Inverse relationship between transmitter release and terminal length in synapses on frog muscle fibers of uniform...

Transmitter release at frog neuromuscular junctions is known to be related positively to nerve terminal length. However, the correlation is inexact, with a wide scatter of data. We have analyzed the endplate potentials (EPPs) of identified frog cutaneous pectoris muscle fibers, correlating release with terminal size and fiber input resistance (Rin). Transmitter release was assayed by quantal analysis of endplate activity in low Ca2+ Ringer solution and by measurement of the EPPs evoked in normal Ringer solution and curare. For fibers of approximately the same Rin, there is an inverse relationship between the level of transmitter release per unit length and total terminal length. Terminals with high levels of release per unit length tend to be shorter than do those which release relatively less transmitter per unit length. Furthermore, if the analysis is restricted similarly to fibers with nearly identical Rin, the total transmitter release of the largest endplates is usually less than that of the shorter terminals in the sample. These findings do not contradict the overall trend of greater release and longer terminals on larger muscle fibers (with lower Rin). Instead, they help explain the variability in measurement of release versus terminal length. The relationship that we find is consistent with the hypothesis that, in the cutaneous pectoris, terminals are induced to grow until an adequate safety factor for transmission is achieved.