摘要Objective To compare the cross-section area (CSA) and the immunoreactivity of conjugated-ubiquitin in soleus extrafusal and intrafusal fibers after simulated-microgravity and to demonstrate the role of muscle spindle in muscle atrophy induced by simulated microgravity. Methods The immunohistochemical technique (ABC) and image analysis were used to assess the conjugated-ubiquitin immunostaining and the cross -sectional area of intrafusal and extrafusal fibers of soleus in simulated-microgravity rats. Results ①Tail-suspension caused a progressive loss of soleus mass. Mean fiber CSA of extrafusal fibers were (7±2)%, (21±4)% and (32±7)% smaller after 3 days, 7 days and 14 days suspension, respectively. While the CAS of intrafusal fibers (bag + chain fibers) were (14±3)% (P＜0.05), (30±7)% (P＜0.01) and (44±10)% (P＜0.01) smaller after 3 days, 7 days and 14 days suspension. ② The immunoreactivity of conjugated-ubiquitin both in extrafusal and intrafusal fibers increased after tail-suspension. The immunoreactivity of intrafusal fibers increased 1 day after suspension and reached the hightest level at 3 days after tail-suspension. The immunoreactivity of extrafusal fibers increased after 3 days suspension and reached the highest level after 7 days tail-suspension, which was lower than that in intrafusal fibers after 3 days tail-suspension. Conclusion These results suggest that soleus atrophy of intrafusal fibers caused by tail-suspension is earlier and more severe than that in extrafusal fibers.