Journal
BIOMEDICAL JOURNAL
Volume 43, Issue 1, Pages 44-52Publisher
ELSEVIER
DOI: 10.1016/j.bj.2019.07.007
Keywords
H-reflex; Motor evoked potential; Pre-synaptic; Soleus; Spinal cord; Spinal cord injury
Funding
- Ministry of Science and Technology, Taiwan [MOST 105-2918-I-182-002, 107-2221-E-182-009-MY3]
- Healthy Aging Research Center at Chang Gung University [EMRPD1I0501]
- Chang Gung Medical Foundation [CMRPD3E0112]
- Neuroscience Research Center, Chang Gung Memorial Hospital, Linkou Medical Center in Taiwan
- United States National Institutes of Health [R01-HD084645, R01-HD082109, K12-HD055931]
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Background: This study used novel human neurophysiologic models to investigate whether the mechanism of rate-sensitive H-reflex depression lies in the pre-synaptic or postsynaptic locus in humans. We hypothesized that pre-synaptic inhibition would suppress la afferents and H-reflexes without suppressing alpha motor neurons or motor evoked potentials (MEPs). In contrast, post-synaptic inhibition would suppress alpha motor neurons, thereby reducing H-reflexes and MEPs. Methods: We recruited 23 healthy adults with typical rate-sensitive H-reflex depression, 2 participants with acute sensory-impaired spinal cord injury (SCI) (to rule out influence of sensory stimulation on supra-spinal excitability), and an atypical cohort of 5 healthy adults without rate-sensitive depression. After a single electrical stimulation to the tibial nerve, we administered either a testing H-reflex or a testing MEP at 50-5000 ms intervals. Results: Testing MEPs were not diminished in healthy subjects with or without typical rate-sensitive H-reflex depression, or in subjects with sensory-impaired SCI. MEP responses were similar in healthy subjects with versus without rate-sensitive H-reflex depression. Conclusions: Results from these novel in vivo human models support a pre-synaptic locus of rate-sensitive H-reflex depression for the first time in humans. Spinal reflex excitability can be modulated separately from descending corticospinal influence. Each represents a potential target for neuromodulatory intervention.
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