Synaptic differentiation of single descending fibers studied by triple
intracellular recording in the frog spinal cord.
Alexander E.Dityatev, and H. Peter Clamann
Department of Physiology, University of Bern, Bhlplatz 5, CH-3012 Bern,
Switzerland.
APStracts 4:293N, 1997.
ABSTRACT
Evoked EPSPs were simultaneously intracellularly recorded in two lumbar
motoneurons located in spinal segments 8-10 in response to intraaxonal
stimulation of a descending fiber. Their mean amplitudes, paired-pulse
facilitation, short- and long-term post-tetanic potentiation were compared to
reveal possible functional differences among synapses formed by one axon on
different postsynaptic targets. The mean amplitudes of EPSPs recorded in two
motoneurons were significantly different in most experiments. This amplitude
difference was related to the location of motoneurons: a) it was 2 fold larger
for motoneurons separated by more than 1 mm than in motoneurons located within
200 ĉm of one another; b) the amplitude of EPSPs recorded in motoneurons
located in the 10th segment was regularly smaller than that recorded in the
9th segment. The estimation of binomial model parameters suggests that the
difference in mean EPSPs amplitude was mostly due to a difference in the
maximal number of quanta prepared for release (binomial parameter n) and mean
release probability rather than to a difference in the quantal size. The
ability of connections formed by a single axon on different motoneurons to
undergo use-dependent synaptic modulations was different on scales of
milliseconds, seconds and tens of minutes as revealed by the measurements of
effects of paired-pulse and tetanic stimulation. The difference in magnitude
of short-term post-tetanic potentiation in connections formed by a single
descending axon was significantly correlated with the difference in mean
probability of release in these connections. Thus, our data revealed a
functional non-uniformity of synapses formed by individual descending fibers
on widely separated motoneurons, most likely innervating different muscles.
This can be one of the mechanisms by which a fine descending control of
recruitment of motoneuronal populations is achieved.
Received 27 June 1997; accepted in final form 17 October 1997.
APS Manuscript Number J546-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 29 October 1997