Different quantal responses within single frog neuromuscular junctions

At frog neuromuscular junctions spontaneous miniature end-plate potentials (m.e.p.p.s) were recorded from several isolated spots within single synapses. This was done by consecutively placing an extracellular glass micro-electrode (focal electrode) at different recording sites, while the intracellular electrode remained in one place. After each set of recordings, muscles were stained to reveal both axon terminals and cholinesterase (ChE) such that the exact position of each recording site could be determined. In many nerve terminal branches a similar quantum size was found at several different spots. In other instances, however, mean quantum amplitudes varied by 10-60% at different spots along the same terminal branches. As a control, individual spots were recorded from repeatedly after repositioning the focal electrode. In these recordings mean m.e.p.p. amplitude varied by only 5-10%. It is concluded that quantum size within a single junction is similar at many spots, but deviates markedly at others. Correlation of this variation with the stained preparations suggested that spots where quanta significantly larger or smaller than normal were recorded were either at ChE rings or at the distal ends of nerve branches, respectively; at different nerve terminal branches within the same junction, quantum amplitudes were similar in many cases but deviated in others. The results are consistent with ultrastructural evidence that frog neuromuscular junctions are non-homogeneous structures which undergo continual remodelling.