As a motor axon grows from the CNS to its target muscle, the terminal has the form of a flattened growth cone with a planar central region, lamellipodia, and filopodia. A mature terminal usually has a stereotyped shape that may be elongated with varicosities, as in several invertebrate species, or have short branches with boutons, as in mammals. We examined inDrosophilathe developmental changes between growth cone and mature terminal using ultrastructural and immunocytochemical methods.The transition period, which occurs 2–3 hr after the first growth cone reaches its target muscle, is marked by the formation of “prevaricosities,” smoothly contoured enlargements of the axons at the point where the nerve trunk first contacts the muscle fiber (MF). There is a 15–30 min ventral-to-dorsal gradient in the formation of prevaricosities on the individual abdominal MFs. Multineuronal innervation of each MF has occurred by this time, and two or more different axons undergo prevaricosity formation while they are intimately intertwined at the nerve entry point (NEP). Presynaptic active zones, both nerve–nerve and nerve–muscle, occur within the prevaricosities along broad contact regions. Synaptotagmin immunoreactive clusters form concurrently.The first varicosities then develop as a result of constrictions of the larger prevaricosities rather than as enlargement of discrete portions of the filopodia or neurites. The prevaricosity stage therefore may include the key steps that lead to the differentiation of functional differences in terminal subtypes as well as those leading to the formation of a stable neuromuscular junction.

SUMMARY1. The membrane of the moth muscle fibre was tested for resting permeability to various ions: it is not permeable to Mg2+ or Ca2+; it is slightly permeable to Na+ and NH4+; it is appreciably permeable to Cl-, but C1-is passively distributed; it is apparently permeable to H+ but effects of HCO3-are not ruled out; and it is primarily permeable to K+.2. Measurement of the internal K+ activity showed that E. is less negative than the resting potential.3. In the presence of DNP, or under anoxia, the membrane potential approaches, EKE; there is a small concomitant decrease in effective membrane resistance.4. An increase in external Ca2+ concentration is accompanied by increased effective membrane resistance and an increase in amplitude of the negative resting potential.5. Cooling the membrane (below room temperature) decreased the amplitude of the resting potential by 4-16 mV per 10°C, and was accompanied by a large increase in effective membrane resistance.6. The experimental results most readily fit the hypothesis that the resting potential of the moth muscle fibre, although the membrane is highly permeable to K+, Cl-and apparently to H+, is primarily maintained by an electrogenic transport process which generates an ionic current across the membrane. The possibility that the concentration gradient of H+ ions is metabolically maintained at a level sufficient to explain the resting potential was considered to be unlikely but could not be directly excluded.

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