Generation and coordination of heartbeat timing oscillation in the medicinal leech. II. Intersegmental coordination.

1. A network of four pairs of interneurons, HN(1)-HN(4), times the heartbeat of the medicinal leech. The following model accounts for the stable phase relationships in the network and the network's response to perturbation, entrainment, and stimulation. 2. There are two centers of oscillation, one the HN(3) pair in the third ganglion, the other the HN(4) pair in the fourth ganglion. 3. The two other pairs, HN(1) and HN(2), couple the oscillation of the HN(3) pair with that of the HN(4) pair, thereby guaranteeing that a coherent time signal is sent to the rest of the heartbeat system. 4. The HN(1) and HN(2) pairs can play this coupling role because they have impulse-initiation sites in the third and fourth ganglia and have input and output connections with the HN(3) and HN(4) pairs. 5. In the context of the network, the intrinsic frequency of the HN(4) pair appears to be higher than that of the HN(3) pair. Coupled, the two pairs strike a stable compromise in which the HN(4) pair leads the HN(3) pair.