Journal of Biological Physics

, Volume 37, Issue 3, pp 317–345 | Cite as

Positive feedback loops sustain repeating bursts in neuronal circuits

  • Wolfgang Otto Friesen
  • Olivia J. Mullins
  • Ran Xiao
  • John T. Hackett
Original Paper


Voluntary movements in animals are often episodic, with abrupt onset and termination. Elevated neuronal excitation is required to drive the neuronal circuits underlying such movements; however, the mechanisms that sustain this increased excitation are largely unknown. In the medicinal leech, an identified cascade of excitation has been traced from mechanosensory neurons to the swim oscillator circuit. Although this cascade explains the initiation of excitatory drive (and hence swim initiation), it cannot account for the prolonged excitation (10–100 s) that underlies swim episodes. We present results of physiological and theoretical investigations into the mechanisms that maintain swimming activity in the leech. Although intrasegmental mechanisms can prolong stimulus-evoked excitation for more than one second, maintained excitation and sustained swimming activity requires chains of several ganglia. Experimental and modeling studies suggest that mutually excitatory intersegmental interactions can drive bouts of swimming activity in leeches. Our model neuronal circuits, which incorporated mutually excitatory neurons whose activity was limited by impulse adaptation, also replicated the following major experimental findings: (1) swimming can be initiated and terminated by a single neuron, (2) swim duration decreases with experimental reduction in nerve cord length, and (3) swim duration decreases as the interval between swim episodes is reduced.


Neuronal circuits Leech Reciprocal excitation Mutual excitation Episodic behavior 





Bursts per episode


Dorsal posterior (nerve)


Central nervous system


Central pattern generator






Recurrent cyclic inhibition


Reciprocally excitatory



Funding was provided by NSF grant IOB-0615631.


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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  • Wolfgang Otto Friesen
    • 1
    • 2
  • Olivia J. Mullins
    • 1
    • 2
  • Ran Xiao
    • 1
  • John T. Hackett
    • 2
    • 3
  1. 1.Department of BiologyUniversity of VirginiaCharlottesvilleUSA
  2. 2.Neuroscience Graduate ProgramUniversity of VirginiaCharlottesvilleUSA
  3. 3.Department of Molecular Physiology and Biological PhysicsUniversity of VirginiaCharlottesvilleUSA

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