Facilitation and the conduction of the nerve action potential at the frog neuromuscular junction

We have measured the time intervals (latencies) between the delivery of paired stimuli and the appearance of the extracellularly recorded nerve terminal action potentials (n.a.p.s) and end-plate currents (e.p.c.s) at different temperatures in the sciatic nerve, sartorius muscle preparation of the frog. When the interstimulus interval was 70 ms, the n.a.p. and e.p.c. latencies following the second stimulus of the pair were shorter than following the first at 10 degrees C and 15 degrees C, and facilitation of the e.p.c. was observed. The shortened latency was also observed at 20 degrees C by Barton and Cohen (1982), who suggested that this may be related to facilitation. At 5 degrees C, however, the n.a.p. and e.p.c. latencies following the second stimulus were longer than following the first, and facilitation was still observed. When a 30 ms interstimulus interval was used, the changes in latency were even greater at the temperatures studied. The changes in e.p.c. latency were due to changes in the conduction velocity of the n.a.p., not changes in synaptic delay. At all temperatures and interstimulus intervals studied, the larger amplitude n.a.p.s were conducted more slowly, however, facilitation was always observed. We conclude that changes in the amplitude and conduction velocity of the nerve action potential do not obviously affect facilitation at the frog neuromuscular junction.