Active phase of frog's end-plate potential.

IT HAS BEEN CONSIDERED that the end-plate potential (e.p.p.) is generated by the brief ionic flux across the end-plate membrane and the later slowly declining phase of the e.p.p. is due to the dissipation of the charge along and across the muscle membrane. This consideration was supported by some authors. Kuffler (21) observed with a single nerve-muscle preparation that the later slowly decaying part of the e.p.p. was destroyed by a propagated muscle impulse and obtained a duration of transmitter action (3-4 msec. at 20°C.) by observing the size of the e.p.p. that was built after the invasion of a propagated muscle impulse. Katz (18) demonstrated that the neuromuscular transmitter produced a brief phase of impedance loss at the end-plate region. Recently Fatt and Katz (10) observed by measuring the displacement of the total charge along and across the muscle membrane during the e.p.p. that the active depolarization process at the end-plate had ceased within 2 msec. On the other hand the time course of the actively depolarizing phase of the e.p.p. was estimated by an analysis of the time course of the e.p.p., it being assumed that the exponentially decaying phase was attributable to the passive repolarization of the muscle membrane (7,19). The purpose of the present experiment was to determine directly the time course of the active phase of the e.p.p. by using the voltage clamp method which was originally described by Hodgkin et al. (14) and was also applied to the squid giant synapse by Tasaki and Hagiwara (29). When the membrane potential is clamped at the resting membrane potential with negative feed-back during the neuromuscular transmission, the electrotonic spread of the charge along the muscle fibre membrane can be eliminated. The feed-back current which flows through the muscle membrane to hold the membrane potential at the resting value is due to the brief electric change at the end-plate, i.e., it will show the active phase of the e.p.p. To simplify the expression, the feed-back current during neuromuscular transmission will be called provisionally the “end-plate current.” A preliminary report of the present experiment appeared in 1958 (27).