Neuronal control of heartbeat in the medicinal leech

Summary

The coordinated activity cycles of the heart excitor motor neurons, or HE cells, of the leech ventral nerve cord responsible for the heart tube constriction rhythm are controlled by a set of heart interneurons, or HN cells. Ganglia 3, 4, 6 and 7 of the ventral cord each contain a pair of such HN cells, which send axons into the posterior connective. On their rearward course, the HN cell axons transit through the neuropil of many posterior ganglia where they inhibit the ipsilateral HE cells. The HN cells maintain an activity rhythm consisting of an active phase, during which they are depolarized and produce a burst of impulses, and an inactive phase during which they are repolarized while receiving a burst of inhibitory synaptic potentials.

The HN cells form part of the neural oscillator that is the source of the heartbeat rhythm, since the phase of the rhythm can be shifted by evoking impulses in an HN cell during the inactive phase of its activity cycle. The HN cell activity rhythms are coordinated intra- and intersegmentally in such a manner that on the peristaltic side there is a progressive rear-to-front progression in the phase of the HN cell activity cycles and that on the non-peristaltic side the activity cycles occur in phase.

A quantitative theoretical analysis shows that the pattern of HN-HE cell synaptic links and the phase relations of the HN cell activity cycles can account for the observed phase relations of the HE activity cycles and hence ultimately also for the heart tube constriction dynamics on both peristaltic and non-peristaltic body sides.