FIGURE 3.

Proprioceptive input to motor neurons is decreased with age. Motor neurons from GS muscle were retrogradely traced using G‐protein‐deleted rabies viral vectors encoding fluorescent proteins (ΔG‐prot. rabies FP). (a) vGlut1 distribution in the lumbar region of a representative spinal cord section. B, Number of motor neurons of the GS motor neuron pool. A minimum of five animals per group was analyzed. (c, d) Representative example of a 3D projection of a motor neuron tree area (C) and the reconstructed version in Imaris (d). All reconstructed vGlut1+ synapses touching motor neurons are in red and the rest light blue. (e) Synaptic density (# synapses/µm²) was calculated by dividing the number of synapses touching the motor neuron by the surface area of the motor neuron. (f) Number of synapses making direct contact to motor neurons. A minimum of 20 motor neurons from a minimum of five different animals per group were analyzed. In (a) scale bar = 200 µm and in (c, d) scale bar = 100 µm. Data are displayed as mean with ±SEM. Using two‐way ANOVA, *p < 0.05; **p < 0.01; ***p < 0.001 indicate statistically significant differences between 7 months controls and aged groups (age effect), while ^# p < 0.05 indicates statistically significant differences between sedentary and exercised animals of the same age group (exercise effect)