1. We spatially localized the origins of quantal currents by recording
simultaneously with two intracellular electrodes and employing the pr
ediction of the one-dimensional cable equations that the time integral
s of the resulting voltage changes fall off exponentially with distanc
e. 2. Miniature endplate potentials (MEPPs) were more frequent near th
e centre of the endplate. In contrast to some work using other methods
, we did not find MEPPs originating at the margins of the endplate to
be strikingly smaller. 3. Spontaneous MEPPs and uniquantal endplate po
tentials (EPPs) were released over the same length of endplate and wit
h the same relative probabilities at different regions. 4. Nicotinic a
gonists decreased evoked quantal output, but did not change the length
over which uniquantal EPPs were generated. We conclude they do not bl
ock nerve conduction in the terminals. 5. Data sets were obtained with
an extracellular electrode and two intracellular electrodes. The extr
acellular electrode was invariably near the centre of the region in wh
ich congruous MEPPs appeared to be generated. However, the range in th
e calculated positions of the synchronous MEPPs was as long as 0.8 mm.
Therefore, it may be possible that extracellular electrodes have a lo
nger recording range than commonly assumed.