Post Connectome Analyses of Circuit Dynamics

Variability, Modulation and Compensation in a Rhythmic Neuronal Circuit

A great deal of effort is now being expended to characterize the connectivity diagrams or “connectomes” underlying circuits in many organisms, as a goal to understand how the nervous system generates behaviors. Experience with small circuits whose connectivity was established years ago shows that the “connectome” is absolutely necessary and completely insufficient to understand how circuit dynamics arise from the intrinsic properties of neurons and their synaptic interactions. I will describe lessons learned about neuronal circuits from a combination of computational and experimental studies on the crustacean stomatogastric ganglion. Of particular recent interest are studies that show that similar circuit outputs can be produced with highly variable circuit parameters. This suggests that the nervous system of each healthy individual has found a set of different solutions that give “good enough” circuit performance. Studies using the rhythmic central pattern generating networks in the crustacean stomatogastric nervous system argue that synaptic and intrinsic currents can vary far more than the output of the circuit in which they are found. As a corollary, to understand circuit performance, merely collecting mean data from many individuals can lead to errors. Instead, it becomes important to measure as many individual network parameters in each individual as possible. These data have implications for the kinds of changes that allow the nervous system to recover function after injury.