DRI

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The Dorsal Ramp Interneuron (DRI) is an interneuron in Tritonia.

Figure 1: Representation of DRI location in the Tritonia CNS. The neurons labeled in the left hemisphere are also found in the right hemisphere of the CNS. Pd = pedal ganglion; Pl = pleural ganglion; Ce = cerebral ganglion; C2 = cerebral neuron 2; DSI = dorsal swim interneuron; S = statocyst. Image adapted from Frost and Katz, 1996.

Basic information

  • NeuronBank AccessionID Tri0002471
  • Names and Aliases: Dorsal Ramp Interneuron, DRI.
  • Species: Tritonia diomedea

There is one DRI in each hemisphere of the pleural ganglion (Figure 1). It has an axon that projects into the ipsilateral cerebral ganglion and across the cerebral commissure into the contralateral cerebropleural ganglion (Figure 2). DRI is necessary and sufficient for evoking the swim motor program in Tritonia. It provides strong excitatory input to the DSIs.

Identification

Figure 2: A carboxyfluorescein fill (arrow) exhibits the DRI axonal morphology. Image adapted from Frost and Katz, 1996.

Anatomy

  • DRI is a bilaterally paired cell found one cell layer below the surface in the pleural ganglion (Figure 1)
  • The DRI axon projects ipsilaterally toward the DSIs and contralaterally across the central commissure (Figure 2).

Physiology

Figure 3: A swim motor pattern triggered by stimulation of pedal nerve 3. DRI fires tonically with brief periods of inactivity during the ventral phase of the swim motor pattern. Figure adapted from Frost and Katz, 1996.
  • DRI fires tonically at rest and produces large, constant latency, nondecrementing monosynaptic EPSPs in the DSIs
  • During the swim motor pattern, DRI fires tonically with brief periods of inactivity during the ventral phase of the rhythm (Figure 3)
  • C2 recruits DRI to excite DSI

Command Neurons

Figure 4: The Tritonia escape swim network. Afferent signals are sent to Tr1 which causes the activation of DRI. DRI then activates the escape swim CPG. Figure adapted from Frost et al., 2001

DRI is considered a command neuron. In this instance it is a gating command neuron, similar to the gating neurons of the Leech swim and crawl motor programs. DRI must be firing in order for Tritonia to swim, and stimulating DRI is sufficient to start the swim, whereas hyperpolarizing DRI will prematurely halt the swim. However, depolarizing DRI to maintain firing will not cause the swim motor pattern to continue indefinitely (Katz, unpublished observations). DRI acts in concert with a trigger neuron, Tr1. When Tr1, is transiently excited a sustained excitation of DRI is initiated, leading to the activation of the swim motor pattern (Figure 4).

References

  1. Frost WN,Katz PS (1996) Single neuron control over a complex motor program. Proc Natl Acad Sci U S A 93:422-6 Tri0002314
  2. Frost WN, Hoppe TA, Wang J, and Tian LM. Swim initiation neurons in Tritonia diomedea. Am.Zool. 41 (4):952-961, 2001. Tri0002816