Physiologically adaptive changes of the L5 afferent neurogram and of the rat soleus EMG activity during 14 days of hindlimb unloading and recovery
2005
SUMMARY The hindlimb unloading rat model (HU, Morey9s 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 L 5 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.
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