Abstract
Posterior root-muscle (PRM) reflexes are short-latency spinal reflexes evoked by epidural or transcutaneous spinal cord stimulation (SCS) in clinical and physiological studies. PRM reflexes share key physiological characteristics with the H reflex elicited by electrical stimulation of large-diameter muscle spindle afferents in the tibial nerve. Here, we compared the H reflex and the PRM reflex of soleus in response to transcutaneous stimulation by studying their recovery cycles in ten neurologically intact volunteers and ten individuals with traumatic, chronic spinal cord injury (SCI). The recovery cycles of the reflexes, i.e., the time course of their excitability changes, were assessed by paired pulses with conditioning-test intervals of 20–5000 ms. Between the subject groups, no statistical difference was found for the recovery cycles of the H reflexes, yet those of the PRM reflexes differed significantly, with a striking suppression in the intact group. When comparing the reflex types, they did not differ in the SCI group, while the PRM reflexes were more strongly depressed in the intact group for durations characteristic for presynaptic inhibition. These differences may arise from the concomitant stimulation of several posterior roots containing afferent fibers of various lower extremity nerves by transcutaneous SCS, producing multi-source heteronymous presynaptic inhibition, and the collective dysfunction of inhibitory mechanisms after SCI contributing to spasticity. PRM-reflex recovery cycles additionally obtained for bilateral rectus femoris, biceps femoris, tibialis anterior, and soleus all demonstrated a stronger suppression in the intact group. Within both subject groups, the thigh muscles showed a stronger recovery than the lower leg muscles, which may reflect a characteristic difference in motor control of diverse muscles. Based on the substantial difference between intact and SCI individuals, PRM-reflex depression tested with paired pulses could become a sensitive measure for spasticity and motor recovery.
Highlights
Posterior root-muscle (PRM) reflexes [1,2] are short-latency spinal reflexes evoked by epidural spinal cord stimulation (SCS) or by skin surface electrodes placed in a configuration to generate a current flow that partially crosses the thecal sac [3,4]
There were no differences between the control-H reflex and the control-PRM reflex nor between the ratios of the control-H reflex to Mmax and of the control-PRM reflex to Mmax
We studied the recovery cycles of H reflexes and PRM reflexes in neurologically intact participants as well as individuals with spinal cord injury (SCI) by paired stimuli at conditioning-test intervals of 20– 5000 ms
Summary
Posterior root-muscle (PRM) reflexes [1,2] are short-latency spinal reflexes evoked by epidural spinal cord stimulation (SCS) or by skin surface electrodes placed in a configuration to generate a current flow that partially crosses the thecal sac (transcutaneous SCS) [3,4]. They result from the stimulation of proprioceptive fibers within posterior rootlets/roots, which reflexly activate motoneurons in the spinal cord [5,6,7,8], are electromyographically recorded as reflex compound muscle action potentials, and can be obtained in practically all lower extremity muscles by a single stimulation pulse with an intensity exceeding the excitation thresholds of the respective lumbar and upper sacral posterior roots [9,10]. Like the H reflex, the PRM reflexes are suppressed by tendon vibration [2,33] and are modulated by passive or active leg movements in a task specific manner [2,9,22,34]
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