Wenning et al. (SFN Abstr 164.4, 2000) described the phase relations of the motor neurons driving constrictions of the two hearts in leech. One heart is driven to constrict synchronously along its length, while the other is driven to constrict in a peristaltic fashion. The hearts alternate between these two coordination modes every 20-40 cycles. The activity of the motor neurons is sculpted by the activity of four identified and one unidentified pair(s) of heart (HN) interneurons. To understand how the heartbeat network generates these patterns in the motor neurons, we need to describe fully the output of all the premotor heart interneurons. We used chains of ganglia, from the head-brain to ganglion 7, which contain the entire heartbeat network. The HN cells were recorded extracellularly with suction electrodes applied to the soma. The HN(4) cell was used as a reference cell, and mainly ipsilateral heart interneurons were recorded. We found that during both the synchronous and the peristaltic modes, the ph...