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Si1 is a Neuron in Melibe and Dendronotus

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, Si1Mel, Si1Den, sint1.
  • Species: Melibe leonina and Dendronotus iris
  • NeuronBank ID: Mel0002265
  • Neurotransmitter: Unknown

There is one Si1 in each hemisphere of the cerebropleural ganglion. The axon projects ipsilaterally to the pedal ganglion.

In Melibe, 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.

In Dendronotus, Si1 does not fire rhythmically during the swim motor pattern, this is caused by differences in the synaptic connectivity.



  • 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.
  • The soma is FMRFamide immunoreactive.
  • Soma has larger diameter than other neurons in its vicinity. It is 30-50microns.
  • The axon as it emerges from the soma projects posteriorly towards the pleural ganglion and then bends sharply laterally.
  • 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.


  • Melibe
    • Si1 is electrically coupled to the ipsilateral Si2, the other member of the swim central pattern generator.
    • Contralateral Si1s mutually inhibit each other, although it is currently unclear whether this is direct synaptic action or indirect effect through electrotonic conduction through Si2.
    • Si1 monosynaptically excites the ipsilateral Si3 while inhibits the contralateral Si3.
    • Si1 is electrically coupled to, and also makes a direct excitatory synapse onto the contralateral Si4.
    • Si1 receives inhibitory synaptic contacts from the contralateral Si3.
    • Si1 monosynaptically excites swim efferents in the ipsilateral pedal ganglion.
  • Dendronotus
    • Each Si1 makes excitatory synaptic contacts onto both left and right Si3.
    • Each Si1 is electrically connected with the contralateral Si1 and with Si2 of both sides.


Simultaneous intracellular recordings from left and right Si1Mel<\sub> 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:
    • Melibe:
      • 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.
    • Dendronotus:
      • Si1 fires tonically during a swim motor pattern.
      • Towards the end of a swim bout, the left and right Si1 neurons begin to burst simultaneously and not in phase with the motor pattern.
      • The membrane potential of Si1 is affected by the intensity of light. When the brain is illuminated, Si1 shows hyperpolarization. Turning the light off leads to excitation of Si1, which often initiates the swim motor pattern by increasing the tonic firing of Si1.


  1. Newcomb JM (2006) Homologous neurons and their locomotor functions in nudibranch molluscs. Ph.D. Dissertation. Georgia State University, Atlanta, GA.
  2. 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.
  3. Sakurai A, Newcomb JM, Lillvis JL, Katz PS (2011) Different Roles for Homologous Interneurons in Species Exhibiting Similar Rhythmic Behaviors, Current Biology 21 (12):1036-1043, 2011.
  4. Sakurai A, Gunaratne CA, Katz PS (2014) Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusc Melibe leonina. J Neurophysiol DOI: 10.1152/jn.00261.2014
  5. Thompson SH, Watson WH (2005) Central pattern generator for swimming in Melibe. J Exp Biol 208: 1347-1361.
  6. Watson WH Lawrence KA, Newcomb JM (2001) Neuroethology of Melibe leonina swimming behavior. Amer Zool 41: 1026-1035.