Sympathetic and parasympathetic neuromuscular junctions in the guinea-pig sino-atrial node

The structure and organization of cholinergic and adrenergic varicosities in the sino-atrial node of the guinea-pig heart was determined by electron microscopy. When random sections of tissue were examined, some varicosities were found in close proximity (less than 90 nm) to a muscle cell, 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 that most varicosities which lost all or part of their Schwann cell wrap formed close appositions with one or more cardiac muscle cells. At the regions of close apposition, the neuromuscular clefts were filled with a single layer of basal lamina, giving neuromuscular separations of about 80 nm. Although evidence of pre-synaptic or post-synaptic thickenings was not found, there was an accumulation of synaptic vesicles towards the regions of close apposition. These observations are discussed in relation to the idea that in a number of different tissues, most autonomic varicosities which lose part of their Schwann cell wrap, form organized neuromuscular junctions and that these junctions may be the sites of neuromuscular transmission.