Spontaneous potentials and fine structure of identified frog denervated neuromuscular junctions

Morphological changes at the neuromuscular junction of the frog during nerve degeneration were correlated with the occurrence of either nerve or Schwann cell-induced miniature endplate potentials in identified edge muscle cells that were subsequently examined in the electron microscope. The initial stages of nerve deterioration produced showers of nerve-induced miniature endplate potentials but showed no apparent change in the vesicle population in nerve terminals. When the miniature endplate potentials ceased, the nerve ending appeared dense and exhibited clumped vesicles, swollen and disrupted mitochondria and membrane whorls. Nerve death was followed by movement and growth of the Schwann cell into the space previously occupied by the nerve terminal. The Schwann cell-muscle junctions were initially silent and no obvious morphological change accompanied the generation of miniature endplate potentials. Schwann cell-induced miniature endplate potentials had frequencies of 10–100/min which were comparable to the frequencies of nerve-induced miniature endplate potentials from similar sized neuromuscular junctions. Some histograms of Schwann cell-induced miniature endplate potentials showed a peak demonstrating a mean quantal size.Only a few vesicles were found in Schwann cells, including the high frequency junctions, so the morphological storage site for the production of miniature endplate potentials is not obvious. The frequencies of miniature endplate potentials evoked by Schwann cells were usually altered in the opposite direction to those evoked by nerves following various challenges, suggesting that the release mechanism for acetylcholine is quite different.