title: Multiple spike initiation zones in a neuron implicated in learning in the leech: a computational model.
abstract: 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.
authors.affiliation: Biology Department and Neuroscience Program, St Olaf College, 1520 St Olaf Ave., Northfield, MN 55057, USA. crisp@stolaf.edu
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Crisp
mesh_terms: D000200:Action Potentials; D000465:Algorithms; D000818:Animals; D002412:Cations / Q000378:metabolism; D054351:Electrical Synapses / Q000502:physiology; D007395:Interneurons / Q000166:cytology / Q000502:physiology*; D007858:Learning; D007865:Leeches / Q000502:physiology*; D008239:Lysine / Q000031:analogs & derivatives; D008839:Microelectrodes; D018613:Microscopy, Confocal; D008959:Models, Neurological; D009046:Motor Neurons / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology
chemical_list: D002412:Cations; C013411:biocytin; D008239:Lysine