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J Appl Physiol (October 11, 2002). doi:10.1152/japplphysiol.01176.2001
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Articles in PresS, published online ahead of print October 11, 2002
J Appl Physiol, 10.1152/jap.01176.2001
Submitted on November 29, 2001
Accepted on October 7, 2002

Effects of sustained electrical stimulation on the excitability of motoneurons innervating paralyzed and control muscles

Jane E Butler1 and Christine K Thomas2*

1 Department of of Neurological Surgery, The Miami Project to Cure Paralysis, University of Miami, Miami, Florida, USA
2 Department of of Neurological Surgery, The Miami Project to Cure Paralysis, University of Miami, Miami, Florida, USA; Department of Physiology and Biophysics, The Miami Project to Cure Paralysis, University of Miami, Miami, Florida, USA

* To whom correspondence should be addressed. E-mail: cthomas{at}miami.edu.

The excitability of thenar motoneurons (reflected by F-wave persistence and amplitude) and thenar muscle force were measured during a stimulation protocol (90 s of 18 Hz supramaximal electrical stimulation of the median nerve) designed to induce muscle fatigue (force decline). Data from muscles (n=15) paralyzed by chronic cervical spinal cord injury were compared to that obtained from control muscles (n=6). The persistence of F-waves in both paralyzed and control muscles increased from ~60% to ~76% during the first 10 s of the fatigue protocol, then declined progressively to ~33% at 90 s. These changes in F-wave persistence suggest similar reductions occur in the excitability of the motoneurons to paralyzed and control motor units after sustained antidromic activation. Despite this, significantly larger force declines occurred in the paralyzed muscles of spinal cord injured subjects (~60%) than in the muscles of control subjects (~15%). These data suggest that the decreases in motoneuron excitability for both the spinal cord injured and control subjects are a result of activity-dependent changes in motoneuron properties that are independent of fatigue related processes in the muscles.




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