Effects of a Contusive Spinal Cord Injury on Spinal Motor Neuron Activity and Conduction Time in Rats

Abstract

Objective. The purpose of this study was to determine the effects of spinal cord injury (SCI) on spike activity evoked in the hindlimb spinal cord of the rat from cortical electrical stimulation.Approach. Adult, male, Sprague Dawley rats were randomly assigned to a Healthy or SCI group. SCI rats were given a 175 kDyn dorsal midline contusion injury at the level of the T8 vertebrae. At four weeks post-SCI, intracortical microstimulation (ICMS) was delivered at several sites in the hindlimb motor cortex of anesthetized rats, and evoked neural activity was recorded from corresponding sites throughout the dorsoventral depths of the spinal cord and EMG activity from hindlimb muscles. Main results. In healthy rats, post-ICMS spike histograms showed reliable, evoked spike activity during a short-latency epoch 10-12 ms after the initiation of the ICMS pulse train (short). Longer latency spikes occurred between ~20-60 ms, generally following a Gaussian distribution, rising above baseline at time LON, followed by a peak response (Lp), and then falling below baseline at time LOFF. EMG responses occurred between . In SCI rats, short-latency responses were still present, longlatency responses were disrupted or eliminated, and EMG responses were never evoked. The retention of the short-latency responses indicates that spared descending spinal fibers, most likely via the corticoreticulospinal pathway, can still depolarize spinal cord motor neurons after a dorsal midline contusion injury. Significance. This study provides novel insights into the role of alternate pathways for voluntary control of hindlimb movements after SCI that disrupts the corticospinal tract in the rat.

Spike-Triggered Intraspinal Microstimulation Improves Motor Performance in an Ambulatory Rat Model of Spinal Cord Injury

“…Spinal cord contusion procedures followed those described in our previous studies (Krizsan-Agbas et al, 2014; Borrell et al, 2020a; Borrell et al, 2020b). Animals underwent a T8 laminectomy and 175 kDyn moderate impact contusion injury using an Infinite Horizon spinal cord impactor (Precision Systems and Instrumentation, LLC, Fairfax Station, VA).…”

“…For placement of the stimulating electrode in the spinal cord, a laminectomy was performed on the T13 vertebrae similar to previous studies (Borrell et al, 2020a; Borrell et al, 2020b). The dura mater was left intact to minimize cerebrospinal fluid leakage.…”

“…The ability of the motor cortex to evoke spinal cord activity below the level of an injury demonstrates that some descending motor pathways to the lumbar cord remain intact after a moderate spinal cord contusion (Borrell et al, 2020a). Furthermore, the administration of ADS between the hindlimb motor cortex and hindlimb spinal cord has shown that ADS causes a plastic change in the intact pathways of rats with a thoracic contusion to the spinal cord (Borrell et al, 2020b).…”

“…These efforts encouraged us to develop an understanding of the connection between the brain and spinal cord after a bilateral thoracic spinal cord contusion that mimics those seen in clinical spinal cord injuries (Krizsan-Agbas et al, 2014; Frost et al, 2015; Borrell et al, 2020a; Borrell et al, 2020b). Through using SCI models, we can create neuromodulation protocols that can more easily translate into clinical use for rehabilitation.…”