functions performed before the injury by lateral cortical motor areas. This is clinically relevant because the lateral surface of the hemisphere receives circulation via the middle cerebral artery (MCA), which is commonly affected in stroke, but the medial surface is served by the anterior cerebral artery and thus is spared in MCA stroke.
We further established recovery of the ability to reach toward, grasp, and manipulate small food objects was closely associated with the number of nerve connections made from the medial cortical motor area onto spinal cord neurons. That is, brain-injured animals with greater numbers of connections onto spinal cord neurons from a medial brain motor area recovered better than animals with fewer such connections.
Recently, we studied the effects of injury to lateral cortical motor areas and lateral sensory processing areas in monkeys. When both areas are injured, recovery of hand and digit function is poorer than when only motor areas are damaged, resulting in movements that are slower and less accurate.
Moreover, this recovery was not associated with increases in the number of connections from medial cortical motor areas onto spinal neurons controlling muscles of the hand and digits. In fact, there were decreases in the number of spinal cord connections, making it difficult to understand how recovery of motor function occurs.
We reasoned, based on research from other laboratories, the motor recovery might occur through an alternative pathway involving connections from medial brain motor areas to neurons in a region of the brainstem that, in turn, make connections onto neurons in the spinal cord. The reticular formation (RF) of the brainstem is such a region.
To investigate this possible alternative recovery pathway, we examined connections from medial motor areas of the brain to neurons in a specific RF nucleus that makes connections onto spinal cord neurons in three groups of monkeys. One group had injuries to lateral brain motor areas, a second group had injury to lateral brain motor and sensory processing areas, and the third group had no brain injuries.
The group with damage to lateral motor areas had similar numbers of connections to the RF neurons from medial brain motor areas as animals with no brain injury. In contrast, the group with injury to lateral brain motor and sensory processing areas had twice the number of RF connections as the other two groups. Similar to the findings described above, recovery of hand and digit motor function was better in brain-injured animals with more connections onto RF neurons than in animals with fewer connections.
We concluded recovery after brain injury affecting lateral brain motor and sensory processing areas, which is common in stroke patients, is at least partially mediated by increases in connections from medial brain motor areas onto neurons located in the brainstem RF.
Overall, these findings strongly suggest stimulating medial brain motor areas to increase connections to neurons in motor areas of the brainstem (after injury to lateral motor and sensory processing areas) and spinal cord (after injury to lateral motor areas alone) may improve recovery from brain injury in human stroke patients.
Anatomically, the blood supplies to the lateral and medial motor areas of the brain arise from different major arteries. Given that many strokes involve the lateral (outside) surface of the brain (supplied by the middle cerebral artery), the medial (inside) surface (supplied by the anterior cerebral artery) is often spared from injury.
Thus, there is great potential for a therapy targeting the medial brain motor areas to improve recovery of hand function after common middle cerebral artery stroke.
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Hand Motor Recovery Following Extensive Frontoparietal Cortical Injury Is Accompanied by Upregulated Corticoreticular Projections in Monkey. Warren G. Darling, Jizhi Ge, Kimberly S. Stilwell-Morecraft, Diane L. Rotella, Marc A. Pizzimenti, Robert J. Morecraft. JNeurosci Jul 2018, 38 (28) 6323-6339; DOI: https://doi.org/10.1523/JNEUROSCI.0403-18.2018