Neuronal mechanisms underlying behavioral switching in a pteropod mollusc

In the pteropod mollusc Clione limacina, wing retraction takes precedence over spontaneous and continuous swimming, a phenomenon here defined as behavioral switching. The wing retraction system is organized as a simple reflex in which wing mechanoreceptors activate a pair of retraction interneurons which in turn excite at least two pairs of retraction motoneurons. Activation of individual mechanoreceptors does not inhibit swimming or trigger wing retraction. A pair of retraction interneurons can fully suppress swimming when induced to fire at physiological frequencies, and may be both sufficient and necessary for swim inhibition. Retraction interneurons monosynaptically inhibiit both swim interneurons and swim motoneurons. Retraction motoneurons inhibit swim motoneurons through a polysynaptic pathway. A model summarizing the neural circuity underlying behavioral switching in Clione is presented. A comparison of this model with the behavioral choice model in Pleurobranchea reveals that the overall neural mechanisms for behavioral choice and behavioral switching are similar as both involve dual function interneurons that not only activate their own motor pathway, but also inhibit the competing motor system. While inhibition is biased toward the afferent side of the competing circuit in behavioral choice, it is biased to the efferent side in behavioral switching