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 alpha-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 microm2) in comparison with the control group (median 1314 microm2). 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.

Physiologically adaptive changes of the L5 afferent neurogram and of the rat soleus EMG activity during 14 days of hindlimb unloading and recovery.

The hindlimb unloading rat model (HU, Morey's model) is usually used to mimic and study neuromuscular changes that develop during spaceflights. This Earth-based model of microgravity induces a muscular atrophy of the slow postural muscle of hindlimbs, such as the soleus, a loss of strength, modifications of contraction kinetics, changes in histochemical and electrophoretical profiles and modifications of the tonic EMG activity. It has been suggested in the literature that some of these neuromuscular effects were due to a reduction of afferent feedback during HU. However, no direct data have confirmed this hypothesis. The aim of this study was to clearly establish if changes of the L5 afferent neurogram are closely related to the soleus EMG activity during and after 14 days of HU. Immediately after HU, the EMG activity of the soleus muscle disappeared and was associated with a decrease in the afferent neurogram. The soleus electromyographic and afferent activities remained lower than the pre-suspension levels until the sixth day of HU and were recovered between the sixth and the ninth day. On the twelfth and fourteenth days, they were increased beyond the pre-suspension levels. During the first recovery day, these activities were significantly higher than those on the fourteenth HU day and returned to the pre-suspension levels between the third and sixth recovery days. To conclude, our study directly demonstrates that the HU conditions cannot be considered as a functional deafferentation, as suggested in the literature, but only as a reduction of afferent information at the beginning of the HU period.