Abstract
The modulation of spinal cord excitability during rhythmic limb movement reflects the neuronal coordination underlying actions of the arms and legs. Integration of network activity in the spinal cord can be assessed by reflex variability between the limbs, an approach so far very little studied. The present work addresses this question by eliciting Hoffmann (H-) reflexes in both limbs to assess if common drive onto bilateral pools of motoneurons influence spinal cord excitability simultaneously or with a delay between sides. A cross-covariance (CCV) sequence between reflexes in both arms or legs was evaluated under conditions providing common drive bilaterally through voluntary muscle contraction and/or rhythmic movement of the remote limbs. For H-reflexes in the flexor carpi radialis (FCR) muscle, either contraction of the FCR or leg cycling induced significant reduction in the amplitude of the peak at the zero lag in the CCV sequence, indicating independent variations in spinal excitability between both sides. In contrast, for H-reflexes in the soleus (SO) muscle, arm cycling revealed no reduction in the amplitude of the peak in the CCV sequence at the zero lag. This suggests a more independent control of the arms compared with the legs. These results provide new insights into the organization of human limb control in rhythmic activity and the behavior of bilateral reflex fluctuations under different motor tasks. From a functional standpoint, changes in the co-variability might reflect dynamic adjustments in reflex excitability that are subsumed under more global control features during locomotion.
Highlights
Changes in the excitability of reflex pathways while remote limbs are performing voluntary rhythmic movements has been extensively described (Loadman and Zehr, 2007; Zehr et al, 2007; de Ruiter et al, 2010; Mezzarane et al, 2011)
During arm cycling with simultaneous bilateral SO contraction, there was an average increase in the right SO H-reflex amplitude of 91.0%, as compared to the respective control condition
These bilateral fluctuations only ceased after a longitudinal bisection of the spinal cord disrupting the action of commissural interneurons that probably mediated the synchronous fluctuations in monosynaptic reflex excitability (Manjarrez et al, 2005)
Summary
Changes in the excitability of reflex pathways while remote limbs are performing voluntary rhythmic movements has been extensively described (Loadman and Zehr, 2007; Zehr et al, 2007; de Ruiter et al, 2010; Mezzarane et al, 2011). Reflex Fluctuations during Rhythmic Movement within the muscle afferent pathway are the main targets of the influences from rhythmic movements of the remote limb These results are evidence of a reciprocal organization between networks involved in rhythmic movements within the spinal cord, i.e., cervical networks affect spinal cord excitability in the lower limbs, and vice-versa, by similar mechanism of reflex modulation. The SO muscle was more active during leg cycling, as can be confirmed by the significant main effect of the factor ‘‘movement’’ (F(1,22) = 12.259, p = 0.002)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
