The effect of postsynaptic block on development of the neuromuscular junction in postnatal rats

The effects of reduced muscle activity on the ultrastructural development of the rat neuromuscular junction (NMJ) have been studied. Soleus muscles of rats were treated with alpha-bungarotoxin (alpha BTx) in order to produce a postsynaptic block of activity between the ages of 10 and 12 days. Muscles of litter mates were treated with saline over the same period. The development of these control and alpha BTx treated muscles was then compared to that of normal muscles from untreated litter mates. During the period between the tenth and twelfth days after birth, normal soleus NMJs show two major ultrastructural changes. 1. The average number of axon terminal profiles present at each endplate decreases. This is thought to reflect the withdrawal of superfluous axons from the endplates. 2. There is an increasing specialization of the postsynaptic structures of the junction: complexity of folding of the muscle junctional membrane increases, as does the accumulation of subjunctional sarcoplasm and muscle nuclei. In soleus muscles treated with alpha BTx, the number of axon terminal profiles observed at the endplates does not decrease, suggesting that elimination of supernumerary axons does not occur. In addition, specialization of the postsynaptic structures of the NMJ is retarded during the period of ACh receptor block.