Conductance relationships across compartments associated with bursting output in a model of a motor neuron in the crustacean cardiac ganglion.

The crustacean cardiac ganglion network coordinates rhythmic contractions of the heart muscle to control the circulation of blood. The specific network of the crab (Cancer borealis) comprises nine cells: five large cell motor neurons (LC1-5) and four small endogenous pacemaker cells (SCs). We report a new three-compartmental biophysical LC model that includes synaptic inputs from SCs onto gap-junction coupled spike-initiation-zone (SIZ) compartments. To determine physiologically viable LC models in this realistic configuration, we sampled maximal conductances from a biologically constrained 9-D parameter space, followed by a selection protocol that had three levels. Our results provide previously unknown structure-function insights related to the crustacean cardiac ganglion large cell, including predictions about morphology, SIZ, and the differential roles of active conductances in the three compartments. An analysis of conductance relationships in model neurons revealed a lack of notable correlations among active conductances in the model population, despite clear reports of such relationships in biological neurons. When combined with the interpretations from other model systems, we hypothesize that modes of bursting driven by a strong presynaptic influence (i.e., "forced" bursting) may not require such conductance relationships, whereas endogenous bursters may require them. We further suggest that conductance relationships in a forced burster neuron will more likely serve to shape the characteristics of the firing pattern in the burst, once generated, rather than contribute to a generative mechanism for bursting itself.NEW & NOTEWORTHY Our results support the hypotheses that, despite being prevalent in biological neurons, correlated membrane conductances are not a necessary feature of synaptically driven bursting in CPG motor neurons and averaging conductances across motor neurons in a network can maintain biologically realistic output. Our results may be most relevant to similar networks, where "forced bursting" is a result of highly interconnected neurons with strong synaptic input organizing the output.