Analysis of impulse adaptation in motoneurons |
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Authors: | Jianghong Tian Tetsuya Iwasaki Wolfgang Otto Friesen |
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Institution: | (1) Department of Biology, University of Virginia, P.O. Box 400328, Charlottesville, VA 22904-4328, USA;(2) Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904-4328, USA;(3) Present address: Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095, USA; |
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Abstract: | 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. |
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