Excitatory Transmission in Insect Neuromuscular Systems

Many, but not all, visceral muscles in insects are innervatedby neurosecretory axons. The neurosecretory junctions with theheart muscle of the American cockroach, Periplaneta americana,show ultrastructural and electrophysiological evidence of chemicallytransmitting synapses, and cytochemical evidence for the presenceof monoamines. Electron microscopy of nerve terminals showsthat synaptic vesicles may be formed directly from electron-dense"neurosecretory" granules Neurotomy of motor axons to skeletal muscles in insects leadsto aggregation and clumping of synaptic vesicles after 48 hours.Treatment of in vitro nerve-muscle preparations with variousrespiratory poisons caused aggregation similar to that developedin neurotomized animals. This suggested that vesicle aggregationin both cases may have resulted from a decrease in availableadenosine triphosphate in the nerve terminal with subsequentalteration in the normal charge density which supports a repulsiveforce between the vesicles.