Miniature End-plate Currents in Voltage-clamped Muscle Fibre

Abstract

THE acetylcholine which is released from presynaptic terminals after an action potential causes an increase in the conductance of the subsynaptic membrane of frog skeletal muscle, and the current which flows across this shunt depolarizes the membrane to give the characteristic end-plate potential (e.p.p.). The time course of this end-plate current (e.p.c.) in the frog has been determined in voltage clamp conditions by Takeuchi and Takeuchi¹. It is now recognized that acetylcholine (ACh) is released from presynaptic terminals in the form of unit packages or quanta². These quanta are released at a low rate from the “resting” terminal and each quantum causes a miniature end-plate potential (m.e.p.p.). After an action potential in the presynaptic terminals, the rate of release of quanta is increased for several milliseconds³. The time course of the end-plate current therefore reflects not only the time course of the conductance change caused by each quantum of ACh but also the time course of the release of the quanta. The time course of the former alone has been measured in the present experiments by recording the miniature end-plate currents (m.e.p.c.s) which normally produce m.e.p.p.s.