Although the nudibranch mollusc Tritonia diomedea orients to the geomagnetic field, the anatomical site and the mechanism of the geomagnetic transducer are not known. Previous work on semi-intact preparations of Tritonia diomedea in which the brain is intact and nerve connections to the periphery are maintained showed that identifiable pedal ganglion neurons Pd5 fired an increased number of action potentials when the horizontal component of the ambient magnetic field was rotated. This response disappeared when all nerves emerging from the brain were cut, suggesting a peripheral locus for the geomagnetic transducer. In the present work, we recorded intracellularly from Pd5 in preparations in which all peripheral nerves were cut except those containing the axons of neurons Pd5 (pedal nerves 2 and 3). These uncut, mixed, sensory-motor trunks innervate the locomotory epithelium of the foot upon which the animal crawls. In this further-reduced preparation, Pd5 again responded to magnetic field rotations with action potentials. To determine the direction of this action potential transmission in response to magnetic field rotations, we analyzed extracellular recordings from nerves containing the Pd5 axons and found that action potentials elicited in Pd5 by magnetic stimuli originate centrally and are transmitted peripherally. In addition, we have explored the behavioral function of Pd5 neurons by simultaneously recording intracellular electrical activity and crawling rate of the semi-intact animal. A significant correlation was found between crawling rate and Pd5 action potential rate. We also found that action potentials in dorsal swim interneurons depolarized both Pd5 and the established locomotion motoneuron Pd21.

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