title: Reflex wind-up in early chronic spinal injury: plasticity of motor outputs.
abstract: In this study we evaluate temporal summation (wind-up) of reflexes in select distal and proximal hindlimb muscles in response to repeated stimuli of the distal tibial or superficial peroneal nerves in cats 1 mo after complete spinal transection. This report is a continuation of our previous paper on reflex wind-up in the intact and acutely spinalized cat. To evaluate reflex wind-up in both studies, we recorded electromyographic signals from the following left hindlimb muscles: lateral gastrocnemius (LG), tibialis anterior (TA), semitendinosus (ST), and sartorius (Srt), in response to 10 electrical pulses to the tibial or superficial peroneal nerves. Two distinct components of the reflex responses were considered, a short-latency compound action potential (CAP) and a longer duration bout of sustained activity (SA). These two response types were shown to be differentially modified by acute spinal injury in our previous work (Frigon A, Johnson MD, Heckman CJ.J Physiol590: 973-989, 2012). We show that these responses exhibit continued plasticity during the 1-mo recovery period following acute spinalization. During this early chronic phase, wind-up of SA responses returned to preinjury levels in one muscle, the ST, but remained depressed in all other muscles tested. In contrast, CAP response amplitudes, which were initially potentiated following acute transection, returned to preinjury levels in all muscles except for Srt, which continued to show marked increase. These findings illustrate that spinal elements exhibit considerable plasticity during the recovery process following spinal injury and highlight the importance of considering SA and CAP responses as distinct phenomena with unique underlying neural mechanisms.NEW & NOTEWORTHYThis research is the first to assess temporal summation, also called wind-up, of muscle reflexes during the 1-mo recovery period following spinal injury. Our results show that two types of muscle reflex activity are differentially modulated 1 mo after spinal cord injury (SCI) and that spinal reflexes are altered in a muscle-specific manner during this critical period. This postinjury plasticity likely plays an important role in spasticity experienced by individuals with SCI.
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; m-johnson16@northwestern.edu.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Johnson
authors.affiliation: Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, Quebec, Canada.
authors.forename: Alain
authors.initials: A
authors.lastname: Frigon
authors.affiliation: Department of Pharmacology-Physiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, Quebec, Canada.
authors.forename: Marie-France
authors.initials: MF
authors.lastname: Hurteau
authors.affiliation: Center for Comparative Medicine, Northwestern University, Chicago, Illinois; and.
authors.forename: Charlette
authors.initials: C
authors.lastname: Cain
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois.|Department of Physical Medicine and Rehabilitation, Physical Therapy and Human Movement Sciences, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.
authors.forename: C J
authors.initials: CJ
authors.lastname: Heckman
mesh_terms: D000818:Animals; D002415:Cats; D019054:Evoked Potentials, Motor; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009473:Neuronal Plasticity; D012018:Reflex; D013119:Spinal Cord Injuries / Q000503:physiopathology*
keywords: motoneuron; reflex; spinal cord injury; wind-up