Motoneuron Recruitment and Motor Evoked Potential Up-Conditioning (MEP) in Spinal Cord Injury (SCI)

Regaining arm/hand function is one of the top priorities of individuals with tetraplegia and is a challenging problem, partly due to the complex nature of upper limb motor function. Through mass practice and conventional therapy, functions can be restored partially, likely through compensation rather than induction of relevant corticospinal plasticity. In such cases, corticospinal recruitment of motoneurons and resulting muscle activation could remain impaired and continue to limit function recovery. In people with incomplete SCI, voluntary activation of the muscles below the injury level is often diminished, due at least partly to impaired motor unit behaviors. Abnormal motoneuron recruitment and motor unit firing, also found in other CNS disorders, would certainly hinder effective production of motor functions. Thus, to enhance upper limb motor rehabilitation beyond what conventional therapy has been able to achieve, a method to improve corticospinal recruitment of a targeted pool of motoneurons would be needed. Here, investigators hypothesize that wrist extensor MEP up-conditioning can improve forearm motor functions in people with cervical SCI by increasing the corticospinal excitability and improving corticospinal recruitment of wrist extensor motoneurons.

Inclusion criteria

• Adult (≥18 yrs old)
• a history of injury to spinal cord at or above C6
• neurologically stable (>1 year post SCI)
• medical clearance to participate
• weak wrist extension at least unilaterally
• expectation that current medication will be maintained without change for at least 3 months.

• Stable use of anti-spasticity medication (e.g., baclofen, diazepam, tizanidine) is accepted. (Because only neurologically stable subjects will enter this study, medication changes will be unlikely.)
• In participants with bilateral wrist extension weakness in whom Extensor Carpi Radialis (ECR) MEP can be elicited in both arms, the more severely impaired arm is studied. In participants with unilateral wrist weakness or in participants with bilateral wrist weakness in whom an ECR MEP can be elicited in only one arm, that arm is studied.

Exclusion criteria

• motoneuron injury
• unstable medical condition
• cognitive impairment (because the studied intervention is a learning-based intervention)
• a history of epileptic seizures
• a pre-existing or confounding neurological condition (e.g., history of MS, Stroke, Parkinson's disease)
• metal implants in the cranium
• implanted biomedical device in or above the chest (e.g., a cardiac pacemaker, cochlear implant)
• no measurable MEP elicited in the ECR
• inability to produce any voluntary ECR EMG activity
• extensive use of functional electrical stimulation to the arm on a daily basis (as it may interfere with or augment the effects of MEP conditioning itself)
• pregnancy (due to changes in posture and potential medical instability)
• inability or unwillingness of subject or legal guardian/representative to give informed consent