This study will determine the temporal organization of corticospinal volleys during reach and grasp movements. Multiple descending volleys in the corticospinal tract generate multiple peaks in muscle responses (indirect (I)-waves). I-waves are a mechanism by which corticospinal neurons are transynaptically activated at periodic intervals of ~1.5 ms. This periodic activation contributes to the recruitment of spinal motoneurons and generation of movement. we will use paired-TMS to examine I-waves in surface EMG recordings from upper-limb muscles during reach and grasp movements. We will also identify motor cortical maps of upper-limb muscles involved in reach and grasp movements. We will use TMS guided by a frameless neuronavigation system to define the size and location of motor cortical maps of upper-limb muscles during reach and grasp movements. We will be able to determine overlaps and functional interactions between distal and proximal arm motor cortical representations. Our preliminary data shows that finger and biceps cortical maps largely overlap during reach and grasp movements in controls but considerable less in patients
Inclusion Criteria: - Inclusion criteria for individuals with SCI: 1. Male and females between 18-85 years, 2. Chronic SCI (≥ 6 months post injury), 3. Cervical injury at C8 or above, 4. Intact or impaired but not absent innervations in dermatomes C6, C7, and C8 using the American Spinal Injury Association sensory scores, and 5. Ability to reach and grasp a small object located at least 8 cm forward, above, and laterally without leaning forward with the trunk Inclusion criteria for healthy controls: 1. Male and females between 18-85 years, 2. Right handed, 3. Ability to reach and grasp a small object located at least 8 cm forward, above, and laterally without leaning forward with the trunk Exclusion Criteria: - Exclusion criteria for individuals with SCI and Healthy Controls: 1. Uncontrolled medical problems including pulmonary, cardiovascular or orthopedic disease 2. Any debilitating disease prior to the SCI that caused exercise intolerance 3. Premorbid, ongoing major depression or psychosis, altered cognitive status 4. History of head injury or stroke 5. Pacemaker 6. Metal plate in skull 7. History of seizures 8. Receiving drugs acting primarily on the central nervous system, which lower the seizure threshold such as antipsychotic drugs (chlorpromazine, clozapine) or tricyclic antidepressants 9. Pregnant females 10. Ongoing cord compression or a syrinx in the spinal cord or who suffer from a spinal cord disease such as spinal stenosis, spina bifida or herniated cervical disk. 11. History of brain tumor and or brain infection