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The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 483-500
Copyright ©2000 by the American Physiological Society
Department of Physiology and Biophysics, School of Medicine, University of Washington, Seattle, Washington 98195
Powers, Randall K. and
Marc D. Binder.
Summation of Effective Synaptic Currents and Firing Rate
Modulation in Cat Spinal Motoneurons. J. Neurophysiol. 83: 483-500, 2000. The aim of this study was to examine
how cat spinal motoneurons integrate the synaptic currents generated by
the concurrent activation of large groups of presynaptic neurons. We
obtained intracellular recordings from cat triceps surae motoneurons
and measured the effects of repetitive activity in different sets of
presynaptic neurons produced by electrical stimulation of descending fibers or peripheral nerves and by longitudinal vibration of the triceps surae muscles (to activate primary muscle spindle Ia afferent fibers). We combined synaptic activation with subthreshold injected currents to obtain estimates of effective synaptic currents at the
resting potential (INrest) and at the
threshold for repetitive discharge
(INthresh). We then superimposed synaptic
activation on suprathreshold injected current steps to measure the
synaptically evoked change in firing rate. We studied eight different
pairs of synaptic inputs. When any two synaptic inputs were activated concurrently, both the effective synaptic currents
(INrest) and the synaptically evoked changes
in firing rate generally were equal to or slightly less than the linear
sum of the effects produced by activating each input alone. However,
there were several instances in which the summation was substantially
less than linear. In some motoneurons, we induced a partial blockade of
potassium channels by adding tetraethylammonium (TEA) or cesium to the
electrolyte solution in the intracellular pipette. In these cells,
persistent inward currents were evoked by depolarization that led to
instances of substantially greater-than linear summation of injected
and synaptic currents. Overall our results indicate that the spatial distribution of synaptic boutons on motoneurons acts to minimize electrical interactions between synaptic sites permitting near linear
summation of synaptic currents. However, modulation of voltage-gated
conductances on the soma and dendrites of the motoneuron can lead to
marked nonlinearities in synaptic integration.
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