P238 Homonymous Ib inhibition during remote muscle contraction

Abstract

Objective Conflicting theoretical models exist regarding the mechanism related to the ability of the Jendrassek maneuvre to reinforce reflex parameters. Our objective was to investigate if eventually inhibition of the spinal Ib motoneurone during remote muscle contraction would contribute to H-reflex enhancement. Method Soleus H-reflex from 26 healthy subjects was evoked on both sides by stimulating the tibial nerve at rest and during bilateral vigorous hadgrip. This procedure was repeated using paired stimulation at interstimulus intervals ranging from 2 ms to 15 ms. Mean conditioned H-wave amplitude expressed as a percentage of its unconditioned value (H-ratio) evoked during handgrip was compared with the H-ratio obtained at the same interstimulus interval at rest. Results The size of H-reflex evoked during handgrip was significantly higher then of that evoked at rest. The H-ratio value plotted against the interstimulus interval showed both at rest and during handgrip a similar decrease that was maximal at 5 ms and lasted less than 10 ms. Discussion The size of the conditioned H-reflex is determined by the balance between test Ia excitation and Ib inhibition elicited by the conditioning stimulus. According to our results the interneurones intercalated in pathways from Ib afferents retain their inhibitory effect on the alpha motoneurone during remote muscle contraction. Conclusion H-reflex enhancement during Jendrassek maneuvre is not due to inhibition of Ib interneurones. Significance Further studies are needed in order to test the role of other spinal circuits or descending drives that produce subthreshold excitation of alpha motoneurones by remote muscle contraction.