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Si3 is a neuron in Melibe

Basic information

Dye fill of Si4 shows xxx.
  • Names and Aliases - Swim Interneuron 4, Si4, Si4Mel.
  • Species: Melibe leonina
  • Neurotransmitter: Unknown.

There is one Si4 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.



  • The Si4 oma located on dorsal surface and in the center of each pedal ganglion around Si2 vicinity.
  • 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.
  • The main axon shows a characteristic "?" mark bend in each pedal ganglion.
  • Fine branches are constrained near the soma and at the end of the axon, showing characteristic T-shape endings in both pedal ganglia.


  • Contralateral Si4s mutually inhibit each other, monosynaptically. The inhibiotry synaptic potentials often reverse to be depolarizing potential because the reversal potential of this mutually-inhibitory synapse is close to the resting potential.
  • Si4 is electrically coupled to the contralateral Si1 and Si2. The connection is stronger with Si1 than Si2.
  • Si4 receives excitatory synaptic input from the contralateral Si1 and Si2.
  • Si4 makes inhibitory synapses onto the contralateral Si3. The synaptic potential has a fast depolarizing phase and a slow hyperpolarizing phase.


Simultaneous intracellular recordings from xx and xxx illustrate the swim motor pattern.
  • Action potentials recorded from the Si4 soma are remarkably shorter (approx. 20-30 mV in height) than those in other swim interneurons; Si4 spikes never overshoot.
  • When a swim motor pattern is not being expressed, Si4 is generally silent.
  • During a swim motor pattern, Si4 fires rhythmic bursts together with the contralateral Si1 and Si2 burst.
  • Injection of a brief square current into one of the Si4 pair can phase shift the swim motor pattern.
  • Injection of steady current into Si4 slightly affect the swim motor pattern.


  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.