Postsynaptic structure may account for variations in miniature endplate current shapes along frog neuromuscular junctions.

Intracellular recordings were made in the presence of 6-9 microM neostigmine with an electrode placed halfway between the distal end and the proximal region of a long simple branch of a frog neuromuscular junction. During the intracellular recording period, an extracellular electrode was placed alternatively at a distal and at a proximal site. The temporal correspondence between the extracellular miniature endplate current and intracellular miniature endplate potential permitted us to identify two miniature endplate potential (MEPP) populations originating, respectively, from the distal and proximal regions. The frequency and intracellular amplitude of MEPPs were higher in the proximal than in the distal region. The rise time and the decay time of the extracellularly recorded MEPC were longer in the proximal region. A significant correlation was observed between the amplitude (in mV) and surface (in mV.ms) of MEPCs. The slope of this regression line was steeper for the proximal region. Furthermore, the correlation coefficient was lower for that region owing to variations of MEPC shape. Our observations suggest that in the presence of anticholinesterase, ACh rebinds more often to acetylcholine receptors in the proximal region because of the presence of longer postjunctional folds.