Original Paper

Journal of Comparative Physiology A

, Volume 196, Issue 2, pp 123-136

First online:

Analysis of impulse adaptation in motoneurons

  • Jianghong TianAffiliated withDepartment of Biology, University of VirginiaDepartment of Mechanical and Aerospace Engineering, University of Virginia
  • , Tetsuya IwasakiAffiliated withDepartment of Mechanical and Aerospace Engineering, University of VirginiaDepartment of Mechanical and Aerospace Engineering, University of California
  • , Wolfgang Otto FriesenAffiliated withDepartment of Biology, University of Virginia Email author 

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Animal locomotion results from muscle contraction and relaxation cycles that are generated within the central nervous system and then are relayed to the periphery by motoneurons. Thus, motoneuron function is an essential element for understanding control of animal locomotion. This paper presents motoneuron input–output relationships, including impulse adaptation, in the medicinal leech. We found that although frequency-current graphs generated by passing 1-s current pulses in neuron somata were non-linear, peak and steady-state graphs of frequency against membrane potential were linear, with slopes of 5.2 and 2.9 Hz/mV, respectively. Systems analysis of impulse frequency adaptation revealed a static threshold nonlinearity at −43 mV (impulse threshold) and a single time constant (τ = 88 ms). This simple model accurately predicts motoneuron impulse frequency when tested by intracellular injection of sinusoidal current. We investigated electrical coupling within motoneurons by modeling these as three-compartment structures. This model, combined with the membrane potential–impulse frequency relationship, accurately predicted motoneuron impulse frequency from intracellular records of soma potentials obtained during fictive swimming. A corollary result was that the product of soma-to-neurite and neurite-to-soma coupling coefficients in leech motoneurons is large, 0.85, implying that the soma and neurite are electrically compact.


Leech Hirudo Circuit Swimming Modeling