Dependence of motoneurone properties on the length of immobilized muscle.

1. The soleus muscle of adult cats was fixed in a shortened or lengthened position for 2 weeks by immobilizing the leg with a plaster cast. To eliminate the effects of reflex activity arising from the muscle, the lumbosacral dorsal roots were sectioned on the side of leg immobilization.2. The soleus muscle showed a significant decrease in weight after immobilization in a shortened position. Immobilization of the muscle in a lengthened position produced no significant changes in its weight.3. Two weeks after transection of the throacic spinal cord, the soleus muscle showed a significant decrease in weight. This decrease was prevented by immobilization of the muscle in a lengthened position. Immobilization in a shortened position caused no further muscle atrophy in cord-transected animals.4. Soleus motoneurones showed a significant decrease in the duration of after-hyperpolarization (a.h.p.) 2 weeks after immobilization of the muscle in a shortened position, whereas immobilization of the muscle in a lengthened position caused no significant changes in the duration of a.h.p.5. A decrease in the duration of a.h.p. of soleus motoneurones induced by cord transection was significantly prevented by immobilization of the muscle in a lengthened position but not by immobilization in a shortened position.6. In a variety of experimental conditions, the mean duration of a.h.p. of soleus motoneurones was significantly correlated with the soleus muscle: body wt. ratio.7. It is concluded that certain motoneurone properties depend upon the condition of the innervated muscle and that the primary factor responsible for the trophic influence is metabolic change in the muscle rather than contractile activity itself.