SYMPATHETIC AND PARASYMPATHETIC NEUROMUSCULAR-JUNCTIONS IN THE GUINEA-PIG SINOATRIAL NODE

The structure and organization of cholinergic and adrenergic varicosit ies in the sino-atrial node of the guinea-pig heart was determined by electron microscopy. When random sections of tissue were examined, som e varicosities were found in close proximity (less than 90 nm) to a mu scle celt, while others appeared to be some distance (greater than 90 nm) from the nearest muscle cell. When the organization of individual varicosities and their relationships with nearby cardiac muscle cells were determined by examining serial sections of tissue, it was found t hat most varicosities which lost all or part of their Schwann cell wra p formed close appositions with one or more cardiac muscle cells. At t he regions of close apposition, the neuromuscular clefts were filled w ith a single layer of basal lamina, giving neuromuscular separations o f about 80 nm. Although evidence of pre-synaptic or post-synaptic thic kenings was not found, there was an accumulation of synaptic vesicles towards the regions of close apposition. These observations are discus sed in relation to the idea that in a number of different tissues. mos t autonomic varicosities which lose part of their Schwann cell wrap, f orm organized neuromuscular junctions and that these junctions may be the sites of neuromuscular transmission.