Si4

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Si3 is a neuron in Melibe

Basic information

Biocytin-filled Si4.
  • 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.

Identification

Anatomy:

  • 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.

Connectivity

  • 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.

Electrophysiology:

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.

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.