Journal of Neurophysiology, Vol 60, Issue 5 1638-1652, Copyright © 1988 by APS
Facilitation and depression in the responses of spinal Renshaw cells to random stimulation of motor axons
U. Windhorst, R. Rissing, J. Meyer-Lohmann, Y. Laouris and U. Kuipers
Zentrum Physiologie und Pathophysiologie, Universitat Gottingen, Federal Republic of Germany.
1. We investigated the responses of cat lumbosacral Renshaw cells to
pseudo-Poison stimulus sequences (of three different mean rates) delivered
to motor axons in ventral roots or various muscle nerves. The Renshaw cell
responses were evaluated by computation of peristimulus time histograms
(PSTHs). 2. PSTHs computed with respect to all the stimuli showed, before
the reference time, near-constant bin contents corresponding to the mean
firing probability (rate), and an initial excitatory component (increase in
discharge probability) after the reference time, followed by a small but
longer-lasting reduction of firing rate. These two response components were
strongly correlated linearly. It is suggested that the postexcitatory rate
reduction is predominantly due to afterhyperpolarization. 3. In general,
Renshaw cell responses to any stimulus in a stimulus train depended upon
the stimulation history. In the averaged record, the response to the second
of a pair of stimuli was affected by the first stimulus independently of
intervening (random) stimuli. Very often, the second response showed a
long-lasting depression (from 25 to greater than 250 ms). In a number of
cases a briefer facilitating effect preceded the depression. 4. These
conditioning effects were largely homosynaptic, i.e., confined to the
particular input channel that was stimulated. This was shown by stimulating
two different nerves (or nerve branches) with independent random patterns
of similar mean rates and determining the cross-conditioning exerted by one
input channel on the excitatory effects of the other. At small intervals
between conditioning and test stimuli of some tens of milliseconds, a
facilitatory effect could often be seen, which almost certainly reflected
spatial summation. However, the subsequent depressant effect was largely
accounted for by the postexcitatory rate reduction consequent to the
conditioning stimulus in the parallel channel. Autoconditioning was still
present. 5. The amount of facilitation and depression as well as their
balance depended on the average Renshaw cell response. This in turn
depended, at each mean stimulus rate, on the strength of synaptic coupling
between an input channel and the cell, and on the mean stimulus rate,
declining with an increase in mean rate. That is, the facilitation
increased and the depression decreased with decreasing synaptic coupling
and increasing mean stimulus rate. 6. Several factors may contribute to
facilitation and depression; these are discussed with respect to their
relative quantitative significance.(ABSTRACT TRUNCATED AT 400 WORDS)
