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

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.