Neural Generation of the Peristaltic and Non-Peristaltic Heartbeat Coordination Modes in the Leech,Hirudo Medicinalis

Abstract

The bilateral paired heart tubes of the leech Hirudo medicinalis are controlled, via excitatory synapses, by a set of bilaterally paired segmental heart motor neurons (HE cells) which are in turn controlled, via inhibitory synapses, by a set of bilaterally paired segmental heart interneurons (HN cells). The HE cells produce rhythmic impulse bursts because their inherent steady discharge is periodically inhibited by the HN cells, most of which produce impulse bursts endogenously. The known synaptic interactions among the HN cells and HE cells can account well for the observed behavior of the hearts. The HE cells are coordinated by the HN cells such that the segmental heart tube sections on one side constrict in a caudorostral sequence (peristalsis), while the segmental heart tube sections on the other side constrict nearly synchronously (non-peristalsis). This difference in the coordination modes of the two hearts is not permanent; reciprocal coordination mode transitions occur every 10–50 heartbeat cycles. Only one member of HN(5) cell pair (the HN cells of the fifth segmental ganglion) is rhythmically active at a time, the other being completely inactive. By coordinating the front and rear HN cells the active HN(5) cell produces non-peristalsis ipsilaterally and peristalsis contralaterally. Reciprocal changes in the activity-inactivity pattern of the HN(5) cell pair are responsible for the reciprocal changes in the coordination mode.