- Names and Aliases - Swim Interneuron 3, Si3, Si3Mel, Si3Den.
- Species: Melibe leonina and Dendronotus iris.
- Neurotransmitter: Unknown.
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.
- 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.
- 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.
- 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.
- 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.
- 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.