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Biological Sciences


Each moment nervous systems are sampling inputs from many sensory sources in parallel and making decisions on what actions, if any, to take. Indeed, it is a universal feature of animal behavior to obtain information from multiple and varied external and intimal sensory systems simultaneously and these in ways that result in adaptive behavioral responses. Visual stimuli are important across all Phyla. The medicinal leech, Hirudo has an intricate visual system with 5 pairs of pigmented cephalic eyes at the anterior margin of the anterior sucker and a set of 7 pairs of simple sensilla including photoreceptors arranged dorsal to ventral in the central annulus of each body segment. Thus, the leech can detect light from many widely spaced locations, anterior to posterior, dorsal to ventral. Hirudo are aquatic predators that are active during daylight, and they use visible light with a maximal sensitivity in the green (540 nm) as one cue to locate prey. Indeed, these leeches become neutral or slightly positively phototactic when hungry. Yet, these organisms are sensitive to sunlight and susceptible to photic damage. I recently showed that these leeches respond to near ultraviolet radiation (UVR) as well as visible light. However, the responses to UVR were very different from visible light, always leading to avoidance regardless of where on the body the UVR was detected. Indeed, when tested in groups or as individuals the UVR responses were similar to withdrawal and avoidance responses evoked by other sensory modalities such as touch. Using electrophysiological recording I show that both UVR and visible light are transduced by photoreceptors in cephalic eyes as well as body wall sensilla. Additionally, a higher order interneuron known to be involved in rapid movements of the body in response to touch or bright visible light flashes was shown to receive UVR activation as well. Furthermore, I show that the UVR pathway to activate this interneuron is not acting via tactile receptor neurons and therefore represents a discrete and parallel input. This system is well positioned as a model to investigate the way in which diverse sensory inputs across modalities are integrated by the central nervous system to produce adaptive behavior.

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