Journal of Neuroscience, Vol 2, 216-224, Copyright © 1982 by Society for Neuroscience
Inverse relationship between transmitter release and terminal length in synapses on frog muscle fibers of uniform input resistance
BM Nudell and AD Grinnell
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.