Ultrastructure of the nerve-muscle junction in the pharynx of the earthworm,Lumbricus terrestris

Summary InLumbricus terrestris the wall of the pharynx is built up from obliquely striated longitudinal and circular muscle layers. The occurrence of perikarya and nerve bundles showing green fluorescence suggests the presence of aminergic innervation in the pharynx. A significant number of chemical synapses were detected in the neuropil among axon terminals. The junctional gap is generally 100–300 nm wide in type I junctions which resemble the cholinergic motor endplates of vertebrate skeletal muscle. A narrow junctional gap of about 25 nm is characteristic of the close contacts in the type II neuromuscular junction. Agranular spherical vesicles, together with small and large dense-cored granules, fill in these axon terminals.