EFFECTIVE SYNAPTIC CURRENT AND MOTONEURON FIRING RATE MODULATION.
Powers, Randall K., and Marc D. Binder.
Department of Physiology & Biophysics, University of Washington, School of
Medicine, Seattle, Washington 98195 USA.
APStracts 2:0141N, 1995.
SUMMARY AND CONCLUSIONS
1 . We used a modified voltage-clamp technique to measure the steady-state
effective synaptic currents (I N ) produced by activating four different input
systems to cat hindlimb motoneurons: Ia afferent fibers, Ia-inhibitory
interneurons, Renshaw interneurons, and contralateral rubrospinal neurons. In
the same motoneurons, we measured the slope of the firing rate-injected
current (f-I) relation in the primary range. We then re-activated these
synaptic inputs during steady, repetitive firing to assess their effects on
motoneuron discharge rate. 2. Our measurements of I N were derived from
recordings made near the resting membrane potential, whereas the effects of
the synaptic inputs on repetitive discharge were evaluated at more
depolarized membrane potentials. Thus, we adjusted the I N values for these
changes in driving force based on estimates of the synaptic reversal
potential and the mean membrane potential during repetitive discharge. 3 . We
found that changes in the steady-state discharge rate of a motoneuron produced
by these synaptic inputs could be reasonably well predicted by the product of
the estimated value of I N during repetitive firing and the slope of the
motoneuron's f-I relation. Although there was a high correlation between
predicted and observed changes in firing rate for our entire sample of
motoneurons (r=0.93; p<0.001), the slope of the relation between predicted
and observed firing rate modulation was significantly greater than 1. 4. The
systematic difference between predicted and observed firing rate modulation
observed in the overall sample was primarily due to the fact that our
predictions underestimated the changes in firing rate produced by Ia
excitation and Ia inhibition. The slope of the relation between observed and
predicted firing rate modulation in response to the rubrospinal and recurrent
inhibitory synaptic inputs did not differ significantly from 1. 5. The
potential sources of error associated with our predictions of firing rate
modulation are discussed and evaluated. The greater than predicted efficacy
of Ia excitation may result from the significant transient component present
in the excitatory synaptic current. The greater than predicted decrease in
firing rate produced by Ia inhibitory synaptic input may indicate that the
effects of injected and synaptic currents on motoneuron discharge rate are not
always equivalent.
Received 30 November 1994; accepted in final form 31 March 1995.
APS Manuscript Number J747-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 9 May 1995.