Fig. 1.

The initial segment and the first paranode of a spinal motor axon. Bar, 0.5 µm. (a) Distal part of the initial segment, where mitochondria accumulated. Microtubular fascicles are observed in the axoplasm (arrows). (b) Distal part of the first paranode. The paranodal axoplasm contains microtubular fascicles (arrows). (c) Endpoint of the initial segment. Beneath the axolemma, dense undercoating is observed (arrowheads). (d) Beginning of the first paranode. The periaxonal cytoplasm contained a great deal of spirally wound microtubules (arrows labelled with Mt).