Phylogenetic and individual variability of neural circuits underlying swimming behaviors in sea slugs
Comparing the roles of homologous neurons in the generation of rhythmic motor patterns provides an unambiguous means to assess the relationship between homology and function in the evolution of behavior. We have explicitly studied this using the neural circuits underlying swimming behaviors of different Nudipleura species (Mollusca, Gastropoda, Heterobranchia). Individual homologous neurons are found across species regardless of the type of behavior that they exhibit. We compared how the central pattern generators (CPGs) underlying the swimming behaviors function and found differences in the neuronal expression of particular serotonin receptors that are related to the production of swimming. These differences were also seen across individual animals within a species that differed in behavior. Furthermore, there are differences in the synaptic connectivity between homologous neurons, causing even homologous behaviors to be produced through different neural mechanisms. Using dynamic clamp, we rewired the circuitry of one species to resemble that of another species to test the role of circuit connectivity in production of homologous swimming behaviors. We also determined that homologous neurons can differ in their position in the motor hierarchy, being a CPG neuron in one species and a neuromodulatory command neuron in another species. Similarly, we found variation among individuals of the same species in synaptic strength which was not evident in the behavior, but which had effects on the susceptibility of the system to a particular lesion. The experiments demonstrate the phylogenetic and individual differences that occur in the neural circuitry underlying behavior.
Dr. Paul Katz is a Professor and Director of Neuroscience at the University of Massachusetts-Amherst.
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