The results of perturbation experiments demonstrate the functional diversity of the interneurons (HN cells) that generate heartbeat in the medicinal leech. 2. HN cells were individually stimulated by single current pulses. The induced activity of HN cells in the first four ganglia (cell pairs HN(1)-HN(4)) reset the rhythm of the interneuron network; induced activity of those in the fifth through seventh ganglia (cell pairs HN(5)-HN(7)) did not. 3. Cells HN(1)-HN(4) can entrain every other interneuron of the network; cells HN(5)-HN(7) cannot. 4. Thus the HN interneuron network includes two distinct subsets: cells HN(1)-HN(4) form the network's timing oscillator; cells HN(5)-HN(7), driven by the timing oscillator, force one of the two coordination states on the heart motor neurons. 5. In general the dynamic behavior of the heart interneuron network was predictable given the web of identified synapses between HN cells. Nevertheless, the unexpected capacity of cells HN(3) and HN(4) to entrain the network shows that there are functional connections still to be found. Burst termination experiments suggest that cells HN(3) and HN(4) inhibit directly the more rostral HN cells. 6. The timing oscillation seems to arise from a balance between the endogenous polarization rhythms of interneurons HN(1)-HN(4) and selective reciprocal inhibition between these same cells.
