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In adult male fiddler crabs, Uca pugnax, there is a marked enlargement of the 1st thoracic ganglion and its nerve root on the side of the major cheliped compared to the side of the minor cheliped. Retrograde uptake of cobalt via the cut ends of the motoneurons revealed a significant hypertrophy of their somata and dendritic fields on the major side of the ganglion compared to the minor side in the male fiddler crabs. (In female fiddler crabs which have two minor chelipeds the motoneurons were similar in size on both sides of the ganglion.) Since the number and distribution of motoneuron somata was relatively constant in the two halves of each ganglion, homologies for individual or groups of neurons could be recognized. The number of axon profiles in a cross-sectional montage of the entire nerve root of the major side in a male fiddler crab was several times greater than that of the minor side in random samples which were appropriately scaled in area. In samples of equal areas the axonal density was similar on the major and minor sides, as was also the range of axon diameters; both signify no difference in size of axons between the contralateral nerve roots. Consequently enlargement of the nerve root on the major side is due to a relative increase in the number of axons. This increase is in sensory fibers since the number of motor fibers are bilaterally constant. Thus neural asymmetry in male fiddler crabs involves hypertrophy of the motoneurons and hyperplasia of the sensory neurons associated with the enlarged condition of the major cheliped.

Lobster neuromuscular terminals releasing comparatively small (low-output type) and large (high-output type) amounts of transmitter but arising from the single excitatory motor axon to the proximal accessory flexor muscle were serially sectioned for electron microscopy. The three-dimensional reconstruction showed the two types of terminals to have a complex branching pattern in which thin branches of the motor axon often enlarged into synapse bearing terminal regions. Quantitative comparison showed that the mean surface area of a synapse is similar in the two types of terminals. However, the low-output terminal has a higher synaptic density and devotes a greater part of its surface area to synapses compared to its high-output counterpart suggesting that transmitter output is not directly related to synaptic area. The mean surface area of a presynaptic dense body is not significantly different between low- and high-output synapses, but there is a significantly greater density of these active zones in the high-output terminal. This results in the ratio of mean dense body area to mean synaptic area being approximately 3 X greater in the high-output synapses than the low-output ones. This significant difference in the surface area of presynaptic dense bodies between low- and high-output synapses correlates with the difference in transmitter output at these two synapses, and implicates the dense bodies in the mechanism of transmitter release at lobster neuromuscular synapses.

The effects of decentralization on the physiology and ultrastructure of neuromuscular terminals were examined by transecting the single excitor axon to the distal accessory flexor muscle in the walking legs of lobsters (Homarus americanus). Decentralization caused a reduction in the amplitude of the excitatory junctional potential without altering the resting potential or input resistance of the muscle fiber thereby suggesting a reduction in transmitter release. Confirmation was obtained by recording of synaptic currents at focal sites which showed failure of transmission and a reduced amplitude on decentralized fibers compared to their intact counterparts on the contralateral leg. The mean quantal content of synaptic transmission decreased approximately 2-7-fold at these decentralized sites compared to their intact counterparts. The ultrastructure of these identified sites was examined with serial section electron microscopy. There are few if any qualitative changes in synaptic ultrastructure between decentralized and control terminals. However, quantitatively there were changes in synaptic ultrastructure which were progressive in nature depending on the severity of the reaction to decentralization. Thus terminals showing a moderate decline in quantal content were characterized by a reduction in the number of presynaptic dense bars and synapses. Terminals showing a severe drop in transmitter release showed in addition to the above changes, a reduction in the size of synapses and terminals. These results show a progression in the loss of the structural parameters controlling transmitter release. Finally synaptic vesicles and mitochondria did not reveal any consistent or marked change with decentralization.

Sensory innervation to the paired claws of the lobster. Homarus americanus, was examined during their differentiation from a bilaterally symmetric state to an asymmetric state of a slender cutter and a stout crusher claw. This was done by estimating the total number and size distribution of axons in the second nerve root which provides most (approximately 90%) of the innervation to the claws and has few, bilaterally distributed motor axons. The paired claws which are undifferentiated and resemble each other in the 1st larval stage correspondingly have nerve roots that are bilaterally symmetric. In early juvenile (4th and 5th) stages when claw type is determined, as well as in subsequent (6th, 7th, 8th, 16th) juvenile stages when the claws gradually differentiate into cutter and crusher types, the paired homologous roots are also similar. It is only in adults that asymmetry in sensory innervation is seen with more axons in the crusher root than in its cutter counterpart. The difference in number of axons between the dimorphic claws is related to differences in surface area between the claws. Thus, bilateral asymmetry in sensory innervation is acquired by the continual but differential addition of axons to the paired claws.

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.