Si3

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Si3 is a neuron in Melibe and Dendronotus.

Basic information

  • Names and Aliases - Swim Interneuron 3, Si3, Si3Mel, Si3Den.
  • Species: Melibe leonina and Dendronotus iris.
  • Neurotransmitter: Unknown.
The Melibe Si3. Calibration bar: 100 micron
The Dendronotus Si3.
















There is one Si3 in each pedal ganglion. The axon projects out the large pedal-pedal connective (PP2). Si3 is a member of the central pattern generator circuit for lateral-flexion swimming. Si3 fires rhythmic bursts during a swim motor pattern.


Identification

Anatomy:

  • Soma located on dorsal surface and in the center of each pedal ganglion.
  • In Melibe, the Si3 soma is located adjacent (medial-posterior) to the Si2 soma.
  • 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.
  • More branches are seen along the main Si3 axon proximal to the soma than Si2.
  • Axon shows T-shaped branching in the contralateral pedal ganglion.

Connectivity

  • Contralateral Si3s mutually inhibit each other, monosynaptically.
  • Si3 makes inhibitory synapses onto the contralateral Si1 and Si2.
  • Si3 receives inhibitory synaptic input from the contralateral Si4.

Electrophysiology:

  • When a swim motor pattern is not being expressed, Si3 generally fires tonically.
  • In Melibe, Si3 fires rhythmic bursts with approximately 25% phase delay from the contralateral Si1 and Si2 burst during the swim motor pattern.
  • In Dendronotus, Si3 bursts with a slight phase advance from the contralateral [Si2 burst.
  • The bursts in contralateral Si3s often overlap in both species.
  • Continual depolarization of an Si3 transiently halts the swim motor pattern.
  • Stimulation of an Si3 can phase shift the swim motor pattern.
  • In Melibe, suppression of Si3 by hyperpolarizing current injection slows down the rhythm.
  • In Dendronotus, Si3 is necessary for swimming, as hyperpolarization of one of the Si2s sufficient to prevent firing will inhibit swimming.

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Simultaneous intracellular recordings from left Si2 (top), right Si3 (middle), and left Si3 (bottom) illustrate the swim motor pattern in Melibe leonina.
Simultaneous intracellular recordings from left Si2 (top), right Si3 (middle), and left Si3 (bottom) illustrate the swim motor pattern in Dendronotus iris.

















References

  1. 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.
  1. Sakurai A and Katz PS (2016) The central pattern generator underlying swimming in Dendronotus iris: A simple half-center network oscillator with a twist. J Neurophysiol DOI: 10.1152/jn.00150.2016.

Identification

Anatomy:

Connectivity

  • Melibe
  • Dendronotus

Electrophysiology:

    • Melibe:
    • Dendronotus:

References