Identifying descending propriospinal influence on lumbosacral motor neuron excitability after spinal cord injury: Effects of ulnar nerve stimulation on MMR amplitude in leg muscles

Society for Neuroscience Chicago 2015 Motoneuron Excitability SCI and Plasticity

Identifying descending propriospinal influence on lumbosacral motor neuron excitability after spinal cord injury: Effects of ulnar nerve stimulation on MMR amplitude in leg muscles

Abstract: Ongoing studies of the effects of spinal cord epidural stimulation in human spinal cord injury (SCI) suggest the presence of descending neural influence which cannot be detected using current methods of SCI evaluation1. Electrophysiological assessments of descending pathways, such as the long propriospinal pathways, provide an alternative technique for assessing residual descending influence on motor neuron excitability below the level of lesion. The present study was designed to determine whether descending influence on lumbosacral motor neuron excitability could be detected via the effects of ulnar nerve stimulation on multi-segmental muscle responses (MMRs) of leg muscles. MMRs were evoked in 14 spinal cord injured participants (3 AIS As, 4 Bs, 4 Cs, and 3Ds) by transcutaneous electrical stimulation over the lumbosacral enlargement while lying in supine. MMRs were recorded via surface electrodes in bilateral rectus femoris, vastus lateralis, medial hamstrings, tibialis anterior, gastrocnemius, soleus, and adductor hallucis brevis muscles. MMRs were conditioned by non-noxious electrical stimulation of the ulnar nerve at a range of condition-test intervals (40-300ms). A functional neurophysiological assessment (FNPA)3 of volitional muscle activation and the International Society for Neurological Classification of Spinal Cord Injury (ISNCSCI) exam2 were also performed to assess volitional supraspinal drive to muscles below the level of injury. Conditioning stimulation delivered to the ulnar nerve resulted in facilitation of MMR amplitudes in at least one muscle group in 10/14 participants, with all muscles being facilitated in 3 participants. The condition-test intervals which most frequently and distinctly caused facilitation ranged from 90-140 ms. MMR modulation revealed descending influence on motor neuron pools below the level of SCI, which was not detectable during voluntary muscle activation attempts (assessed by FNPA and ISNCSCI exam) in 5 participants; MMR modulation was always seen in individuals (n=6) who demonstrated volitional activation of leg muscle(s). This method of assessing descending propriospinal influence on lumbosacral motor neuron excitability allows quantification of pathway-specific contributions to spinal excitability in multiple bilateral leg muscles, which are not detected by other methods. Results of this study, in conjunction with ongoing experiments involving reticulospinal, vestibulospinal, and corticospinal pathway modulation of MMRs, may aid in understanding the neurophysiology mediating voluntary movement observed in the presence of epidural stimulation after human SCI.

Disclosures: D.A. Atkinson: None. D.G. Sayenko: None. A. Mink: None. K. Gurley: None. V. Smith: None. Y.P. Gerasimenko: None. S.J. Harkema: None.

Support: Frazier Rehab Institute and Kentucky One Health Kentucky Spinal Cord & Head Injury Research Trust Grant no. 11-7 National Institute of General Medical Sciences Grant 8 P30 GM-103507 Helmsley Foundation Grant #2011PG-MED011 Commonwealth of Kentucky Challenge of Excellence Trust Fund Owsley Brown Frazier Chair in Neurological Rehabilitation Endowment Russian Foundation for Basic Research Grant 13-04-12030 ofi-m

Authors: *D. A. ATKINSON1,4, D. G. SAYENKO5, A. MINK2,4, K. GURLEY6, V. SMITH3, Y. P. GERASIMENKO5,7, S. J. HARKEMA3,4; 1Dept. of Anatom. Sci. and Neurobio., 2Dept. of Physiol. and Biophysics, 3Dept. of Neurosurg., Univ. of Louisville, Louisville, KY; 4Neurosci. Collaborative Ctr., Frazier Rehab Inst., Louisville, KY; 5Dept. of Integrative Biol. and Physiol., UCLA, Los Angeles, CA; 6Louisianna State UniversityHealth Sci. Ctr., New Orleans, LA; 7Pavlov Inst. of Physiol., St. Petersburg, Russian Federation

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