Miniature end‐plate currents and potentials generated by quanta of acetylcholine in glycerol‐treated toad sartorius fibres

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Abstract
1. Part of the end‐plate region of glycerol‐treated toad sartorius muscle fibres in a hypertonic solution was voltage‐clamped using two microelectrodes. The control was adequate for recording miniature end‐plate currents (m.e.p.c.s) in the vicinity of the electrodes only, at clamp potentials from — 20 to — 100 mV. At any potential, the peak amplitude of m.e.p.c.s varied widely but their mean amplitude was linearly related to clamp potential. The equilibrium potential, obtained by extrapolation, was more positive than in normal fibres.2. The growth phase of m.e.p.c.s was linear and rapid (< 0·7 msec). The decay phase was exponential. The time constant of decay of m.e.p.c.s was affected by temperature, and increased as the membrane potential was increased. At 20° C, the time constant of decay ranged from 0·8 to 3·8 msec at membrane potentials from — 20 to — 100 mV.3. The mean conductance change caused by a quantum of acetylcholine was 5·5 × 10−8 mho.4. Voltage responses to rectangular current injections through one electrode were recorded with three other electrodes in the end‐plate region of glycerol‐treated fibres. Miniature end‐plate potentials (m.e.p.p.s) were also recorded with the same four electrodes.5. The decrement of both DC voltage responses and m.e.p.p.s along a fibre was exponential but the m.e.p.p. ‘space constant’ was significantly shorter than the DC space constant.