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Polysensory Interneuronal Projections to Foot Contractile Pedal Neurons in Hermissenda

Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, Texas

Submitted 25 September 2008; accepted in final form 1 December 2008

A Pavlovian-conditioning procedure may produce modifications in multiple behavioral responses. As an example, conditioning may result in the elicitation of a specific somatomotor conditioned response (CR) and, in addition, other motor and visceral CRs. In the mollusk Hermissenda conditioning produces two conditioned responses: foot-shortening and decreased locomotion. The neural circuitry supporting ciliary locomotion is well characterized, although the neural circuit underlying foot-shortening is poorly understood. Here we describe efferent neurons in the pedal ganglion that produce contraction or extension of specific regions of the foot in semi-intact preparations. Synaptic connections between polysensory type I_b and type I_s interneurons and identified foot contractile efferent neurons were examined. Type I_b and type I_s interneurons receive synaptic input from the visual, graviceptive, and somatosensory systems. Depolarization of type I_b interneurons evoked spikes in identified tail and lateral foot contractile efferent neurons. Mechanical displacement of the statocyst evoked complex excitatory postsynaptic potentials (EPSPs) and spikes recorded from type I_b and type I_s interneurons and complex EPSPs and spikes in identified foot contractile efferent neurons. Depolarization of type I_b interneurons in semi-intact preparations produced contraction and shortening along the rostrocaudal axis of the foot. Depolarization of I_s interneurons in semi-intact preparations produced contraction of the anterior region of the foot. Taken collectively, the results suggest that type I_b and type I_s polysensory interneurons may contribute to the neural circuit underlying the foot-shortening CR in Hermissenda.