Animal-to-animal variability in the phasing of the crustacean cardiac motor pattern: an experimental and computational analysis

The cardiac ganglion (CG) of Homarus americanus is a central pattern generator (CPG) that consists of two oscillatory groups of neurons: the "small cells" (SCs) and the "large cells" (LCs). We show that the SCs and LCs begin their bursts nearly simultaneously, but end their bursts at variable phases. This variability contrasts with many other CPG systems in which phase is well- maintained. To determine both the consequences of this variability and how CG phasing is controlled, we modeled the CG as a pair of Morris-Lecar oscillators coupled by electrical and excitatory synapses, and constructed a database of 15,000 simulated networks using random parameter sets. These simulations, like our experimental results, displayed variable phase relationships, with the bursts beginning together but ending at variable phases. The model suggests that the variable phasing of the pattern has important implications for the functional role of the excitatory synapses. In networks in which the two oscillators had similar duty cycles, the excitatory coupling functioned to increase cycle frequency. In networks with disparate duty cycles, it functioned to decrease network frequency. Overall, we suggest that the phasing of the CG may vary without compromising appropriate motor output, and that this variability may critically determine how the network behaves in response to manipulations.