A 3D map of the hindlimb motor representation in the lumbar spinal cord in Sprague Dawley rats

Abstract

Objective Spinal cord injury (SCI) is a devastating neurological trauma with a prevalence of about 282,000 people living with an SCI in the United States in 2016. Advances in neuromodulatory devices hold promise for restoring function by incorporating the delivery of electrical current directly into the spinal cord grey matter via intraspinal microstimulation (ISMS). In such designs, detailed topographic maps of spinal cord outputs are needed to determine ISMS locations for eliciting hindlimb movements. The primary goal of the present study was to derive a topographic map of functional motor outputs in the lumbar spinal cord to hindlimb skeletal muscles as defined by ISMS in a rat model. Approach Experiments were carried out in nine healthy, adult, male, Sprague Dawley rats. After a laminectomy of the T13-L1 vertebrae and removal of the dura mater, a four-shank, 16-channel microelectrode array was inserted along a three-dimensional (200 µm) stimulation grid. Trains of three biphasic current pulses were used to determine evoked movements and EMG activity. Via fine wire electromyographic (EMG) electrodes, Stimulus-Triggered Averaging (StTA) was used on rectified EMG data to determine response latency. Main results Hindlimb movements were elicited at a median current intensity of 6 µA, and thresholds were significantly lower in ventrolateral sites. Movements typically consisted of whole leg, hip, knee, ankle, toe, and trunk movements. Hip movements dominated rostral to the T13 vertebral segment, knee movements were evoked at the T13-L1 vertebral junction, while ankle and digit movements were found near the rostral L1 vertebra. Whole leg movements spanned the entire rostrocaudal region explored, while trunk movements dominated medially. StTAs of EMG activity demonstrated a latency of ~4 ms. Significance The derived motor map provides insight into the parameters needed for future neuromodulatory devices.

Activity Dependent Stimulation Increases Synaptic Efficacy In Spared Pathways In An Anesthetized Rat Model Of Spinal Cord Contusion Injury

“…EMG fine-wire electrode implantation. The implantation and verification of fine-wire EMG electrodes into hindlimb muscles was conducted as previously described (Borrell et al, 2017). EMG electrodes were implanted in four hindlimb muscles: the lateral gastrocnemius (LG), tibialis anterior (TG), vastus lateralis (VL), and biceps femoris (BF).…”

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

“…EMG electrodes were implanted in four hindlimb muscles: the lateral gastrocnemius (LG), tibialis anterior (TG), vastus lateralis (VL), and biceps femoris (BF) (Borrell et al, 2017). Briefly, each EMG electrode consisted of a pair of insulated multi-stranded stainless-steel wires exposed approximately 1 mm, with the implanted end of the wire folded back on itself ('hook' electrode).…”

“…Using custom software (Matlab; The Mathworks, Inc., Natick, MA), stimulustriggered averages (StTAs) of rectified EMG of were analyzed to determine muscle activation (Hudson et al, 2015;Borrell et al, 2017). For each of the implanted muscles, EMG signals were recorded for at least 10 ICMS stimulus trains to obtain StTAs over a 220 ms time window.…”

“…Previously derived 3-dimensional ISMS-evoked topographic maps were used to guide the location of the spinal opening to obtain stimulation sites in the hindlimb spinal cord (Padmanabhan and Singh, 1979;Borrell et al, 2017). The dura mater was removed using fine forceps and small scissors to allow microelectrode penetration.…”

“…Each stimulus consisted of three, 200 µs biphasic cathodal pulses delivered at 300 Hz repeated at 1/sec. These ISMS parameters are commonly used for ISMS mapping procedures (Sunshine et al, 2013;Shahdoost et al, 2014a;Borrell et al, 2017).…”