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1 School of Health and Sport Sciences, Osaka Univertsity, Toyonaka City, Osaka, Japan; Graduate School of Medicine, Osaka Univertsity, Suita City, Osaka, Japan
2 Faculty of Integrated Human Studies, Kyoto University, Kyoto City, Kyoto, Japan
3 School of Dentistry, University of California, Los Angels, Los Angels, California, USA
4 Graduate School of Medicine, Osaka Univertsity, Suita City, Osaka, Japan
5 School of Health and Sport Sciences, Osaka Univertsity, Toyonaka City, Osaka, Japan
6 Faculty of Dentistry, Osaka University, Suita City, Osaka, Japan
7 National Center for Neurology and Psychiatry, Kodaira City, Tokyo, Japan
* To whom correspondence should be addressed. E-mail: ohira{at}space.hss.osaka-u.ac.jp.
Responses of electromyogram (EMG) in soleus muscle and both afferent and efferent neurograms at the L5 segmental level of spinal cord were investigated during acute and chronic unloading induced by hindlimb suspension and/or tenotomy in adult rats. The soleus EMG and afferent neurogram decreased 88% and 37%, respectively, relative to those at quadrupedal posture on the floor, following acute hindlimb suspension that causes passive shortening of soleus due to ankle plantarflexion. However, the afferent neurogram (p<0.05) and soleus EMG (p>0.05) recorded on the floor increased following tenotomy of synergists. Further, the afferent input was inhibited, when the soleus EMG disappeared after tenotomy of soleus. The afferent neurogram and EMG of the soleus showed correlated responses to a variety of treatments, suggesting that the afferent neurogram recorded at the L5 segmental level reflects the neural input associated with the activity level of the soleus predominantly. The level of efferent neurogram decreased following acute hindlimb suspension, but was not influenced significantly by tenotomy of synergists and/or soleus itself. The EMG and afferent neurograms remained low up to the 4th day, but recovered to the pre-experimental levels within 14 days, due to reorganization of sarcomere number and length, as well as the shortening of muscle fiber length and recovery of tension development. It is suggested that the levels of EMG and afferent neurogram associated with antigravity muscle are closely related to the tension development of the muscle.
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