Ultrastructural synaptic features differ between α- and γ-motoneurons innervating the tibialis anterior muscle in the rat

Authors

  • Ronaldo M. Ichiyama,

    1. Department of Physiological Science, University of California, Los Angeles, California 90095
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  • Jonas Broman,

    1. Department of Experimental Medical Science, Lund University, S-221 00 Lund, Sweden
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  • V. Reggie Edgerton,

    1. Department of Physiological Science, University of California, Los Angeles, California 90095
    2. Department of Neurobiology, University of California, Los Angeles, California 90095
    3. Department of Neurology, University of California, Los Angeles, California 90095
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  • Leif A. Havton

    Corresponding author
    1. Brain Research Institute, University of California, Los Angeles, California 90095
    2. Department of Neurology, University of California, Los Angeles, California 90095
    • Department of Neurology, David Geffen School of Medicine at UCLA, 710 Westwood Plaza, Los Angeles, CA 90095-1769
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Abstract

We investigated the synaptology of retrogradely labeled spinal motoneurons after injection of horseradish peroxidase into the tibialis anterior (TA) muscle of adult rat. In total, 32 TA motoneurons were investigated in the electron microscope and demonstrated a bimodal size distribution with cell diameter peaks at 40 μm and 20 μm, likely representing α- and γ-motoneurons, respectively. Both α- and γ-motoneurons were apposed by S- and F-type synaptic boutons, whereas only α-motoneurons demonstrated inputs by the large M- and C-type boutons. The proportion of cell body membrane covered by synaptic inputs was surprisingly indistinguishable between α-motoneurons (72.2%) and γ-motoneurons (63.5%). The ratio between the number of F- and S-type boutons in apposition with the motoneuron cell body (F/S ratio) and the ratio between the soma membrane coverage provided by F- and S-type boutons were both significantly higher in α- than in γ-motoneurons. When comparing our data with previous findings in other species, we conclude that rat TA α-motoneurons are similar to cat and primate α-motoneurons with regard to synaptic terminal morphology, frequency, and distribution. However, rat γ-motoneurons show a markedly higher total synaptic coverage and frequency than cat γ-motoneurons, although both species exhibit appositions made by the same synaptic types and similar ratios between inhibitory and excitatory inputs. J. Comp. Neurol. 499:306–315, 2006. © 2006 Wiley-Liss, Inc.

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