The heartbeat control system of medicinal leeches: heart motor neurons and the heartbeat central pattern generator. (A) Bilateral circuit diagram including all the identified heart interneurons (HNs) of the central pattern generator (CPG) showing the inhibitory connections from the HNs of the leech heartbeat CPG onto heart (HE) motor neurons. (B) Circuit diagram of the identified HNs of the core CPG showing their synaptic interconnections. The two possible states of the heartbeat CPG are illustrated, one with the left switch interneuron quiescent and the right switch interneuron active (corresponding to left synchronous), and the other with the left switch interneuron active and the right switch interneuron quiescent (corresponding to left peristaltic). In (A) and (B), large filled circles represent cell bodies and associated input processes. Lines indicate cell processes, small filled circles indicate inhibitory chemical synapses and diode symbols indicate electrical connections. HNs that have similar input and output connections are lumped together for ease of presentation. (C) Simultaneous recordings of a bilateral pair of front premotor interneurons (HN(3)), a bilateral pair of middle premotor interneurons (HN(7)) and the bilateral pair of switch interneurons (HN(5)) during a switch in coordination mode from left synchronous to left peristaltic as indicated in the circuit diagrams in (B). Body side indicated by R or L in the HN index.