Targeted reinnveration was shown in upper limb amputees to remap motor representations in the brain that were closely matched to presumed pre-amputation states, according to recently published study results.
“Despite numerous reports of cortical reorganization, other studies have shown that the motor representations of the missing limb persist following amputation, although not necessarily in their original areas…the resulting movement representation of the missing limb may move out of the original cortical area and into neighboring areas,” the researchers wrote. “Similarly, the magnetic stimulation of motor cortical areas previously linked to the missing limb was also able to elicit contractions or movements from residual muscles adjacent to the site of amputation. These results beg the question of whether the persistent representation of the missing limb could return to its original area, perhaps via surgical interventions that attempt to restore motor function.”
Researchers selected three participants with upper limb amputations to undergo targeted reinnervation (TR) at least 6 months following injuries. All participants had non-painful phantom limb sensations and were consistent users of upper arm prostheses. To determine motor cortical representations of the missing and intact limbs, researchers used cued motor tasks performed both 1 week before surgery and after TR and recorded high-density EEG signals during performance of the tasks. All amputees performed shoulder abduction, elbow flexion or extension and hand closing or opening with both sides of the body.
Before TR, when participants attempted voluntary movement with the missing limb, the researchers noted movement representations of the missing limb shifted out of the original cortical area of the brain into neighboring areas. For one transhumeral participant, “the elbow representation moved to the hand regions of the cortex, while the hand representations moved more laterally to that,” the researchers wrote. In the two shoulder disarticulation amputees, the hand representations in the brain shifted medially closer to shoulder representations. However, after TR, missing limb representations returned closer to their original locations.
“If you have nerves in the periphery that reattach to muscle, you start to use the original circuits that were there in the first place and were able to control your elbow, wrist and fingers. So by doing targeted reinnervation these circuits come back alive,” Julius P.A. Dewald, PT, PhD, chair and professor of the department of physical therapy and human movement sciences in the Feinberg School of Medicine and professor in Biomedical Engineering in the McCormick School of Engineering at Northwestern University, told O&P Business News. “The light goes on again, if you will, and you regain control of that muscle and you have signals that you can pick up in the reinnervated muscle. You may not have the same resolution anymore, but you start to get reorganization of the motor map that you have in the cortex.”
Plasticity of the nervous system
Although shifts in representations after peripheral injury and cortical re-mapping after TR are not well understood, the most frequently accepted mechanisms believed to be responsible for changes in the primary motor cortex after peripheral injury is the unmasking of latent synaptic connections that provide a possible foundation on which plasticity can occur, according to the researchers. They noted recent studies have shown that motor output maps can rapidly change following procedures such as transient deafferentation or peripheral nerve transection, indicating an “underlying framework for rapid brain plasticity.”
“The overall take home message is that the nervous system is very plastic and once you hook up the periphery, cortical reorganization can happen in the time it takes for the nerve to find the muscle fiber in the periphery,” Dewald said. “This study allows us to answer fundamental questions about neural plasticity in general and brain plasticity related to the motor cortex more specifically.” — by Casey Tingle
Disclosure: Dewald has no relevant financial disclosures.