Spinal cord was carefully flushed using saline, and lumbar spinal cord was dissected for further analysis. Each gastrocnemius muscle from each animal was dissected and gently stretched for 10 s before being immersed in fixing solution. Both tissues were fixed in 2.5% glutaraldehyde in 0.1 mol/l cacodylate buffer, pH = 7.4. The fixed material was sectioned at the Stanford Electron Microscopy Facility. Sections were taken between 75 and 80 nm, picked up on formvar/carbon‐coated 75 mesh Ni grids and stained for 20 s in 1:1 saturated uracetate (≈7.7%) in acetone followed by staining in 0.2% lead citrate for 3–4 min for contrast. For each spinal cord, sample two tissue blocks (volume of 5 mm3) were cut to obtain an average of 20 grids. Each grid included at least five cells, which were analyzed along non‐serial sections. Motor neurons were selected based on classic morphological features (multipolar cells with dispersed nuclear chromatin and prominent nucleoli). Mitochondrial samples were observed in a JEOL 1230 transmission electron microscope at 80 kV, and photographs were taken using a Gatan Multiscan 791 digital camera.