Rhythmic movements in both vertebrate and invertebrate animals are generated by neuronal circuits residing in the central nervous system. Research in our laboratory and those of other investigators have elucidated some of the principles underlying these circuits. We plan now to extend our research to the mechanisms through which these rhythm generating circuits are controlled via brain neurons by using the brain and ventral nerve cord of an invertebrate, Hirudo medicinalis, as a model system. Specifically, we will study the interaction by which neurons in the brain of this animal act to initiate swimming movements. Our approach will be a) to identify brain neurons which can act to initiate swimming activity, then b) to describe the interactions of these neurons with cells of the ventral nerve cord, and finally, c) to investigate the function of these interactions for swim initiation. In particular, we will investigate the means by which brain neurons control swimming activity via hormone release and via "command," swim-initiating neurons of ventral nerve cord. Our long range objective is to understand the physiological mechanisms by which the nervous system generates and controls movements in humans and other animals. The approach taken here is to study thoroughly the relatively simple movements of a favorable invertebrate model. Much of our knowledge concerning the detailed workings of the nervous system is derived from invertebrate models. We believe that the discoveries made in the course of this research project also will contribute importantly to this body of knowledge.