Abstract
1. Nerve terminals in two different muscles of the frog, the sartorius and cutaneous pectoris (c.p.), have been found to differ sharply in safety factor. This difference is shown to be attributable to corresponding disparities in the amount of transmitter released, without evident correlated morphological differences. 2. In Ringer containing 0.3 mM-Ca2+ and 1 mM-Mg2+, quantal content of c.p. junctions exceeded that of sartorius junctions by 3-4 times. 3. When quantal content was corrected for nerve terminal size, c.p. terminals still released 2-4 times more transmitter per unit terminal length. 4. Light and electron microscopic examination of junctional morphology in the two 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. It seems likely, therefore, that the greater release at c.p. junctions is due to a 'physiological' difference between the two populations of terminals. 5. No evidence could be found that action potential invasion of the terminal was less complete in the sartorius than in the c.p. 6. The dependence of evoked and spontaneous release on Ca2+ concentration was of similar slope for terminals in the two muscles, but of different absolute value, consistent with the observed difference in release.