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DSI is a Neuron in Tritonia.

Basic information

Serotonin immunohistochemistry shows the locations of the DSIs (circled).
  • NeuronBank AccessionID Tri0001043
  • Names and Aliases- Dorsal Swim Interneuron, DSI, DSI-A, DSI-B,C, or just swim interneuron.
  • Species: Tritonia diomedea
  • Neurotransmitter: Serotonin (5-HT)

There are three DSIs in each hemisphere of the cerebropleural ganglion. All three are serotonin-immunoreactive. They have axons that project contralaterally to the pedal ganglion. The DSIs play an important role in the swim central pattern generator. They have both synaptic and neuromodulatory actions on other neurons in the CPG. They fire rhythmic bursts of action potentials during the swim motor pattern. For more information see [1]. The DSIs also accelerate ciliary crawling by synapsing onto efferent neurons in the contralateral pedal ganglion.


The DSIs are homologous to the CeSP-A neurons in other Nudibranchs. They are also homologous to neurons in a similar position across all gastropods. Here are the names of identified homologues in other species. Positive identifications have been made in:

(Ref: Newcomb and Katz, 1997)

5-HT immunoreactivity alone suggests that they are present in many if not most opisthobranchs (Heterobranchia)

A drawing of a DSI with dendritic fields in the cerebral ganglia, projecting to the contralateral Pedal ganglion. Its axon projects out one of the Pedal connectives. Based on Getting et al.(1980)
Simultaneous intracellular recordings of two DSIs, DRI, and C2. Notice that the DSIs burst synchronously during the swim. Their action potentials are smaller at the onset of the motor pattern.



  • Serotonergic Neuron in CeSP cluster with contralateral projecting axon.
  • Generally the most lateral three neurons in the cluster are DSIs.
  • In the dissection microscope, with some reflected illumination, the DSIs appear somewhat translucent with a small pigment spot.
  • Cell fills show the DSIs exiting Pedal Nerve 5, which is the smaller of the two Pedal-Pedal connectives. But, electrophysiological data suggests that at least one DSI is present in Pedal Nerve 6. This might be a DSI-A vs B,C difference but hasn't been resolved.


  • Upon impaling a DSI with a sharp electrode, they are readily distinguished from surrounding neurons by their irregular spiking activity and the sharp afterhyperpolarization that follows each spike.
  • If 2 or more DSIs are impaled, regardless of whether they are ipsilateral to each other or contralateral, they will receive simultaneous spontaneous IPSPs.
  • The DSIs recruit inhibition onto themselves, so depolarization of one tends to inhibit the others.
  • The most unambiguous feature is the pattern of activity during a swim motor pattern.

Distinguishing DSI-A from DSI-B,C: There are three DSIs on each side of the cerebral ganglion. They are called DSI-A,B,C. DSI-B and DSI-C are indistinguishable. DSI-A differs from DSI-B,C.

  • The DSIs can also be recognized by their electrical coupling. DSI-B,C are electrically coupled both ipsilaterally and contralaterally. DSI-A is electrically coupled to its contralateral counterpart.
  • There are some synaptic differences between DSI-A and DSI-B,C.


  1. Fickbohm DJ, Lynn-Bullock CP, Spitzer N, Caldwell HK, Katz PS (2001) Localization and quantification of 5-hydroxytryptophan and serotonin in the central nervous systems of Tritonia and Aplysia. J Comp Neurol 437:91-105.
  2. Hill ES, Sakurai A, and Katz PS (2008) Transient enhancement of spike-evoked calcium signaling by a serotonergic interneuron. J.Neurophysiol. 100 (5):2919-2928.
  3. Getting PA (1977) Neuronal organization of escape swimming in Tritonia. J Comp Physiol A 121:325-342.
  4. Getting PA (1981) Mechanisms of pattern generation underlying swimming in Tritonia. I. Neuronal network formed by monosynaptic connections. J Neurophysiol 46:65-79.
  5. Getting PA, Lennard PR, Hume RI (1980) Central pattern generator mediating swimming in Tritonia. I. Identification and synaptic interactions. J Neurophysiol 44:151-164.
  6. Hume RI, Getting PA (1982) Motor organization of Tritonia swimming. II. Synaptic drive to flexion neurons from premotor interneurons. J Neurophysiol 47:75-90.
  7. Katz PS, Fickbohm DJ, Lynn-Bullock CP (2001) Evidence that the swim central pattern generator of Tritonia arose from a non-rhythmic neuromodulatory arousal system: Implications for the evolution of specialized behavior. Am Zool 41:962-975.
  8. Katz PS, Frost WN (1995a) Intrinsic neuromodulation in the Tritonia swim CPG: Serotonin mediates both neuromodulation and neurotransmission by the dorsal swim interneurons. J Neurophysiol 74:2281-2294.
  9. Katz PS, Frost WN (1995b) Intrinsic neuromodulation in the Tritonia swim CPG: The serotonergic dorsal swim interneurons act presynaptically to enhance transmitter release from interneuron C2. J Neurosci 15:6035-6045.
  10. Katz PS, Getting PA, Frost WN (1994) Dynamic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit. Nature 367:729-731. Tri0002301
  11. McClellan AD, Brown GD, Getting PA (1994) Modulation of swimming in Tritonia: Excitatory and inhibitory effects of serotonin. J Comp Physiol A 174:257-266.
  12. Newcomb JM, Fickbohm DJ, Katz PS (2006) Comparative mapping of serotonin-immunoreactive neurons in the central nervous systems of nudibranch molluscs. J Comp Neurol 499:485-505.
  13. Newcomb JM, Katz PS (2007) Homologues of serotonergic central pattern generator neurons in related nudibranch molluscs with divergent behaviors. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 193:425-443.
  14. Sakurai A and Katz PS. (2008) A serotonergic interneuron evokes both state-dependent and state-independent neuromodulatory actions. Neuroscience Meeting Planner.Washington, DC: Society for Neuroscience 574.3.
  15. Tian LM, Kawai R, Crow T. (2006) Serotonin-immunoreactive CPT interneurons in Hermissenda: identification of sensory input and motor projections. J.Neurophysiol. 96 (1):327-335.