Abstract
Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.
Highlights
After spinal cord injury (SCI), trivial cutaneous stimuli on the leg are an easy way to trigger local spasms which can spread to many muscles including the arms [1]
Ascending, descending and contralateral interlimb reflex responses were evoked in muscles of participants with SCI and able-bodied control participants with both superficial peroneal nerve (SPN) and radial nerve (RN) stimulation
In contrast to the higher incidence of ascending interlimb reflexes evoked by SPN stimulation in participants with SCI, we found the descending interlimb reflex responses in leg muscles that were evoked by RN stimulation occurred significantly less in the SCI group (6% and 0% for ipsilateral and contralateral legs, respectively) than in the control group (20% and 10% for ipsilateral and contralateral legs, respectively, p = 0.012)
Summary
After spinal cord injury (SCI), trivial cutaneous stimuli on the leg are an easy way to trigger local spasms which can spread to many muscles including the arms [1]. Pre-existing ascending and descending propriospinal connections may become more prominent after SCI [6] where, in the absence of descending corticospinal connections, the overall level of spinal cord excitability appears to increase compared to that in able-bodied controls [15,16,17,18] after the initial period of spinal shock [19]. This may be a result of increased spinal excitation and/or decreased spinal inhibition [15,18,20,21,22], or changes in the intrinsic properties of the cells [13]
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