Abstract
Recently we reported in humans that electrical stimulation of the wrist extensor muscle extensor carpi radialis (ECR) could facilitate or suppress the H reflex elicited in flexor carpi radialis (FCR), for inter-stimulus intervals (ISIs) of 30 ms or 70 ms, respectively. The facilitation at 30 ms may be produced by both flexor afferents and extensor Ib afferents acting on a spinal circuit; the origin of the suppression at 70 ms is less certain. In this study, we investigated possible descending inputs to these systems. We used magnetic stimulation of the contralateral primary motor cortex, and click sound stimulation, to activate the corticospinal and the reticulospinal tracts respectively, and measured the effects on the H reflex conditioned by ECR stimulation. Corticospinal inputs reduced both the 30 ms facilitation and 70 ms suppression, indicating corticospinal inhibition of both circuits. By contrast, we failed to show any effect of clicks, either on the H reflex or on its modulation by ECR stimulation. This suggests that click-activated reticulospinal inputs to these circuits may be weak or absent.
Highlights
Assessing the function of spinal circuits non-invasively in humans can be a challenging task, several spinal circuits as well as ways of assessing them have been described in the literature—e.g., reciprocal inhibition (Day et al, 1984), recurrent inhibition (Bussel and PierrotDeseilligny, 1977), cutaneomuscular reflexes (Jenner and Stephens, 1982), amongst others
Post hoc t-tests indicated that transcranial magnetic stimulation (TMS) had a significantly smaller effect on the H-reflex conditioned by extensor carpi radialis (ECR) stimulation than on the H reflex alone, for early facilitation delays (EFDs) 6 and 7 ms
Post hoc t-tests indicated that TMS had a greater effect on the H-reflex conditioned by ECR stimulation than on the H-reflex alone, for inter-stimulus intervals (ISIs) 25 and 40 ms
Summary
Assessing the function of spinal circuits non-invasively in humans can be a challenging task, several spinal circuits as well as ways of assessing them have been described in the literature—e.g., reciprocal inhibition (Day et al, 1984), recurrent inhibition (Bussel and PierrotDeseilligny, 1977), cutaneomuscular reflexes (Jenner and Stephens, 1982), amongst others. Cavallari et al (1985) proposed that the action of Ib fibers from wrist extensors could be detected on the wrist flexor H reflex following radial nerve stimulation. This effect was not consistent between subjects, and was superimposed on the more pronounced suppression produced by Ia afferents
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