Submitted on December 4, 2007
Accepted on January 26, 2008
Short-term plasticity of spinal reflex excitability induced by rhythmic arm movement
Bahar J-Rohbakhsh1 and E. Paul Zehr2*
1 RehabNeuroLab, Universithy of Victoria, Victoria, Canada
2 Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, Canada
* To whom correspondence should be addressed. E-mail: pzehr{at}uvic.ca.
Rhythmic arm movement reduces Hoffmann (H-) reflex amplitudes in leg muscles by modulation of presynaptic inhibition in group Ia transmission. To date only the acute effect occurring during arm movement has been studied. We hypothesized that the excitability of soleus H-reflexes would remain suppressed beyond a period of arm cycling conditioning. Subjects used a customized arm ergometer to perform rhythmic 1Hz arm cycling for 30 min. H-reflexes were evoked before, during and after arm cycling via stimulation of the tibial nerve in the popliteal fossa. The most important finding was that the H-reflex amplitudes were significantly suppressed during and up to 20 min after arm cycling had been terminated. Thus, remote arm cycling can induce adaptive plasticity in the soleus H-reflex pathway that persists beyond the period of conditioning. In an additional experiment the prolonged effect of arm cycling combined with cutaneous superficial radial (SR) nerve stimulation was investigated. Cutaneous stimulation cancelled the prolonged suppression of H-reflex amplitude induced by arm cycling. Since SR nerve stimulation facilitates soleus H-reflex via reducing the level of Ia presynaptic inhibition, persistence in presynaptic inhibitory pathways is suggested as the underlying neural mechanism. The simplest explanation of this observation is plateau potential-like behaviour of interneurons mediating presynaptic inhibition of Ia afferent transmission.
