The roles of endogenous membrane properties and synaptic interaction in generating the heartbeat rhythm of the leech, Hirudo medicinalis.

1. Inhibitory synapses among the central neurones involved in the generation of the heartbeat rhythm of the leech were blocked by either low Cl- physiological saline or presynaptic hyperpolarizing current. 2. Low Cl- saline reversibly blocked inhibitory postsynaptic potentials (IPSPs) from the HN cells onto both other HN cells and HE cells but did not block electrical coupling among HN cells. 3. The rhythmic bursts of impulses in HE cells were abolished when IPSPs were blocked by either low Cl- saline or hyperpolarization of HN cells. 4. The rhythmic bursts of impulses in HN cells were not abolished (except in cell HN(5)) when IPSPs were blocked by low Cl- saline, but phase relations became unfixed (unless the cells were electrically coupled). 5. Both brief depolarizing and hyperpolarizing current pulses reset the rhythm of HN cells whose IPSPs were blocked by low Cl- saline. 6. The results indicate that the motor neurones to the heart (HE cells) produce rhythmic impulse bursts because their steady discharge is periodically inhibited by the HN interneurones. The pattern generated by the HN cells originates from an endogenous rhythm co-ordinated by the inhibitory interactions and electrical coupling between these cells.