Membrane structures and physiology of an immature synapse

Summary

Immature synapses, developing moth neuromuscular junctions, were studied using electrophysiological and ultrastructural techniques and were compared with synapses from the flight muscles of adult moths. Neuromuscular junctions, formed by short side branches of the single fast motor axon, were assessed for functional state by stimulating the nerve and recording the endplate potential intracellularly from the muscle fibre. The muscle was then fixed and prepared for scanning, thin-section, and freeze-fracture microscopy.

The immature stage differs from the adult by having very small (average 7.8 m V, compared with 20–30 mV), long duration ejp's that fatigue rapidly. The immature junctions are, however, only 13% shorter than those of the adult. Within the junction, the nerve terminal comes into direct contact with the muscle membrane in a series of oval patches separated by glial processes. These regions of apposition or ‘plaques’ in the immature synapse are about half the diameter of the adult plaques. In freeze-fractured material, the nerve terminal membrane in the plaque region bears an irregular band of particles on the cytoplasmic leaflet; the length of the band is essentially the same in the immature synapse as in the adult. This band marks the location of the active zone, an electron dense bar of the same length in thin section. The apposing external leaflet of the muscle membrane bears a patch of postsynaptic particles; the patch is much smaller than in the adult plaque. These immature patches, presumably representing clusters of receptors, range in size from a dozen particles to a hundred or more. We consider it likely that a lack of postsynaptic receptors may partially explain the very small ejp in the developing synapse, but that other factors may also be limiting.

Desmosome-like contacts between glial cells and the muscle fibre were observed. Small wisps of electron dense material appear to bridge the extracellular space between the nerve terminal and the muscle fibre or between the glial processes and the muscle fibre in some locations. They are found in the same regions of the neuromuscular junction as small groups of large particles, suggesting that these two features are different aspects of the same structure. From their location one could hypothesize that they have either a mechanical function of stabilizing the glial invaginations, or a role in communication between the three types of developing cell.