Multiple spike initiation zones in a neuron implicated in learning in the leech: a computational model

Invert Neurosci. 2009 Mar;9(1):1-10. doi: 10.1007/s10158-008-0084-4. Epub 2009 Jan 14.


Sensitization of the defensive shortening reflex in the leech has been linked to a segmentally repeated tri-synaptic positive feedback loop. Serotonin from the R-cell enhances S-cell excitability, S-cell impulses cross an electrical synapse into the C-interneuron, and the C-interneuron excites the R-cell via a glutamatergic synapse. The C-interneuron has two unusual characteristics. First, impulses take longer to propagate from the S soma to the C soma than in the reverse direction. Second, impulses recorded from the electrically unexcitable C soma vary in amplitude when extracellular divalent cation concentrations are elevated, with smaller impulses failing to induce synaptic potentials in the R-cell. A compartmental, computational model was developed to test the sufficiency of multiple, independent spike initiation zones in the C-interneuron to explain these observations. The model displays asymmetric delays in impulse propagation across the S-C electrical synapse and graded impulse amplitudes in the C-interneuron in simulated high divalent cation concentrations.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Action Potentials*
  • Algorithms
  • Animals
  • Cations / metabolism
  • Electrical Synapses / physiology
  • Interneurons / cytology
  • Interneurons / physiology*
  • Learning*
  • Leeches / physiology*
  • Lysine / analogs & derivatives
  • Microelectrodes
  • Microscopy, Confocal
  • Models, Neurological*
  • Motor Neurons / physiology*
  • Synaptic Transmission / physiology


  • Cations
  • biocytin
  • Lysine