Neuromuscular junctions in limulus polyphemus.

The unorthodox neuromuscular junctions (NMJs) of the horseshoe crab, Limulus polyphemus, were examined with transmission electron microscopy and confocal microscopy. Large (90-120 nm) electron-dense vesicles ~ 40nm contact the muscle at the point of insertion on tendons. Small (40nm) clear vesicles are found in NMJs on sarcolemmal evaginations at 40 percent of the distance between origin and insertion. Both NMJ types are immunoreactive to antisynapsin, a monoclonal antibody to fruit-fly synapse protein