Non‐linear summation of end‐plate potentials in the frog and mouse.

Abstract

Endplate potentials (e.p.p.) and endplate currents (e.p.c.) were recorded intracellularly from muscle fibers of frog and mouse at various levels of curarization to determine the relation between the potential change and the underlying synaptic conductance change over a wide range of e.p.p. amplitudes. In frog muscle fibers the e.p.p.-e.p.c. relation was linear for e.p.p. amplitudes up to about 10 mV. Beyond 10 mV the rate of increase of e.p.p. amplitude became progressively smaller as the e.p.c. amplitude increased. The equation proposed by Martin to correct for this non-linearity consistently over-corrected the e.p.p. amplitudes. When synaptic potentials and currents of long duration were produced by ionophoresis of ACh [acetylcholine] onto the endplate, the voltage-current relation showed greater non-linearity than with nerve-evoked responses. Correction of the synaptic potential amplitudes resulted in a linear relation. The relation between e.p.p. and e.p.c. amplitudes in mouse muscle showed a greater non-linearity than in frog muscle and over-correction by the equation was correspondingly smaller. Theoretical voltage-current relations were calculated for various membrane models and compared with the physiological relations. Mouse muscle relations correlated with a point synaptic contact on an infinite cable; those from frog muscle were consistent with a simple resistive-capacitative model with no cable extending from the synaptic region. Several correction factors for non-linear summation are discussed.