Spontaneous and uniquantal-evoked endplate currents in normal frogs are indistinguishable.

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1. A recent paper concludes that the shapes of spontaneous and uniquantal-evoked signals are different. The signals were recorded with extracellular electrodes, often in the presence of neostigmine. Differences were reported between the voltage-time integrals, and between the decay times of spontaneous and evoked signals. 2. These results disagree with earlier studies using the two-electrode voltage clamp technique. 3. We recorded miniature-endplate currents (MEPCs) and uniquantal-endplate currents (u-EPCs) in a low-Mg(2+)-Ca2+ solution, sometimes with neostigmine present. Evoked quantal outputs were estimated by the method of failures, so we could reject the appropriate number of the largest evoked releases. The twenty-nine experiments showed that there were no consistent differences between the current-time integrals or half-decay times (t1/2), regardless of whether or not neostigmine was present. 4. When recording simultaneously with intracellular and extracellular electrodes, on average about 25% of the miniatures were seen in both recordings. On average, 63% of the endplate potentials were also seen in both recordings. Extracellular recording may not give the precise localization generally assumed. 5. We again conclude that quanta released by nerve stimulation and spontaneously are indistinguishable at normal frog neuromuscular junctions.

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