To survive, animals must execute proper behaviors at appropriate times. Animals are constantly bombarded with a plethora of sensory stimuli and must choose which of these stimuli to as important and which to ignore. By presenting an animal with two stimuli simultaneously, each of which elicits a different behavior when presented alone, we can rank-order an animal's behavioral repertoire from most preferred to least preferred. This ranking of behaviors has been termed a behavioral hierarchy. We use the leech as a model system to explore how animals place different priorities on different behaviors due to its simple nervous system, its readily identifiable neurons, and its amenability to reduced preparations. In Chapter 2 we show that feeding suppresses all competing behaviors within the leech nervous system by reducing the sensory excitation that normally drives these behaviors. Neurons involved in the ingestion phase of feeding generate a descending inhibition that travels down the nerve cord and inhibits the tactile sensory fibers of the pressure mechanosensory neurons (P cells). We show that this inhibition is mimicked in isolated ganglia by the application of the neuromodulator serotonin. We propose that the leech uses sensory gating mediated through serotonin in the decision making process to inhibit competing behaviors during feeding. In Chapter 3 we show the distention incurred during feeding targets neurons downstream of the sensory neurons, most likely the swim gating-neurons or central pattern generating cells. Increasing levels of distention result in a gradual decrease in the number of swim cycles elicited in response to stimulation and an increase in the swim period. We propose that the feeding induced suppression of swimming is distributed within the nervous system and mediated by at least two distinct mechanisms or decision-making modules: ingestion prevents the initiation of swimming by targeting the sensory neurons that drive it, whereas distention inhibits the maintenance of swimming by targeting downstream neurons. We believe that distributed decision-making networks may be universal in the nervous systems of all complex animals

We characterized the behavioral responses of two leech species, Hirudo verbana and Erpobdella obscura, to mechanical skin stimulation and examined the interactions between the pressure mechanosensory neurons (P cells) that innervate the skin. To quantify behavioral responses, we stimulated both intact leeches and isolated body wall preparations from the two species. In response to mechanical stimulation, Hirudo showed local bending behavior, in which the body wall shortened only on the side of the stimulation. Erpobdella, in contrast, contracted both sides of the body in response to touch. To investigate the neuronal basis for this behavioral difference, we studied the interactions between P cells. Each midbody ganglion has four P cells; each cell innervates a different quadrant of the body wall. Consistent with local bending, activating any one P cell in Hirudo elicited polysynaptic inhibitory potentials in the other P cells. In contrast, the P cells in Erpobdella had excitatory polysynaptic connections, consistent with the segment-wide contraction observed in this species. In addition, activating individual P cells caused asymmetrical body wall contractions in Hirudo and symmetrical body wall contractions in Erpobdella. These results suggest that the different behavioral responses in Erpobdella and Hirudo are partly mediated by interactions among mechanosensory cells.