The Recognition of the Distribution Features of Corticospinal Neurons by a Retrograde Trans-synaptic Tracing to Elucidate the Clinical Application of Contralateral Middle Trunk Transfer

Peripheral nerve transfers for dysfunctions in central nervous system injuries: a systematic review

BACKGROUND

The review highlights recent advancements and innovative uses of nerve transfer surgery in treating dysfunctions caused by central nervous system (CNS) injuries, with a particular focus on spinal cord injury (SCI), stroke, traumatic brain injury, and cerebral palsy.

METHODS

A comprehensive literature search was conducted regarding nerve transfer for restoring sensorimotor functions and bladder control following injuries of spinal cord and brain, across PubMed and Web of Science from January 1920 to May 2023. Two independent reviewers undertook article selection, data extraction, and risk of bias assessment with several appraisal tools, including the Cochrane Risk of Bias Tool, the JBI Critical Appraisal Checklist, and SYRCLE's ROB tool. The study protocol has been registered and reported following PRISMA and AMSTAR guidelines.

RESULTS

Nine hundred six articles were retrieved, of which 35 studies were included (20 on SCI and 15 on brain injury), with 371 participants included in the surgery group and 192 in the control group. These articles were mostly low-risk, with methodological concerns in study types, highlighting the complexity and diversity. For SCI, the strength of target muscle increased by 3.13 of Medical Research Council grade, and the residual urine volume reduced by more than 100 ml in 15 of 20 patients. For unilateral brain injury, the Fugl-Myer motor assessment (FMA) improved 15.14-26 score in upper extremity compared to 2.35-26 in the control group. The overall reduction in Modified Ashworth score was 0.76-2 compared to 0-1 in the control group. Range of motion (ROM) increased 18.4-80° in elbow, 20.4-110° in wrist and 18.8-130° in forearm, while ROM changed -4.03°-20° in elbow, -2.08°-10° in wrist, -2.26°-20° in forearm in the control group. The improvement of FMA in lower extremity was 9 score compared to the presurgery.

CONCLUSION

Nerve transfer generally improves sensorimotor functions in paralyzed limbs and bladder control following CNS injury. The technique effectively creates a 'bypass' for signals and facilitates functional recovery by leveraging neural plasticity. It suggested a future of surgery, neurorehabilitation and robotic-assistants converge to improve outcomes for CNS.

Constraint-Induced Movement Therapy Modulates Neuron Recruitment and Neurotransmission Homeostasis of the Contralesional Cortex to Enhance Function Recovery after Ischemic Stroke

Stroke often results in long-term and severe limb dysfunction for a majority of patients, significantly limiting their activities and social participation. Constraint-induced movement therapy (CIMT) is a rehabilitation approach aimed explicitly at enhancing upper limb motor function following a stroke. However, the precise mechanism remains unknown. This study explores how CIMT may alleviate forelimb paralysis in ischemic mice, potentially through structural and functional remodeling of brain regions beyond the infarct area, especially the contralateral cortex. We demonstrated that CIMT recruits neurons from the contralesional cortex into the network that innervates the affected forelimb, as evidenced by PRV retrograde nerve tracing. Additionally, we investigated how CIMT influences synaptic plasticity in the contralateral cortex by evaluating synaptic growth marker levels and neurotransmission's homeostatic regulation. Our findings uncover a rehabilitative mechanism by which CIMT treats ischemic stroke, characterized by increased recruitment of neurons from the contralateral cortex into the network that innervates the affected forelimb, facilitated by homeostatic regulation of neurotransmission.

Longitudinal optogenetic mapping reveals enhanced motor control by the contralesional cortex after traumatic brain injury in mice

Traumatic brain injury (TBI) is a significant cause of human disability, and understanding its spontaneous recovery pattern after injury is critical for potential treatments. However, studies on the function of the contralesional cortex after TBI have mostly focused on acute-phase changes, and long-term dynamic changes in the control of the affected limb by the contralesional cortex are less understood. To unravel long-term adaptations in the contralesional cortex, we developed a mouse model of TBI and used longitudinal optogenetic motor mapping to observe the function of contralesional corticospinal neurons (CSNs) projecting to the unilateral seventh cervical (C7) segment of the spinal cord. We injected a retrograde adeno-associated virus (AAV) expressing channelrhodopsin-2 to optogenetically stimulate and map the functional connections of the motor-sensory cortex. We validated the effectiveness of transcranial optogenetic stimulation for functional mapping and observed a general increase in the control of the affected limb by the contralesional cortex over time. Using retrograde labeling techniques, we showed that TBI does not affect the distribution of C7-CSNs but alters their function, and the labeled CSNs are concentrated in the caudal and rostral forelimb areas. Our findings provide new insights into harnessing contralesional cortical plasticity to improve treatment for affected limbs. This study sheds light on the long-term adaptations in the contralesional cortex after TBI, paving the way for potential clinical applications of optogenetic stimulation to improve motor control and rehabilitation outcomes.

Plasticity of the Central Nervous System Involving Peripheral Nerve Transfer

Peripheral nerve injury can lead to partial or complete loss of limb function, and nerve transfer is an effective surgical salvage for patients with these injuries. The inability of deprived cortical regions representing damaged nerves to overcome corresponding maladaptive plasticity after the reinnervation of muscle fibers and sensory receptors is thought to be correlated with lasting and unfavorable functional recovery. However, the concept of central nervous system plasticity is rarely elucidated in classical textbooks involving peripheral nerve injury, let alone peripheral nerve transfer. This article is aimed at providing a comprehensive understanding of central nervous system plasticity involving peripheral nerve injury by reviewing studies mainly in human or nonhuman primate and by highlighting the functional and structural modifications in the central nervous system after peripheral nerve transfer. Hopefully, it will help surgeons perform successful nerve transfer under the guidance of modern concepts in neuroplasticity.