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The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 86-97
Copyright ©2002 by the American Physiological Society
Department of Physiology, The University of Arizona, College of Medicine, Tucson, Arizona 85724-5051
Hornby, T. George,
Jennifer C. McDonagh,
Robert M. Reinking, and
Douglas G. Stuart.
Effects of Excitatory Modulation on Intrinsic Properties of
Turtle Motoneurons. J. Neurophysiol. 88: 86-97, 2002. The purpose of this study was to quantify the
effects of excitatory modulation on the intrinsic properties of
motoneurons (MNs) in slices of spinal cord taken from the adult turtle.
Responses were noted following application of an excitatory modulator:
serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane
dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs
was divided into 2 groups, on the basis of whether MNs did (28/44) or
did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such
acceleration indicates the development of a plateau potential (PP).
Excitatory modulation lowered the MNs' resting potential, increased
input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to
the left. For both MN groups, the relative efficacy of excitatory
modulation on both non-PP and PP MNs was generally in the following
order: combined application > 5-HT 
muscarine > tACPD. In many instances, the effects of modulation differed
significantly for non-PP versus PP MNs, the most pronounced being in
their I-f relation. To describe this difference, it was
necessary to measure a two-phase relation. In PP MNs, excitatory
modulation considerably increased the slope of the first (initial)
phase and flattened the second (later) phase of this relation. The
latter result bore similarities to that obtained in a previous study,
which addressed MN firing behavior during fictive locomotion of the
high-decerebrate cat.
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