The proximal portions of axons of large anterior-horn cells were investigated in the lumbar cords of 10 normal human autopsy cases. Light-microscopically, 81 myelinated axons were observed to be connected with the cell body. Of the 81 axons, 78 emanated from the cell body and 3 others originated in the proximal part of primary dendri-tes. As for normal-looking neurons (n = 77), the length of the axon hillock plus initial segment was 64.0 ± 12.3 µm (average ± SEM) ranging from 47.5 to 110.0 µm, while the diameter of the thinnest portion of the initial segment was 2.40 ± 0.30 µm (average ± SEM), ranging from 1.32 to 3.92 µm. Electron-microscopically, the predominant organelles of the axon hillock were mitochondria, neurofilaments which merged into the axon and occasional granular endoplasmic reticulum. A few synaptic boutons were found on the surface of the axon hillock. The cell membrane of the initial segment consisted of a layer of electron-dense material (undercoating). The cytoplasm contained many neurofilaments, running parallel to the longitudinal axis of the initial segment. Among the neurofilaments, lysosomes, smooth endoplasmic reticulum, dense bodies and vesicular profiles as well as mitochondria were seen. At the beginning of the myelin sheath, the axoplasm contained mitochondria, many neurofilaments and occasional ly
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