The leech heartbeat central pattern generator (CPG) consists of seven bilateral pairs of heart interneurons that coordinate the activity of heart motor neurons. In the CPG, front and rear premotor interneurons are coordinated differently on the two sides by intervening switch interneurons. We have extended a previous network model that included the front premotor interneurons to construct a model of the entire heartbeat CPG. In our CPG model, we implemented known synaptic and neuronal properties. In contrast to the front premotor interneurons, the switch interneurons fire in accelerating spike frequency bursts and only on the synchronous side. The switch interneurons were modeled using a three conductance neuron model that was tuned to fit experimental data. The rear premotor interneurons show similar activity to and were modeled in the same manner as the front premotor interneurons (Hill et al. 2001). To reproduce experimentally observed CPG phasing data (Morris & Calabrese, SFN Abstr 2001), we varied syn...