Frequency and amplitude gradients of spontaneous release along the length of the frog neuromuscular junction.

Experiments were done at the neuromuscular junctions (NMJs) of the cutaneous pectoris muscle of the frog to test the validity of the "spatial decay method" which uses simultaneous recording with two intracellular electrodes, each located in the muscle fiber near a distal end of the terminal. The miniature endplate potential (MEPP) peak amplitude recorded by each electrode is used to calculate the position of the release site producing it as well as its initial amplitude in front of its release site. The validity of the method was tested with a third focal electrode located between the two intracellular electrodes, at about 60-100 microns from one of the intracellular electrodes, the probability of spontaneous transmitter release was not uniform along the NMJ either in low or in normal Ca2++ concentration. The release sites located in the region close to the first point of contact between the axon and the muscle fibre, usually close to the center of the NMJ, were spontaneously more active than the distal release sites. The degree of the steepness of proximodistal release gradients varied among different junctions. The less active regions tended to produce smaller MEPPs than did the more active ones. In fact, a correlation was found between the logarithm of MEPP frequency in a given region and the mean corrected MEPP amplitude in that region. These results also show that MEPP frequency was modulated to a greater extent than MEPP amplitude along the frog nerve terminal. The proximodistal gradient in MEPP frequency may be related to corresponding gradients in density and length of release sites along the junction, whereas MEPP amplitude gradients may be related to gradients in the length of release sites and/or postjunctional folds.