Neuromuscular plasticity following limb immobilization.

The effects of immobilization on the ultrastructure of the rat neuromuscular junction of type I and type II muscle fibres were studied both qualitatively and quantitatively. Muscle fibre areas were measured as well. The plantaris muscle was immobilized in a shortened position by applying a plaster cast for three weeks. Immobilized muscles were then compared to normal litter mates. Both type I and type II immobilized muscle fibres atrophied. Endplates from type II muscle fibres exhibited greater amounts of degeneration than type I endplates. Degeneration consisted of nerve terminal disruption, exposed junctional folds, postsynaptic areas which contained little or no postjunctional folds, and subjunctional sarcoplasmic masses. In addition to degeneration, the type II endplates also exhibited regeneration in the same endplate consisting of small terminals associated with large expanses of junctional folds, several small terminals occurring within the same primary synaptic cleft, and several axons wrapped by the same Schwann cell. These observations suggest terminal axonal regeneration. Our results demonstrate that limb immobilization produces muscle atrophy as well as denervation-like changes at the neuromuscular junctions which leads to terminal axonal sprouting and an ultrastructural remodelling.