Behavioral experiments have demonstrated that diverse animals can sense the Earth’s magnetic field and use it as a cue for guiding movements over both short and long distances. Little is known, however, about the neural circuitry that underlies magnetic orientation behavior.
Most research on magnetic orientation has focused on vertebrate animals such as migratory birds and sea turtles. Although such animals have proven to be excellent subjects for behavioral experiments, they are not ideal for neurobiological research, in part because the complexity of the vertebrate nervous system makes cellular-level investigations of neural circuitry challenging.
The marine mollusc Tritonia diomedea represents a favorable model system for studying how the nervous system detects magnetic cues, processes them, and generates appropriate motor responses. Behavioral experiments have demonstrated that Tritonia can orient magnetically. In addition, this animal has large, individually identifiable brain cells and a relatively simple nervous system amenable to cellular-level electrophysiological analyses.
Intracellular electrophysiological recordings have demonstrated that three bilaterally symmetric pairs of identifiable neurons respond with altered electrical activity to changes in Earth-strength magnetic field. Two of these pairs, known as the Pd5 and Pd6 neurons, are excited by changes in the direction of the ambient field. The third pair, known as the Pd7 cells, is inhibited by the same magnetic stimuli that excite the others. All of these cells presumably function in the neural circuitry underlying magnetic orientation behavior.
The six magnetically responsive neurons in Tritonia represent the first individually identifiable cells known to respond to Earth-strength magnetic fields in any animal. Recent evidence suggests that at least some of these cells are involved in the motor output of the magnetic orientation circuitry. The Pd5 and Pd6 neurons probably control or modulate the activity of cilia on which the slug crawls and thus influence the direction toward which the animal moves. Given the relative simplicity of the Tritonia nervous system, it may eventually be possible to characterize, at the level of individual neurons, the entire neural circuitry that gives rise to magnetic orientation behavior in this neuroethological model animal.
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