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Si2 is a Neuron in Melibe.

Basic information

Drawing of a dye fill of Si2 (blue) shows its location in the pedal ganglion and morphology of its projections.
  • Names and Aliases - Swim Interneuron 2, Si2, sint2.
  • Species: Melibe leonina
  • Neurotransmitter: Unknown

There is one Si2 in each pedal ganglion. The axon projects out the large pedal-pedal connective (PP2). Si2 is an integral component of the central pattern generator that produces lateral-flexion swimming. Si2 fires rhythmic bursts during a swim motor pattern that are in phase with ipsilateral swim neurons and antiphase with contralateral swim neurons.



  • Soma located on dorsal surface and in the center of each pedal ganglion.
  • Projections into neuropil of pedal ganglion and also out large pedal-pedal connective (PP2 according to nomenclature in Newcomb et al, 2006) to contralateral pedal ganglion.


Simultaneous intracellular recordings from left Si2 and right Si1 illustrate alternating bursts (grey rectangle) of contralateral swim interneurons during swim motor pattern.
  • When a swim motor pattern is not being expressed, Si2 generally fires irregularly with occasional periods of sustained firing or relative silence. Sustained firing of Si2 correlates with the animal turning towards that side. In contrast, sustained silence is associated with turning in the contralateral direction.
  • During a swim motor pattern, Si2 fires rhythmic bursts in phase with ipsilateral swim neurons and out of phase with the contralateral Si2 and other contralateral swim neurons. The bursts in contralateral Si2s do not overlap.
  • Continual depolarization of an Si2 results in a sustained lateral flexion to the same side.
  • Si2 is necessary for swimming, as hyperpolarization of one of the Si2s sufficient to prevent firing will inhibit swimming.
  • Stimulation of an Si2 can phase shift the swim motor pattern.
  • Contralateral Si2s mutually inhibit each other, although it is currently unclear whether this is mono- or polysynaptic.
  • Si2 is electrically coupled to the ipsilateral Si1, the other member of the swim central pattern generator.
  • Si2 monosynaptically excites swim efferents in the ipsilateral pedal ganglion and monosynaptically inhibits swim efferents in the contralateral pedal ganglion.


  1. Newcomb JM, Fickbohm DJ, Katz PS (2006) Comparative mapping of serotonin-immunoreactive neurons in the central nervous systems of nudibranch molluscs. J Comp Neurol 499: 485-505.
  2. Thompson SH, Watson WH, III (2005) Central pattern generator for swimming in Melibe. J Exp Biol 208: 1347-1361.
  3. Watson WH, III, Lawrence KA, Newcomb JM (2001) Neuroethology of Melibe leonina swimming behavior. Amer Zool 41: 1026-1035.