Intersegmental coordination of leech swimming: comparison of in situ and isolated nerve cord activity with body wall movement.

Recordings of motoneuron activity during swimming, obtained from leech ventral nerve cords in situ, were compared with films of swimming leeches and with recordings of motoneuron activity from isolated nerve cords. It was found that the intersegmental phase lag in body movement is greater than the phase lag of in situ neuronal activity, which is in turn greater than the phase lag of isolated nerve cord activity. We conclude that peripheral neuronal or mechanical effects, as well as sensory feedback to the central pattern generator, contribute to the movement pattern of the intact swimming leech.