Growth-related features of lobster neuromuscular terminals

Neuromuscular terminals of the low-output type formed by the single excitor axon to the limb distal accessory flexor muscle in the lobster Homarus americanus were studied with serial section electron microscopy. This type of innervation was compared between a small and a large lobster where a two-fold difference in mean quantal content of synaptic transmission was found. Several growth-related changes in the fine structure of these low-output synaptic terminals were seen. First, there was a proliferation of multiterminal innervation consisting of an increase in the number of nerve terminals, synapses and presynaptic dense bars between the small and large lobster. Also the mean surface area of the synapses increased significantly in the large compared to the small lobster. Second, synapses possessed distinct areas of non-specialized membrane or perforations which showed a growth-related increase in their number per synapse between small and large lobsters. Such perforations also occurred in the high-output synapses but only amongst the larger synapses of the older lobster. It is proposed that these perforations subdivided synapses into smaller functional units for membrane recycling as they provide a ready source of non-synaptic axolemma for nearby active sites (dense bars). Third, the branch point between subsidiary and principal terminals as well as the ending of a terminal is composed of synaptic membrane which is presumably involved respectively in the sprouting and elongation of nerve terminals during growth. Altogether these observations signify both qualitative and quantitative changes in identified neuromuscular terminals with growth.