Constancy and variability in the output of a central pattern generator

• Authors: Brian J. Norris, Emory University; Angela Wenning, Emory University; Terrence Michael Wright, Emory University; Ronald Calabrese, Emory University
• Journal Title: Journal of Neuroscience Nursing
• Volume: Volume 31, Number 12
• Publisher: Society for Neuroscience | 2011-03-23, Pages 4663-4674
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1523/JNEUROSCI.5072-10.2011
• Subjects: Biology, Neuroscience
• Keywords: leech; synaptic strength; motor neurons; network model

Abstract

Experimental and corresponding modeling studies have demonstrated a 2–5 fold variation of intrinsic and synaptic parameters across animals, while functional output is maintained. These studies have led to the hypothesis that correlated, compensatory changes in particular parameters can at least partially explain the biological variability in parameters. Using the leech heartbeat CPG, we selected three different segmental motor neurons that fire in a functional phase progression but receive input from the same four premotor interneurons. Previous work suggested that the phase progression arises because the pattern of relative strength of the four inputs varies systematically across the segmental motor neurons. Nevertheless, there was considerable animal-to-animal variation in the absolute strengths of these connections. We tested the hypothesis that functional output is maintained in the face of variation in the absolute strength of connections because relative strengths onto particular motor neurons are maintained. We found relative strength is not strictly maintained across animals even as functional output is maintained, and animal-to-animal variations in relative strength of particular inputs do not correlate strongly with output phase. In parallel with this variation in synaptic strength, the firing phase of the premotor inputs to these motor neurons varies considerably across individuals. We conclude that the number (four) of inputs to each motor neuron, which each vary in strength, and the phase diversity of the temporal pattern of input from the CPG diminish the influence of individual inputs. We hypothesize that each animal arrives at a unique solution for how the network produces functional output.