The neural control of alternate heartbeat coordination states in the leech,Hirudo medicinalis

Abstract

The bilateral paired hearts of the medicinal leech are controlled by a set of segmental heart motor neurons (HE cells) which are in turn controlled, via inhibitory synapses, by a set of segmental heart interneurons (HN cells). The HE cells fire in rhythmic impulse bursts because their steady discharge is periodically inhibited by the HN cells. 1) With the identification of an additional pair of HN cells in segmental ganglion 1 and the elucidation of several synaptic connections between HN cells a more complete heart control circuit diagram is now available. This circuit diagram accounts for the observed activity cycles of the various HN and HE cells and consequently for the behavior of the hearts themselves. 2) The bilaterally paired HE cells are coordinated by the HN cells such that the segmental heart tube sections on one side constrict in a caudorostral sequence to produce a rear-to-front peristalsis, while the segmental heart tube sections on the other side constrict nearly synchronously (non-peristaltically). This difference in the coordination modes of the two hearts is not permanent, and reciprocal coordination mode transitions occur every 10–50 heartbeat cycles. Cell HN(5) is phasically active on the side of the non-peristaltic heart tube and completely inactive on the side of the peristaltic heart tube. Reciprocal changes in the activity-inactivity pattern of the HN (5) cell pair are responsible for the observed spontaneous reciprocal changes in coordination mode. When cell HN (5) is made to be active or inactive, by intracellularly injected current, similar but unilateral changes, in coordination mode occur.