ESCH, T.*; KRISTAN, W.B.: How the leech decides to swim or crawl

In our studies of behavioral choice in the medicinal leech, we have been examining the role that neurons whose somata are located in the anterior brain play in producing behaviors. We found that one neuron, located in sub-packet R3b, has complex effects on behavioral choice in that it can elicit different behaviors under different experimental conditions. In isolated nerve cords, depolarization of this cell normally produces either the crawling or the swimming motor pattern. In semi-intact preparations, depolarization of the cell stimulates crawling, swimming, or a hybrid of these behaviors. Studies in semi-intact preparations indicate that sensory feedback is important in the decision to swim or crawl. Stimulation of the head brain neuron in low fluid levels elicits crawling, while in high fluid levels the same stimulation elicits swimming. The hybrid behavior is elicited in intermediate fluid levels. Different fluid levels similarly affect the decision to swim, crawl, or produce a hybrid behavior following stimulation in the fully intact leech. These data, along with data from other neurons, suggest that different locomotory behaviors are controlled by an overlapping set of neurons, and that sensory information provides important information about which behavior to produce.