C2 neuron

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Basic information

Figure 1: Simultaneous intracellular recordings of C2, DSI, VSI and VFN, exhibiting the swim motor pattern. Notice that C2 fires during the dorsal flexion phase. 5 Hz, 3 sec stimulation was given to PdN3 as indicated by arrows.
  • NeuronBank AccessionID Tri0002380
  • Names and Aliases Cerebral Interneuron 2, C2
  • Species Tritonia diomedea
  • Neurotransmitter possibly FMRFamide
  • Homologous to A1 in Pleurobranchaea
  • Identified in Melibe leonina, Hermissenda crassicornis, and Flabellina iodinea <ref name="Lillvis12">Lillvis JL, Gunaratne CA, and Katz PS. Neurochemical and neuroanatomical identification of central pattern generator neuron homologues in nudipleura molluscs. PLoS.ONE. 7 (2):e31737, 2012.[1]</ref>

C2 (cerebral interneuron 2, also known as CeW) is one of the members of the Tritonia escape-swim central pattern generator (CPG). There is one C2 in each cerebral ganglion. C2 fires bursts of action potentials during the dorsal phase of the swim motor pattern <ref name="Getting77">Getting PA. Neuronal organization of escape swimming in Tritonia. J.Comp.Physiol.A 121:325-342, 1977.</ref><ref>Taghert PH, Willows AOD (1978) Control of a Fixed Action Pattern by Single, Central Neurons in the Marine Mollusk, Tritonia diomedea. J Comp Physiol 123: 253-259.</ref> (Figure 1) and makes synaptic connections with the other two members of the escape-swim CPG <ref>Getting PA, Lennard PR, Hume RI (1980) Central Pattern Generator Mediating Swimming in Tritonia. I. Identification and Synaptic Interactions. J Neurophysiol 44(1): 151-164.</ref>

C2 is peptidergic <ref name="SnowA">Snow RW (1982) Evidence for a Peptide-Mediated Neurotransmission in a Molluskan Brain. J Neurobiology 13(3): 267-277</ref>, although the identity of its peptide neurotransmitter is currently unknown. The neuron is immunoreactive for FMRFamide <ref>Longley RD, Willows AOD (1985) Immunocytochemical localization of serotonin, FMRFamide, BPP, and SCPB in nudibranch mollusks. Soc Neurosci Astr 11: 943.</ref>, but it may not express the true FMRFamide peptide.



Figure 2: C2 has a contralateral axon projection (Figure: Getting et al., 1980)
Figure 3: Both C2s filled with Lucifer Yellow. Notice the contralateral projection to PdN6 (Image: Evan Hill)
  • The 50-100 micron soma of C2 appears white when illuminating from above and somewhat pink when illuminating below the CNS.
  • C2 projects to the contralateral pedal ganglion where it exhibits extensive branching. The axon continues to travel through pedal nerve 6 (PdN6; pedal-pedal connective) (Figure 2, 3) and terminates in the ipsilateral (to its soma) pedal ganglion.
  • C2 has fine, hair-like branches in both the ipsi- and contralateral cerebral ganglion.


  • C2 is generally silent at rest, with spontaneous EPSPs.
  • The action potential of C2 has very little after-hyperpolarization.

Interactions with other escape-swim CPG neurons:

Figure 4: A C2 stimulus produced all 4 synaptic actions in the pedal motor neuron, Pd5. Figure: <ref name="Snow82b">Snow RW (1982) Characterization of the Synaptic Actions of an Interneuron in the Central Nervous System of Tritonia. J Neurobiology 13(3): 251-266.</ref>.
  • C2 is weakly electrically coupled to its contralateral counterpart <ref name="Getting81">Getting PA (1981) Mechanisms of Pattern Generation Underlying Swimming in Tritonia. I. Neuronal Network Formed by Monosynaptic Connections. J Neurophysiol 46(1): 65-79.</ref>.
  • C2 both excites and inhibits the ipsilateral DSI. The excitation is fast in onset and small in amplitude, while the inhibition is larger and amplitude and has a delayed onset. In some cases the excitatory component is not present. <ref name="Getting81"/>
  • C2 inhibits and excites the ipsilateral VSI. The inhibitory component is fast in onset and small in amplitude, while the excitatory component is larger in amplitude and has a delayed onset. In some cases, the inhibitory component is not present <ref name="Getting81"/>.
  • C2 is excited by the ipsilateral DSI <ref name="Getting81"/>.
  • C2 is inhibited by the ipsilateral VSI <ref name="Getting83">Getting PA (1983)Mechanisms of Pattern Generation Underlying Swimming in Tritonia. III. Intrinsic and Synaptic Mechanisms for Delayed Excitation. J Neurophysiol 49(4): 1036-1050.</ref>.

Additional Inputs:

  • C2 is excited by neurons in the anterior half of the S-cell cluster <ref>Getting PA (1976) Afferent Neurons Mediating Escape Swimming of the Marine Mollusc, Tritonia. J Comp Physiol 110: 271-286.</ref>. This excitation is polysynaptic <ref>WN Frost, TA Hoppe, J Wang, and L M Tian. Swim initiation neurons in Tritonia diomedea. Am.Zool. 41 (4):952-961, 2001.</ref>.

Additional Outputs:

  • C2 polysynaptically inhibits Pleural neurons 2, 6, 7, and 8 <ref name="SnowA"/>
  • C2 has variable monosynaptic effects on motor neurons in the pedal ganglion. The effects can be seen in isolation, or in some instances all 4 effects can be recorded from a single follower neuron (Figure 4).
    • The effects on pedal follower neurons are a fast inhibitory PSP, a fast excitatory PSP, a slow inhibitory PSP, and a slow excitatory PSP <ref name="SnowA"/>.