Physiological regulation of synaptic effectiveness at frog neuromuscular junctions.

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Abstract
Nerve terminals in the sartorius and cutaneous pectoris (c.p.), muscles of the frog, differ sharply in safety factor. This difference is attributable to corresponding disparities in the amount of transmitter released, without evident correlated morphological differences. In Ringer containing O.3 mM Ca2+ and 1 mM Mg2+, quantal content of c.p. junctions exceeded that of sartorius junctions by 3.4 times. When quantal content was corrected for nerve terminal size, c.p. terminals still released 2.4 times more transmitter per unit terminal length. Light microscopic and EM examination of junctional morphology in the 2 muscles revealed no significant difference in the spacing of presynaptic active zones, the width of synaptic contact, or the density of presynaptic vesicles and mitochondria. The greater release at c.p. junctions is apparently due to a physiological difference between the 2 populations of terminals. No evidence could be found that action potential invasion of the terminal was less complete in the sartorius than in the c.p. The dependence of evoked and spontaneous release on Ca2+ concentration was of similar slope for terminals in the 2 muscles, but of different absolute value, consistent with the observed difference in release.