Degenerative changes in the function of neuromuscular junctions of Manduca sexta during metamorphosis

In Manduca sexta the decline in neuromuscular function during metamorphic degeneration was compared in two muscles which differed characteristically with regard to pre- and postsynaptic physiological properties. In both muscles, morphological evidence indicated that a significant number of the active zones within the population of neuromuscular junctions on a given fiber were nonfunctional. Nevertheless, the degenerating nerve terminals were able to produce an above-threshold excitatory junction potential (EJP) which was facilitated in a manner characteristic of the muscle being observed. Abnormal findings during the early stages of degeneration included a larger than normal EJP, a decline in EJP amplitude over a 20 min period even with low frequencies of stimulation, an increase in EJP duration, a decline in muscle fiber resting potential amplitude with age, a decrease or disappearance of post-tetanic potentiation and long-term facilitation, and an increased likelihood that the motor nerve would fail to conduct a stimulus. The two muscles were qualitatively similar but quantitatively different with regard to these degenerative changes. It is suggested that this combination of relatively normal function with abnormal properties might be associated with the withdrawal of glial processes from the neuromuscular junctions, changes in the cable properties associated with shrivelling of the muscle fibers, and a decline in the metabolic functions supporting both muscle fiber resting potentials and those underlying transmitter synthesis, mobilization and release.