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Si1 is a Neuron in Melibe.

Basic information

Dye fill of Si1 (red) shows its morphology and relation to surrounding serotonergic (green) neurons known as the CeSP cluster.
  • Names and Aliases - Swim Interneuron 1, Si1.
  • Species: Melibe leonina
  • Neurotransmitter: Unknown

There is one Si1 in each hemisphere of the cerebropleural ganglion. The axon projects ipsilaterally to the pedal ganglion. Si1 is an integral component of the central pattern generator that produces lateral-flexion swimming. Si1 fires rhythmic bursts during a swim motor pattern that are in phase with ipsilateral swim neurons and antiphase with contralateral swim neurons.



  • Non-serotonergic neuron that is surrounded by serotonergic CeSP neurons and projects to ipsilateral pedal ganglion.
  • Soma located just posterior and lateral to the tentacular lobe, and just medial to the three serotonergic CeSP-A neurons.
  • Soma has larger diameter than other neurons in its vicinity.
  • The axon projects ipsilaterally to the pedal ganglion, with fine branches projecting anteriorally in both the cerebral and pedal ganglia. The axon continues out the larger of the two pedal-pedal connectives (PP2 according to nomenclature in Newcomb et al, 2006) to the contralateral pedal ganglion.


Simultaneous intracellular recordings from left and right Si1 illustrate alternating bursts (grey rectangle) during swim motor pattern.
  • When a swim motor pattern is not being expressed, Si1 is generally silent.
  • During a swim motor pattern, Si1 fires rhythmic bursts in phase with ipsilateral swim neurons and out of phase with the contralateral Si1 and other contralateral swim neurons. The bursts in contralateral Si1s do not overlap.
  • In semi-intact preparations, continual depolarization of an Si1 results in a sustained lateral flexion to the same side.
  • Si1 is necessary for swimming, as hyperpolarization of one of the Si1s sufficient to prevent firing will inhibit swimming.
  • Stimulation of an Si1 can phase shift the swim motor pattern.
  • Contralateral Si1s mutually inhibit each other, although it is currently unclear whether this is mono- or polysynaptic.
  • Si1 is electrically coupled to the ipsilateral Si2, the other member of the swim central pattern generator.
  • Si1 monosynaptically excites swim efferents in the ipsilateral pedal ganglion.