Excitatory Transmission in Insect Neuromuscular Systems

Abstract

Many, but not all, visceral muscles in insects are innervated by neurosecretory axons. The neurosecretory junctions with the heart muscle of the American cockroach, Periplaneta americana , show ultrastructural and electrophysiological evidence of chemically transmitting synapses, and cytochemical evidence for the presence of monoamines. Electron microscopy of nerve terminals shows that synaptic vesicles may be formed directly from electron-dense “neurosecretory” granules Neurotomy of motor axons to skeletal muscles in insects leads to aggregation and clumping of synaptic vesicles after 48 hours. Treatment of in vitro nerve-muscle preparations with various respiratory poisons caused aggregation similar to that developed in neurotomized animals. This suggested that vesicle aggregation in both cases may have resulted from a decrease in available adenosine triphosphate in the nerve terminal with subsequent alteration in the normal charge density which supports a repulsive force between the vesicles.