Membrane specialization at an insect myoneural junction.

Myoneural junctions were examined in the asynchronous basalar flight muscle of the beetle Pachnoda ephippiata. The outer surface of the postjunctional membrane exhibits an array of prominent projections spaced at approximately 200 A intervals which arise directly from the outer dense lamina of the plasma membrane and extend part way across the junctional cleft. The projections follow irregularities in the contour of the postjunctional membrane precisely and they end abruptly near the edge of the junctional region. No separation can be resolved between the projections and the underlying trilaminar plasma membrane after a variety of preparative methods, and the projections therefore appear to be a component part of the membrane. This specialization, which is distinctly different from that at desmosomes and hemidesmosomes, occurs nowhere else on the surface of the muscle and is interpreted as a mosaic of specialized membrane subunits which probably include the receptor sites for the transmitter.