Journal of the Autonomic Nervous System, 44 (1993) 1-16 1 © 1993 Elsevier Science Publishers B.V. All rights reserved 0165-1838/93/$06.00 JANS 01399 Sympathetic and parasympathetic neuromuscular junctions in the guinea-pig sino-atrial node * Julia K. Choate, Megan Klemm and G.D.S. Hirst Department of Zoology, University of Melbourne, Parkville, Australia (Received 23 October 1992) (Revision received and accepted 25 January 1993) Key words: Sympathetic; Parasympathetic; Varicosity; Neuromuscular junction; Sino-atrial node Abstract 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, givingneuromuscular 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. Introduction There are many suggestions that autonomic varicosities do not form specialized neuroeffector junctions [9,12,13,19]. In most studies examining the organisation of autonomic varicosities, any neuronal tissue which contained synaptic vesicles and was exposed through a break in the Schwann Correspondence to: G.D.S. Hirst, Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia. * Preliminary results of this study have been presented to a meeting of the Australian Physiological and Pharmacologi- cal Society, Monash University, Melbourne, September 1991. cell wrap was considered to be a potential site of neurotransmitter release. In the heart it has been suggested that three types of neuromuscular junc- tions exist. These are: (i) junctions which have intimate relationships (here pre- and post-junc- tional membranes are separated by less than 25 nm and the separating layer of basal lamina is absent); (ii) junctions at which there is a close apposition between the pre- and post-junctional membranes (at these junctions the cleft is 25-100 nm wide and is filled with a single layer of basal lamina); and (iii) junctions with en passant rela- tionships where the separation is over 100 nm and up to several /xm [6]. Similar neuromuscular junctions have been observed in other tissues. For example, profiles of individual varicosities inner-