The Tritonia Swim Central Pattern Generator

Tritonia diomedea is a sea slug that escapes from predatory starfish by rhythmically flexing its entire body in the dorsal and ventral directions. This escape swim behavior is produced by a central pattern generator (CPG), without needing sensory feedback. There are several features of the neural basis for this response that make it of particular interest for neuroscientists. One is that the CPG is a network oscillator; bursting arises as an emergent property of the neurons and their connectivity. Another interesting feature is that the CPG contains state-dependent, intrinsic neuromodulation: one of the CPG neurons uses the neurotransmitter serotonin (5-HT) to modulate the strength of synapses made by the other CPG neurons under certain conditions. This CPG seems to have evolved from a nonoscillatory network. Finally, there are novel mechanisms for plasticity during learning and in response to injury.

Neural Control of Swimming in Nudipleura Molluscs

This article compares the neural basis for swimming in sea slugs belonging to the Nudipleura clade of molluscs. There are two primary forms of swimming. One, dorsal/ventral (DV) body flexions, is typified by Tritonia diomedea and Pleurobranchaea californica. Although Tritonia and Pleurobranchaea evolved DV swimming independently, there are at least two homologous neurons in the central pattern generators (CPGs) underlying DV swimming in these species. Furthermore, both species have serotonergic neuromodulation of synaptic strength intrinsic to their CPGs. The other form of swimming is with alternating left/right (LR) body flexions. Melibe and Dendronotus belong to a clade of species that all swim with LR body flexions. Although the swimming behavior is homologous, their swim CPGs differ in both cellular composition and in the details of the neural mechanisms. Thus, similar behaviors have independently evolved through parallel use of homologous neurons, and homologous behaviors can be produced by different neural mechanisms.

Multiple Orientation Circuits Converging on the Pd7 Cells in Tritonia diomedea

Magnetoreception is a sophisticated orientation mechanism, involving a magnetoreceptor connected to the nervous system with signal amplification. The mollusk Tritonia diomedea is a good model to investigate the behavioral and neural responses to the magnetic field. The mollusk inhibits all unnecessary activities and focuses on an available cue during orientation. Although Pd7 cells are inhibited by magnetic pathway, it was excited by another stimulus, water streams plus food odor. Two sensory pathways connected to Pd7 through the same or different circuits were tested. The action potential activity through Pd7 was compared in these different stimulations. The changes in Pd7 activity indicate a response of enhanced electrical activity to water streams plus food odor stimulus, and Pd7 activity can be excited by at least one of these stimuli. These results indicate an inverse relationship between magnetic orientation and feeding.