Neuromodulation of the intrinsic stimulus current-spike frequency relationship of spinal motoneurons in the adult turtle

Abstract

Shortly following the advent of intracellular (IC) recording from spinal cord (SC) motoneurons (MNs) in the anesthetized cat, Eccles (1957) proposed that MNs behave passively in response to synaptic input. It is now known, however, that repetitive MN discharge is subject to the influence of endogenous neurotransmitters and neuromodulators that alter sub- and/or supra-threshold ionic conductances. Much of the literature in this field is anecdotal and focussed on ionic mechanisms. Further research is therefore required on the robustness of neuromodulatory effects, their significance during natural and fictive movements, and their generalization across vertebrate species. Accordingly, the purpose of this study was to quantitate the effects of neuromodulation on MN properties determined from the SC slice of the adult turtle. The present work is divided into four parts. The first (Chapter 2) summarizes the literature on MN behavior as determined from a variety of vertebrate preparations. The second (Chapter 3) provides an evaluation of the robustness of the electrophysiological measurements made from turtle SC MNs and compares them to analogous results from the lamprey and cat. The third part (Chapter 4) reports on the MNs' responses to three excitatory and one inhibitory neuromodulator. The MN population was divided into two groups on the basis of their propensity to generate plateau potentials (PPs). In PP MNs, the slope of the stimulus current-spike frequency relation was flattened to an extent comparable to recent findings in the decerebrate cat preparation. The fourth component of the study (Chapter 5) provides a comparison of MN behavior in a variety of preparations across vertebrate species, with particular emphasis on the functional efficacy of the PP. In summary, the present work has provided a new opening in the study of neuromodulation on vertebrate spinal MN properties by quantifying the effects of such modulation. In addition, the work has added further evidence supporting an evolutionary conservation of MN properties across vertebrates, with a particular emphasis on the functional significance of the PP as a key determinant of MN discharge.