Ultrastructural studies of neuromuscular junctions in visceral and skeletal muscles of the chaetognath Sagitta setosa.

The ultrastructural characteristics of the neuromuscular junctions were studied in oesophageal (visceral) muscle and in four skeletal muscles of the head and trunk in Sagitta. Three types of neuromuscular junctions were encountered. The first is made up of nerve terminals which synapse with the surface of the muscle fiber, in a deep or in a slight depression. The second is characterized by muscle fiber protrusions that cross the connective tissue and form synapses with nerve endings; in this type, numerous post-junctional membrane folds are noted. In the third type, the synaptic cleft is very large (greater than 0.2 micron) and contains bundles of connective fibers. Nerve endings are partially ensheathed in glial cells; they contain mostly clear synaptic vesicles, though some dense-cored vesicles are noted. In many muscle fibers post-junctional membrane thickenings are also observed. All observed neuromuscular junctions resemble chemical synapses. Chaetognaths thus show a great variety of neuromuscular junction ultrastructure as do for instance Arthropods.