Aging produces striking morphological changes in NMJs of the mouse sternomastoid muscle. Confocal images of a young (2 months of age) NMJ (A) and an old, fragmented NMJ (B) (bottom panels; 26 months of age). In both cases, a motor nerve terminal (NT) is present on the surface of the muscle fiber, formed by branches of a single motor axon. In the young junction (A), the NT branches form smooth contacts with the muscle fiber surface where a high concentration of AChRs are accumulated into largely continuous gutters, giving the appearance of a pretzel. The synapse has a small number of glial cells (tSCs) that cover the branches of the NT. In contrast, the NMJ in the old animal (B) has AChRs that are fragmented into small islands that are each apposed by a varicosity in the NT; varicosities in the NT are commonly linked by thin neurites. The tSCs appear more active and are commonly seen to be extending short processes to the sides of the NT. NT and SCs were labeled by transgenic expression of fluorescent proteins (CFP and GFP, respectively), and AChRs with a subsaturating concentration of Alexa 647 bungarotoxin. Scale bar, 10 μm.