Neuronal basis of leech swimming: separation of swim initiation, pattern generation, and intersegmental coordination by selective lesions.

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Abstract
In leeches [Hirudo medicinalis, Macrobdella decora and M. setertia] the intersegmental axons of identified oscillator interneurons, which participate in pattern generation and intersegmental coordination, are located in the paired, lateral interganglionic connectivities; identified swim-initiating interneuron axons are located in the medial connective (Faivre''s nerve). By cutting particular connectives, it is possible to interrupt the intersegmental projections of either of these 2 functional classes of interneurons. The effects of such cuts on the swim-motor patterns produced by brainless, isolated nerve cords and single ganglia were studied using intracellular and extracellular recording techniques. Midbody section of Faivre''s nerve, containing swim-initiating interneuron axons, had no effect on intersegmental swim-cycle coordination; it impaired only slightly the ability of intracellular stimulation of a swim-initiating interneuron in 1 end of the nerve cord to evoke the swim-motor pattern distal, and proximal, to the cut. After midbody section of both lateral connectives containing the oscillator interneuron axons, swim-initiating interneuron stimulation, delivered in either end of the nerve cord, elicited swim episodes simultaneously in both ends. The swim patterns produced by the 2 ends exhibited different swim-cycle periods and the swim cycles were no longer phase locked. These results show that axons in the lateral connectives are necessary and sufficient to mediate intersegmental coordination, whereas the integrity of swim-initiating interneuron axons is neither necessary nor sufficient for coordination. Experiments were performed to determine the smallest piece of nerve cord capable of producing the swim-motor pattern. Totally isolated pairs of ganglia produced swim activity, whereas single ganglia did not. When single ganglia were left attached to the remaining nerve cord by 1 or both adjacent Faivre''s nerves, they produced rhythmic swim impulse bursts in response to swim-initiating interneuron stimulation elsewhere in the nerve cord. Interganglionic oscillator interneuron connections are not necessary for the generation of rhythmic swim activity by single ganglia.