The Journal of Neurophysiology Vol. 81 No. 3 March 1999, pp. 1135-1146
Copyright ©1999 by the American Physiological Society
Quantal Secretion and Nerve-Terminal Cable Properties at
Neuromuscular Junctions in an Amphibian (Bufo marinus)
G. T.
Macleod,
L.
Farnell,
W. G.
Gibson, and
M. R.
Bennett
The Neurobiology Laboratory, Institute for Biomedical Research, The
Department of Physiology and The School of Mathematics and Statistics,
University of Sydney, New South Wales 2006, Australia
Macleod, G. T.,
L. Farnell,
W. G. Gibson, and
M. R. Bennett.
Quantal secretion and nerve-terminal cable
properties at neuromuscular junctions in an amphibian (Bufo
marinus). The effect of a conditioning depolarizing
current pulse (80-200 µs) on quantal secretion evoked by a similar
test pulse at another site was examined in visualized motor-nerve
terminal branches of amphibian endplates (Bufo marinus).
Tetrodotoxin (200 nM) and cadmium (50 µM) were used to block
voltage-dependent sodium and calcium conductances. Quantal release at
the test electrode was depressed at different distances (28-135 µm)
from the conditioning electrode when the conditioning and test pulses
were delivered simultaneously. This depression decreased when the
interval between conditioning and test current pulses was increased,
until, at an interval of ~0.25 ms, it was negligible. At no time
during several thousand test-conditioning pairs, for electrodes at
different distances apart (28-135 µm) on the same or contiguous
terminal branches, did the electrotonic effects of quantal release at
one electrode produce quantal release at the other. Analytic and
numerical solutions were obtained for the distribution of transmembrane
potential at different sites along terminal branches of different
lengths for current injection at a point on a terminal branch wrapped
in Schwann cell, in the absence of active membrane conductances.
Solutions were also obtained for the combined effects of two sites of
current injection separated by different time delays. This cable model
shows that depolarizing current injections of a few hundred
microseconds duration produce hyperpolarizations at ~30 µm beyond
the site of current injection, with these becoming larger and occurring
at shorter distances the shorter the terminal branch. Thus the effect
of a conditioning depolarizing pulse at one site on a subsequent test
pulse at another more than ~30 µm away is to substantially decrease
the absolute depolarization produced by the latter, provided the
interval between the pulses is less than a few hundred microseconds. It
is concluded that the passive cable properties of motor nerve terminal
branches are sufficient to explain the effects on quantal secretion by a test electrode depolarization of current injections from a spatially removed conditioning electrode.
