Animal-to-animal variability in the phasing of the crustacean cardiac motor pattern: an

40 The cardiac ganglion (CG) of Homarus americanus is a central pattern generator (CPG) that 41 consists of two oscillatory groups of neurons: the “small cells” (SCs) and the “large cells” (LCs). 42 We show that the SCs and LCs begin their bursts nearly simultaneously, but end their bursts at 43 variable phases. This variability contrasts with many other CPG systems in which phase is well44 maintained. To determine both the consequences of this variability and how CG phasing is 45 controlled, we modeled the CG as a pair of Morris-Lecar oscillators coupled by electrical and 46 excitatory synapses, and constructed a database of 15,000 simulated networks using random 47 parameter sets. These simulations, like our experimental results, displayed variable phase 48 relationships, with the bursts beginning together but ending at variable phases. The model 49 suggests that the variable phasing of the pattern has important implications for the functional role 50 of the excitatory synapses. In networks in which the two oscillators had similar duty cycles, the 51 excitatory coupling functioned to increase cycle frequency. In networks with disparate duty 52 cycles, it functioned to decrease network frequency. Overall, we suggest that the phasing of the 53 CG may vary without compromising appropriate motor output, and that this variability may 54 critically determine how the network behaves in response to manipulations. 55 56