The fine structure of neuromuscular junctions in the segmental muscles of the blowfly larva

Abstract

In the larva of Phormia terrae-novae the terminal regions of the motor axons are naked and lie on the surface of the muscle cell in grooves formed by indentations of the sarcolemma. The axolemma and sarcolemma are closely apposed. The area of unapposed axolemma which faces the haemolymph is isolated from it by only a thin covering of basement-membrane material. Glycogen particles, mitochondria, neurosecretory vesicles, and synaptic vesicles are present in the presynaptic axons. Beneath the nerve terminals, the sarcoplasm contains a system of convoluted membranes, the subsynaptic reticulum, which is invaginated from the sarcolemma. Two anatomical types of junctional contact occur between the apposed sarcolemma and axolemma. Junctions of the first type have none of the structural features attributed to synaptic contacts. The axolemma is wrinkled in outline and appears to extrude vesicular material from its surface. It is suggested that these junctions might subserve a trophic function. Junctions of the second type are thought to be synaptic contacts. The axolemma and sarcolemma are separated by a narrow electron-opaque cleft that is traversed by septate desmosomes. Synaptic vesicles are associated with electron-dense bodies which are juxtaposed to the cytoplasmic surface of the axolemma. The physiological significance of some of the structural features of the insect neuromuscular junction is discussed.