Some Observations Concerning the End-Plate Potential

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Abstract
The properties of the motor end-plate were studied by recording the end-plate potential (e. p. p.) and the end-plate current (e. p. c.) by means of an intracellular microelectrode and by measuring the equilibrium potential for the e. p. p. in Na-lack and Cl-lack (Cl ions are replaced with NO3, SO4 and I ions) Ringer solution. The e. p. c. measured by the voltage-clamp method in the end-plate region is the current required to keep the membrane potential at the resting level during neuromuscular transmission and represents the temporal change of shunt resistance across the membrane potential. The time course of the e. p. c. when compared with that of the e.p. p. showed a characteristic difference. Experiments were made to show whether a potential similar to the e. p. p. could be reconstructed at the membrane if the shunting resistance is changed according to the time course of the e. p. c. The calculations were made using an analog computor on an electrically equivalent circuit of the end-plate region. The time course of the computed e. p. p.''s under various conditions was similar to that of the natural e. p. p.''s. The relationship between the magnitude of the e. p. p. and the intensity of the e. p. c, and the time course between them under various curarine concentration were discussed. When the membrane potentials under the clamped condition were shifted at various levels for 200 msec and indirect stimulations were applied the relationships between the membrane currents at the e.p. c.''s were obtained. The effective resistance of the resting membrane calculated from those relationships was 200-600 K [OMEGA] . A straight line connecting the peak amplitude of the e. p. c.''s plotted against various clamped voltages of the membrane crossed the abscissa at 50 to 80 mV, while the resting potential was usually -70 to -90 m V. Thus the equilibrium potential for the e. p. p. was -10 to -30 m V of the membrane potential, and the minimum value of the shunting resistance across the membrane potential in the end-plate region, calculated from the tangents formed by these relationships, was 600K[OMEGA] to 2M [OMEGA] under minimum curarized conditions. vVhen the external Na ions were replaced by sucrose-Ringer solution, the e. p. p. and the e. p. c. were sometimes reversed. The equilibrium potential in Na-lack solution shifted near to the normal resting potential and the resting potential went up towards zero by 20 to 30 mV. The mechanism for the production of the reversed e. p. p. and the reversed e. p. c. were discussed from these two potential levels, and it was concluded that the same mechanism produces the inhibitory synaptic potential obtained from other tissues. The effects of various anions, such as NO3, SO4 and I, on the e. p. p. and the e. p. c. were investigated. The resistance of the resting membrane was increased by about twiceby full replacement of Ringer solution with Cl-lack solution. The amplitude of the e. p. p. was enlarged by NO3 but not by SO4. From the relationships between the magnitude of the e. p. p.''s and the intensity of e. p. c.''s in sucrose and NO3 solution, it was clear that the amplitude of e.p.p. in NO3 increased without an increase of the intensity of the e.p. c. Thus the increase of the e. p. p. in NO3 solution is due to the resistance increase of the resting membrane.