title: Periodic modulation of repetitively elicited monosynaptic reflexes of the human lumbosacral spinal cord.
abstract: In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral spinal cord at 2 Hz evokes unmodulated reflexes in the lower limbs, while stimulation at 22-60 Hz can generate rhythmic burstlike activity. Here we elaborated on an output pattern emerging at transitional stimulation frequencies with consecutively elicited reflexes alternating between large and small. We analyzed responses concomitantly elicited in thigh and leg muscle groups bilaterally by epidural stimulation in eight motor-complete spinal cord-injured individuals. Periodic amplitude modulation of at least 20 successive responses occurred in 31.4% of all available data sets with stimulation frequency set at 5-26 Hz, with highest prevalence at 16 Hz. It could be evoked in a single muscle group only but was more strongly expressed and consistent when occurring in pairs of antagonists or in the same muscle group bilaterally. Latencies and waveforms of the modulated reflexes corresponded to those of the unmodulated, monosynaptic responses to 2-Hz stimulation. We suggest that the cyclical changes of reflex excitability resulted from the interaction of facilitatory and inhibitory mechanisms emerging after specific delays and with distinct durations, including postactivation depression, recurrent inhibition and facilitation, as well as reafferent feedback activation. The emergence of large responses within the patterns at a rate of 5.5/s or 8/s may further suggest the entrainment of spinal mechanisms as involved in clonus. The study demonstrates that the human lumbosacral spinal cord can organize a simple form of rhythmicity through the repetitive activation of spinal reflex circuits.
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria; ursula.hofstoetter@gmail.com.
authors.forename: Ursula S
authors.initials: US
authors.lastname: Hofstoetter
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria; Institute of Analysis and Scientific Computing, Vienna University of Technology, Vienna, Austria; and.
authors.forename: Simon M
authors.initials: SM
authors.lastname: Danner
authors.affiliation: Neurological Center, Maria Theresien Schloessel, Otto Wagner Hospital, Vienna, Austria.
authors.forename: Brigitta
authors.initials: B
authors.lastname: Freundl
authors.affiliation: Neurological Center, Maria Theresien Schloessel, Otto Wagner Hospital, Vienna, Austria.
authors.forename: Heinrich
authors.initials: H
authors.lastname: Binder
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria;
authors.forename: Winfried
authors.initials: W
authors.lastname: Mayr
authors.affiliation: Institute of Analysis and Scientific Computing, Vienna University of Technology, Vienna, Austria; and.
authors.forename: Frank
authors.initials: F
authors.lastname: Rattay
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria;
authors.forename: Karen
authors.initials: K
authors.lastname: Minassian
mesh_terms: D000293:Adolescent; D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography; D005260:Female; D006801:Humans; D007866:Leg / Q000503:physiopathology; D008161:Lumbosacral Region; D008297:Male; D018482:Muscle, Skeletal / Q000503:physiopathology; D012018:Reflex / Q000502:physiology*; D013116:Spinal Cord / Q000503:physiopathology*; D013119:Spinal Cord Injuries / Q000503:physiopathology*; D055815:Young Adult
keywords: human; repetitive nerve stimulation; rhythm generation; spinal cord stimulation; spinal reflexes