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Standardizing The Use Of Somatosensory Evoked Potentials In The Rat Hindlimb Model
Max C. Herman, N/A.
Vassar College, Poughkeepsie, NY, USA.
The regenerative peripheral nerve interface (RPNI) consists of transferred skeletal muscle reinnervated by a peripheral nerve. A sensory RPNI must possess the ability to transmit viable electrochemical signals to the brain. Somatosensory evoked potentials (SSEPs) can provide information on peripheral nerve health as observed through averaged electrocorticograms (ECoGs) of the contralateral area of the somatosensory cortex following electrical stimulation of peripheral nerves. The purpose of this study was to establish an SSEP protocol for use in the rat hindlimb consisting of stimulation parameters and expected cortical responses for various peripheral nerve branches.
One week prior to testing, nine subjects underwent cortical electrode placement surgery. An incision was made along the midline of the skull, skin and connective tissue were cleared to expose bregma. Two titanium microscrews (KLS Martin) were implanted over the hindlimb somatosensory cortex, located 1.8-2.2 mm posterior and 2.4-2.8 mm lateral to bregma. One ground screw was implanted 2.0 mm anterior to the coronal and 2.0 mm lateral to the sagittal suture over the right motor cortex. Dental cement was used to secure the screws. One week after screw placement, the tibial, peroneal, and sural nerves were exposed. A stimulation hook electrode was placed around each nerve and electrophysiological recordings were taken from both the contralateral and ipsilateral ECoG screws (Figure 1). Stimulation intensities ranged from 25-1000 μA. Maximum amplitude and latency were recorded for the minimum, median, and maximum stimulation intensities for each nerve.
SSEPs recorded from both the contralateral and ipsilateral screw electrodes were compared in five subjects. Tibial nerve SSEPs recorded from the contralateral side were significantly higher in amplitude when compared to the ipsilateral side (at minimum intensity: p=.003, maximum intensity: p=.048). For the sural nerve, contralateral SSEPs were also significantly higher in amplitude than the ipsilateral SSEPs (min. intensity: p=.025, median intensity: p=.014, max. intensity: p=.011). No significant differences were found between the responses in the contralateral and ipsilateral SSEPs from the peroneal nerve.
Within this study, we have developed and validated an SSEP protocol for analysis of ECoG representation of peripheral nerve stimulation. SSEPs generated from the tibial and sural nerves using this protocol showed significantly greater amplitudes from the contralateral ECoG screw compared to ipsilateral screw. This study demonstrates that this SSEP procedure can be used in the rat hindlimb to effectively test the viability and health of sensory and mixed nerves. Future studies will use this protocol to test the cortical electrophysiological representation of reinnervated nerves in our sensory RPNI model.
This work was sponsored by the Defense Advanced Research Projects Agency (DARPA) MTO under the auspices of Dr. Jack Judy through the Space and Naval Warfare Systems Center, Pacific Grant/Contract No. N66001-11-C-4190
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