Rapid neuromuscular remodeling following limb immobilization.

The effect of immobilization on endplate morphology of the rat soleus muscles was studied qualitatively and quantitatively. The endplate was visualized by light microscopic zinc iodide osmium (ZIO) staining and by electron microscopy. The soleus muscle was immobilized by pinning of ankle and knee joints at right angles for 5 days. Immobilized muscles were then compared to the contralateral side and to normal litter mates. After 5 days of partial disuse, muscle fibers atrophied and nerve terminal area increased in ZIO-determined measurements. Neuromuscular junctions (NMJs) of disuse muscle fibers visualized by electron microscopy exhibited greater amounts of degeneration than either contralateral or control NMJs. Degeneration consisted of nerve terminal disruption, exposed junctional folds, and postsynaptic areas which contained little or no postjunctional folds. Regeneration also occurred in the same NMJs, consisting of small terminals associated with large expansion of junctional folds, several small terminals occurring within the same primary synaptic cleft, and several axons wrapped by the same Schwann cell. These observations demonstrate, for the first time, that partial disuse for only 5 days produces muscle atrophy as well as denervation-like changes at the NMJ, which leads to terminal sprouting within the endplate area and remodeling.