Windup of Flexion Reflexes in Chronic Human Spinal Cord Injury: A Marker for Neuronal Plateau Potentials?

  1 Department of Physical Medicine and Rehabilitation and   2 Department of Physical and Biomedical Engineering, Northwestern University Medical School, Chicago, IL 60611;   3 Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois 60611; and   4 Department of Biom...

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Bibliographic Details
Published in:Journal of neurophysiology 2003-01, Vol.89 (1), p.416-426
Main Authors: Hornby, T. G, Rymer, W. Z, Benz, E. N, Schmit, B. D
Format: Article
Language:English
Subjects:
Adult
Aged
Baclofen - therapeutic use
Chronic Disease
Electric Stimulation
Electromyography
Female
Humans
Interneurons - physiology
Male
Middle Aged
Motor Neurons - physiology
Movement - physiology
Muscle Contraction - physiology
Muscle Relaxants, Central - therapeutic use
Muscle Spasticity - drug therapy
Muscle Spasticity - physiopathology
Muscle, Skeletal - innervation
Muscle, Skeletal - physiology
Reaction Time - physiology
Reflex, Abnormal - physiology
Spinal Cord - cytology
Spinal Cord - physiopathology
Spinal Cord Injuries - physiopathology
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Summary:  1 Department of Physical Medicine and Rehabilitation and   2 Department of Physical and Biomedical Engineering, Northwestern University Medical School, Chicago, IL 60611;   3 Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois 60611; and   4 Department of Biomedical Engineering, Marquette University, Milwaukee, Wisconsin 53201-1881 Hornby, T. G., W. Z. Rymer, E. N. Benz, and B. D. Schmit. Windup of Flexion Reflexes in Chronic Human Spinal Cord Injury: A Marker for Neuronal Plateau Potentials?. J. Neurophysiol. 89: 416-426, 2003. The physiological basis of flexion spasms in individuals after spinal cord injury (SCI) may involve alterations in the properties of spinal neurons in the flexion reflex pathways. We hypothesize that these changes would be manifested as progressive increases in reflex response with repetitive stimulus application (i.e., "windup") of the flexion reflexes. We investigated the windup of flexion reflex responses in 12 individuals with complete chronic SCI. Flexion reflexes were triggered using trains of electrical stimulation of plantar skin at variable intensities and inter-stimulus intervals. For threshold and suprathreshold stimulation, windup of both peak ankle and hip flexion torques and of integrated tibialis anterior electromyographic activity was observed consistently in all patients at inter-stimulus intervals 3 s. For subthreshold stimuli, facilitation of reflexes occurred only at intervals 1 s. Similarly, the latency of flexion reflexes decreased significantly at intervals 1 s. Patients that were receiving anti-spasticity medications (e.g., baclofen) had surprisingly larger windup of reflex responses than those who did not take such medications, although this difference may be related to differences of spasm frequency between the groups of subjects. The results indicate that the increase in spinal neuronal excitability following a train of electrical stimuli lasts for 3 s, similar to previous studies of nociceptive processing. Such long-lasting increases in flexion reflex responses suggest that cellular mechanisms such as plateau potentials in spinal motoneurons, interneurons, or both, may partially mediate spinal cord hyperexcitability in the absence of descending modulatory input.
ISSN:0022-3077
1522-1598
DOI:10.1152/jn.00979.2001