Soleus motoneuron excitability after rat hindlimb unloading using histology and a new electrophysiological approach to record a neurographic analogue of the H-reflex

Hindlimb unloading (HU) induces neuromuscular disturbances in slow postural muscles such as the soleus. Among these perturbations, a reorganization of soleus motor units and decreases in peripheral afferent feedback have been reported and could change the α-motoneuron excitability. The purpose of this study was, therefore, to examine the distribution of soleus motoneuron soma sizes and the neurographic analogue of the H-reflex (NAH) induced by the soleus afferent excitation after 14 HU days. With this aim in view, we have elaborated a new electrophysiological method, discounting antidromic motor potentials, in order to record a neurographic analogue of the H-reflex, induced by soleus nerve stimulation at the L5 ventral root level instead of the electromyographic H-reflex response. Our results showed that, from a morphological point of view, the distribution of motoneuron soma sizes was modified and was shifted towards smaller sizes after HU (median 1065 μm2) in comparison with the control group (median 1314 μm2). The electrophysiological part of this study showed that thresholds and the maximal intensity of the NAH were respectively reduced by 30.5% and 27% after HU. No significant difference was obtained for the latency and the maximal amplitude of the NAH. Modifications of motoneuron excitability could be involved in the reflex adaptations observed after HU. However, changes in the presynaptic inhibition levels after HU cannot be discounted.