EXPERIMENTAL EVALUATION OF INPUT-OUTPUT MODELS OF MOTONEURON DISCHARGE.
Randall K., D. B. Powers and Marc D. Binder.
Department of Physiology & Biophysics, University of Washington, School of
Medicine, Seattle, Washington 98195 USA.
APStracts 2:0262N, 1995.
SUMMARY AND CONCLUSIONS
1. We measured the modulation of the background firing rate of cat spinal
motoneurons produced by simulated, repetitive excitatory postsynaptic
potentials (EPSPs) to test the accuracy of several proposed motoneuron input-
output functions. Rhythmic discharge was elicited in the motoneurons by
injecting suprathreshold current steps of 1 - 1.5 s in duration. On alternate
trials, trains of short (0.5 - 5 ms) current pulses were superimposed upon the
current steps to simulate the effects of trains of individual EPSPs. The
increase in firing rate (DF) due to the addition of the pulses was calculated
as the difference in motoneuron discharge rate between trials with and without
the superimposed pulse trains. 2. In the same motoneurons, we were able to
study the effects of changes in pulse frequency, duration and amplitude, as
well as changes in the background discharge rate. A sub-linear relationship
between pulse rate and DF was observed, with DF rising relatively steeply with
increasing pulse frequency at low pulse rates and saturating at high pulse
rates. A similarly-shaped relation was observed between DF and pulse duration.
In contrast, DF generally increased in a greater than linear fashion with
increasing pulse amplitude. 3. In previous studies, we had demonstrated that
when a relatively constant synaptic input is produced by high-frequency
synaptic activity, DF is approximately equal to the product of the net
synaptic current reaching the soma (IN) and the slope of the motoneuron's
steady-state frequency-current relation (f/I). In the present study, this
input-output function consistently underestimated the observed DF,
particularly for low input rates, indicating that the transient current pulses
are more effective in modulating motoneuron discharge than an equivalent
amount of constant current. 3. Other investigators have proposed input-output
functions derived from the relation between synaptic potential amplitude and
the magnitude of the peak of a crosscorrelogram compiled from the discharge of
the pre-and post-synaptic neurons. These functions consistently overestimated
the observed DF, particularly for high pulse rates. This overestimation may
result in part from the fact that the effects of a synaptic potential (or
current pulse) on postsynaptic discharge probability also include a period of
decreased firing probability. Moreover, the crosscorrelation function may
depend on the arrival rate of synaptic potentials (or current pulses). 4.
Another proposed input-output function based on a simple threshold-crossing
model of the motoneuron with a fixed spike threshold predicts firing rates
that were often close to the observed DF. However, the model did not reproduce
the observed relations between DF and input pulse rate or pulse duration. 5.
The deficiencies of the basic threshold-crossing model may arise from the fact
that it does not incorporate variations in membrane conductance and firing
threshold that occur in real motoneurons. A more complete motoneuron model
that incorporates both of these features was able to replicate the observed
changes in firing rate associated with changes in input pulse frequency and
duration.
Received 8 May 1995; accepted in final form 16 August 1995.
APS Manuscript Number J313-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 15 September 1995.