The role of muscle activity in the differentiation of neuromuscular junctions in slow and fast chick muscles.

The differentiation of neuromuscular junctions of multiply innervated, slow anterior latissimus dorsi (ALD) and focally innervated, fast, posterior latissimus dorsi (PLD) muscles was studied in normal and curarized chick embryos. At 16 days of incubation, fibres of both muscles are contacted by several axon profiles, the number of which falls with age. In 18-day-old embryos individual endplates in ALD are usually contacted by three axon profiles, whereas in PLD, endplates are contacted only by a single large terminal profile. At this time, there is already a significant accumulation of cell organelles in the postsynaptic area. Treatment of embryos with curare during the 7th and 12th day of incubation delays the differentiation of the neuromuscular junction in both muscles. The paralysis dramatically affects the decrease of the number of axon profiles at individual endplates in both muscles. At 16 days the number of axon profiles was greater in embryos treated with curare than in the untreated controls. At 18 days when the number of axon profiles normally decreases, the endplates of both types of curarized muscles have an even greater number of axon profiles than at 16 days. Endplates in curarized PLD had up to 13 and in curarized ALD up to 12 axon profiles. The effects of curare gradually wore off and when the movements of the embryos again became more vigorous, the normal differentiation of neuromuscular junctions continued. At 21 days of incubation many embryos recover from curare and show endplates of normal appearance in both muscles. These results suggest that activity of the muscle is essential for the maturation of the neuromuscular junctions.