Introduction
Motor output following spinal cord injury may be severely disrupted due to changes in the descending and segmental controls. We have recently begun to use long trains of percutaneous electrical stimuli over lower limb muscles in patients with traumatic spinal cord injury (SCI) to look for evidence that involuntary “extra contractions” can develop beyond those due to the activation of motor axons beneath the stimulating electrodes. It has recently been proposed that these extra contractions reflect plateau-like behaviour in spinal interneurones and motoneurones [e.g. 1, 2]. Furthermore, plateau potentials contribute to spasticity in a rat model of spinal cord injury [e.g. 3].
Methods
Studies have been conducted on 14 subjects with spinal cord injury (3-152 months post-injury). Five patients had clinically complete lesions. Surface electrodes were applied over the plantar- and dorsiflexors muscles of the ankle. Several stimulus patterns incorporating high frequencies (more than 50 Hz) and lasting beyond 5 seconds were used. Stimulation was provided by special constant current stimulators under computer control. Subjects were reminded to remain relaxed.
Evidence of extra contractions in addition to the force generated by direct activation of motor axons was based on marked asymmetry of force with respect to stimulation, progressively rising force during stimulation of constant amplitude and frequency, and force remaining high after stimulation frequency had returned to the control level following a high-frequency burst.
Results
Twelve of the 14 patients showed evidence of this involuntary behaviour following one or more of the stimulus patterns. This occurred more frequently for triceps surae than tibialis anterior. In addition, some force or electromyographic activity outlasted the stimulation in most patients. There was no apparent correlation between the level or completeness of injury and the ability to induce the behaviour. Eleven patients exhibited hyperreflexia and reported spontaneous spasms, but there was no obvious association with the extra contractions and presumed plateau-like behaviour.
Discussion
We suggest that the discharge of neurones within the spinal cord can contribute to the extra contractions evoked by electrical stimulation over muscles in spinal cord injured subjects. However, this involuntary behaviour may be harder to obtain in intact healthy subjects. The ability to control this behaviour requires further investigation.
References:
[1] Collins, D.F., D. Burke, and S.C. Gandevia, Large involuntary forces consistent with plateau-like behavior of human motoneurons. J Neurosci, 2001. 21(11): p. 4059-65.
[2] 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, 2003. 89(1): p. 416-26.
[3] Bennett, D.J., Y. Li, P.J. Harvey, and M. Gorassini, Evidence for plateau potentials in tail motoneurons of awake chronic spinal rats with spasticity. J Neurophysiol, 2001. 86(4): p. 1972-82.