Quantitative assessment of forelimb motor function after cervical spinal cord injury in rats: Relationship to the corticospinal tract

Approximately 50% of human spinal cord injuries (SCI) are at the cervical level, resulting in impairments in motor function of the upper extremity. Even modest recovery of upper extremity function could have an enormous impact on quality of life for quadriplegics. Thus, there is a critical need to develop experimental models for cervical SCI and techniques to assess deficits and recovery of forelimb motor function. Here, we analyze forelimb and forepaw motor function in rats after a lateral hemisection at C5 and assessed the relationship between the functional impairments and the extent of damage to one descending motor system, the corticospinal tract (CST). Female Sprague-Dawley rats were trained on various behavioral tasks that require the forelimb, including a task that measures gripping ability by the hand (as measured by a grip strength meter, GSM), a food reaching task, and horizontal rope walking. After 8 weeks of post-injury testing, the distribution of the CST was evaluated by injecting BDA into the sensorimotor cortex either ipsi- or contralateral to the cervical lesion. Complete unilateral hemisection injuries eliminated the ability to grip and caused severe impairments in food retrieval by the forepaw ipsilateral to the lesion. There was no indication of recovery in either task. In cases in which hemisections spared white matter near the midline, there was some recovery of forelimb motor function over time. Assessment of rope climbing ability revealed permanent impairments in forelimb use and deficits in hindlimb use and trunk stability. Sensory testing using a dynamic plantar aesthesiometer revealed that there was no increase in touch sensitivity in the affected forelimb. For the cases in which both histological and behavioral data were available, spared forelimb motor function was greatest in rats in which there was sparing of the dorsal CST.